A review of nitrogen utilisation
efficiency in the dairy cow, and what
milk producers can do now to improve it
J.M. Moorby and H.R. McConochie
September 2002
Table of Contents
SUMMARY ....................................................................................................................................................3
INTRODUCTION .........................................................................................................................................4
OBJECTIVE ..................................................................................................................................................6
FORAGE EFFECTS ON NITROGEN UTILISATION EFFICIENCY ..................................................6
EFFECTS OF FERTILISER NITROGEN APPLICATION ON NITROGEN UTILISATION EFFICIENCY ..........................6
IMPROVED RUMEN NUTRIENT BALANCE AT GRAZING ..................................................................................7
EFFECTS OF ENSILING GRASS ON NITROGEN UTILISATION EFFICIENCY .........................................................9
EFFECTS OF FEED SUPPLEMENTS ON NITROGEN UTILISATION EFFICIENCY ..................10
ACTION OF SUPPLEMENTS ..........................................................................................................................10
EFFECTIVE SUPPLEMENTS FOR IMPROVING NITROGEN UTILISATION EFFICIENCY .......................................13
MILK PROTEIN OUTPUT AND NITROGEN UTILISATION EFFICIENCY ..................................16
MODE OF ACTION OF SUPPLEMENTARY AMINO ACIDS ................................................................................18
Energy balance ....................................................................................................................................19
Amino acid profile................................................................................................................................19
EFFECTS OF DAIRY COW MANAGEMENT ON NITROGEN UTILISATION EFFICIENCY ....20
MILKING FREQUENCY AND BST.................................................................................................................20
MILK UREA NITROGEN ...............................................................................................................................21
EFFECTS OF FORAGE CONSERVATION ON NITROGEN UTILISATION EFFICIENCY ........22
GRASS SILAGE ...........................................................................................................................................22
Crop maturity .......................................................................................................................................23
Wilting ..................................................................................................................................................24
Silage additives ....................................................................................................................................24
LEGUME SILAGES.......................................................................................................................................26
Nitrogen utilisation efficiency of ruminants fed forage and grain legumes .........................................27
Tannin-containing legumes ..................................................................................................................29
Non-tannin containing legumes ...........................................................................................................30
Harvest management of legumes and nitrogen utilisation efficiency ...................................................33
RECOMMENDATIONS ............................................................................................................................34
FORAGE .....................................................................................................................................................34
Grazed forages .....................................................................................................................................34
Conserved forages ...............................................................................................................................35
RUMEN ENVIRONMENT ..............................................................................................................................36
PERIPHERAL AND MAMMARY TISSUE METABOLISM ...................................................................................36
DAIRY COW MANAGEMENT........................................................................................................................37
APPENDIX 1 ...............................................................................................................................................38
REFERENCES ............................................................................................................................................41
2
Summary
Nitrogen utilisation efficiency (NUE) of dairy cattle on UK dairy farms is typically 20 –
30%. This means that 70 – 80% of dietary nitrogen is excreted, costing the dairy farmers
in lost revenue, and contributing to environmental pollution. There are a number of
things that the dairy farmer can do immediately to reduce N excretion from his cows,
most of which are related to feeding the animal. The greatest loss of dietary N results
from an imbalance between the supply of N and fermentable energy sources in the rumen,
leading to the absorption of ammonia by the cow, a large proportion of which is excreted
in urine. Crop management, cow management, silage making processes, and accurate
diet formulation can all contribute to improving dairy cow NUE.
Post rumen inefficiencies in the utilisation of dietary N also occur, the most important of
which is the use of amino acids for gluconeogenesis instead of for protein synthesis.
Another cause of post-rumen inefficiency is an imbalanced supply of amino acids in
relation to requirements for milk protein synthesis. The impact of both of these processes
can be reduced through accurate diet formulation, ensuring that appropriate supplements
are provided to the animals.
Specific recommendations for actions by dairy farmers which would lead to
improvements in nitrogen utilisation efficiency by dairy cows include:

Reduce levels of N fertiliser application to grazing swards

Use grass varieties in new swards that contain high concentrations of water
soluble carbohydrates

Move cows to fresh pastures after afternoon milking

Rapid wilt silage crops and use an effective bacterial inoculant to maximise
residual water soluble carbohydrate concentrations and reduce proteolysis

Supply appropriate feed supplements (containing starches or sugars) to allow
rumen microbes to make use of rapidly available N fractions of the diet

Consider the use of supplements containing rumen-protected methionine and
lysine with maize-based diets

Use total-mixed rations to allow more accurate diet formulations

Use milk urea concentrations as an indicator of inefficient use of feed nitrogen.
3
Introduction
In comparison with other ruminant and non-ruminant food production systems in the
United Kingdom, the dairy industry is the most important contributor to nitrogen
pollution (Castillo et al., 2000b). Nitrogen losses in agriculture are responsible for both
atmospheric pollution, by volatilisation and denitrification of ammonia and nitrous oxide,
and hydropheric pollution, by the leaching of nitrates into water. Intensification of the
agricultural industry throughout Europe, characterised by the use of higher levels of
nitrogen fertiliser and feed protein, corresponds to an increase in atmospheric pollution.
Atmospheric ammonia inputs have increased 50% since 1950, and the agricultural sector
is responsible for about 40% of anthropogenic nitrous oxide emissions (Morard, 1999).
Hydropheric nitrate pollution is the subject of European directives that stipulate that the
nitrate concentration of water must not exceed 50 mg nitrate/litre. However, this has been
found to be exceeded in many waterways in the UK and is likely to be a consequence of
leaching of surplus agricultural nitrogen which can amount to 40% of the nitrogen
applied to agricultural land (Jarvis, 1994; Ledgard et al., 1998).
The 1991 Nitrate Directive, which is in the process of being fully implemented in
England through the designation of Nitrate Vulnerable Zones (NVZ), concern over
emissions of greenhouse gases, and current government interest in environmental
protection, are all likely to result in greater regulation regarding the use of nitrogen and
the production and disposal of nitrogenous by-products. Such regulation is likely to incur
additional costs of production on the dairy industry. Implementation of the 1991 Nitrates
Directive alone is forecast to cost the agricultural industry £26-36 million per year
(DEFRA, 2001). Future regulation regarding nitrate pollution may follow that of the
Netherlands, which involves a tax on nitrogen balance surpluses. The prospect of such a
situation occurring in the UK increases the importance of improving nitrogen utilisation
efficiency (NUE) on dairy farms. However, one must not ignore the potential financial
benefits of improving NUE in the milk production system as improvements would be
associated with either higher returns or reduced feed costs. Throughout this review NUE
has been calculated as follows:
NUE (%) = [N used for productive purposes (milk, growth) / N intake] x 100.
In general, NUE on the dairy farm is very low (20 to 30 %) and in very intensive
situations it can be even lower. Korevaar (1992) reported that in the Netherlands whole
farm NUE levels were as low as 14 % in intensive systems. Bussink and Oenema (1998)
calculated that whole farm NUE of up to 46 % is possible and whole farm experiments
conducted in the United States confirm this: using an integrated approach to nitrogen
utilisation on mixed crop and dairy farms, NUE of 42 and 47% have been realised (Dou
et al., 1996).
There are two main areas of production that affect NUE. Firstly, in recent years there has
been a shift in European farming practices from mixed to specialised intensive milk
production operations. According to Weissbach and Ernst (1994), the loss of integration
between livestock and crop production has led to a situation in which grassland alone
cannot utilise the nitrogen excreted by livestock, and this in turn has led to increased
losses of N and the importation of additional nitrogen onto the farm in the form of feed
and fertiliser. Secondly, ruminants are inherently inefficient converters of nitrogen into
protein. An analysis of nitrogen balance experiments conducted on 91 diets and 580
dairy cows in different countries (Castillo et al., 2000b) revealed that the average
efficiency of utilisation of N intake for milk production is 28%. Analysis of additional N
balance data (100 dairy cow diets and 27 sheep diets; Appendix 1) agrees with this, with
a mean NUE of 26%. The greatest proportion of nitrogen loss occurs in the rumen in
which ammonia produced from the microbial catabolism of proteins and amino acids and
which is not incorporated into microbial protein, is absorbed across the rumen wall and
either excreted as urea in urine or recycled back to the gut in saliva or across the intestinal
mucosa. This inefficiency, through losses in urine, is the predominant reason for the poor
NUE of the dairy farm.
We estimate that improvements in NUE from 26% to 35% would be worth in excess of
£100 million per year to the UK dairy industry and could cut surplus nitrogen production
by 49 thousand tonnes.
5
Objective
The purpose of this review is to highlight the areas of the whole milk production system
including nutrition and forage conservation, where the inefficiencies of nitrogen
utilisation exist. The effect of inefficiencies on milk production and milk quality will
also be examined. The review will conclude with a number of suggestions for improving
NUE on dairy farm in the short term.
Forage effects on nitrogen utilisation efficiency
In the UK, fresh and conserved grass constitutes a considerable proportion of the dairy
cow’s diet. The quality of the forage protein, in terms of the proportions of true protein to
non-protein nitrogen (NPN), can have a major bearing on the utilisation efficiency of the
nitrogenous fractions of grass and grass silage. In fresh herbage true protein constitutes
between 75% to 90% of the nitrogen fraction, however during ensilage 45% to 60% of
this may be degraded to NPN (Woolford, 1984). Pasture management practices such as
N application and maturity of herbage at harvest have been shown to affect the ratio of
true protein to NPN, its degradability, and the concentrations of other important fractions
such as water soluble carbohydrates (WSC; Peyraud et al., 1997). Although much of the
protein entering the rumen is degraded into NPN and incorporated into microbial protein,
this process tends to be the point at which inefficiencies occur, leading to poor NUE in
the dairy cow. Van Straalen et al. (1993) found that 70% of the crude protein (CP) in
grass silage is degraded in the rumen compared to around 58% of the CP in fresh ryegrass
and clover. Dietary supplementation can influence the fate of N released in the rumen,
and NUE has been found to increase from 17% to 30% in dairy cows offered herbage
with starchy or fibrous concentrate supplements (Valk et al., 1990).
Effects of fertiliser nitrogen application on nitrogen utilisation efficiency
Intensification within the dairy industry has led to an increase in the rate of fertiliser N
being applied to grassland in order to realise higher stocking rates. Nitrogen balance data
from a number of experiments which explored increased rates of N fertiliser application
on grassland (Astigarraga et al., 1993; Delagarde et al., 1997; Peyraud et al., 1997),
showed that NUE decreased with increasing N application rate. This is probably due to
an increase in the rumen degradability of the forage nitrogen as a result of increased N
6
application and a reduction in the concentration of WSC (Peyraud and Astigarraga,
1998). Van Vuuren et al. (1992) showed that rumen ammonia concentrations in dairy
cows offered herbage receiving 500 kg N/ha/year was higher than that of animals offered
herbage receiving 275 kg N/ha/year. Likewise Astigarraga et al. (1993) showed that
higher rates of fertiliser N application increased rumen ammonia concentrations and as a
result increased blood urea concentrations. Although some of this N is utilised through
the recycling of urea to the rumen, much N is lost in urine. The inability of rumen
microbes to convert rumen available N into protein is the main reason for the poor NUE
associated with heavily fertilised herbage. This is not simply a response to increased CP
intake and the ratio between true protein and NPN in the CP. Of equal or greater
importance is the dietary supply of fermentable carbohydrates (FC) and the balance of
these with CP in the herbage. There is an inverse correlation between FC and CP in fresh
grass, which is directly related to the rate of N fertiliser application (Peyraud and
Astigarraga, 1998). As higher rates of N fertiliser are applied, grass CP concentration
increases and water soluble carbohydrate (WSC) concentrations decrease, which can
Nitrogen use efficiency (%)
result in a reduction in NUE (Figure 1).
40
35
30
25
20
15
0
50
100
150
200
250
300
N application, kg/ha of pasture
Figure 1. Relationship between N fertiliser application to grazing pastures and nitrogen
use efficiency in the dairy cow (data from Keady and Murphy, 1998, Keady et al., 1998
and Peyraud et al., 1997).
Improved rumen nutrient balance at grazing
Beever and Siddons (1986) suggested that N losses arising from ammonia absorption
from the rumen are a consequence of the imbalance that exists between the degradation
7
rates of ammonia-producing herbage N, and energy yielding organic matter (the so-called
asynchrony in supply of energy and nitrogen). The effect of improved balance between
dietary N and FC has been characterised in experiments studying the effect of offering
cows perennial ryegrass bred for increased WSC concentrations on milk and milk protein
production (Miller et al., 2001b; Moorby and Miller, unpublished data). In these
experiments NUE was increased by up to 7% above a control variety of ryegrass, and
overall efficiency of milk protein production was as high as 37%, apparently in response
to the additional WSC present in the herbage. Work carried in vivo (Lee et al., 2002b)
and in vitro (Miller et al., 2001b; Lee et al., 2002a) has demonstrated that incubation of
grass with a high WSC concentration produced a lower concentration of ammonia and
increased the production of propionate in rumen liquor compared with control grass.
Diurnal changes in grass WSC concentrations (Figure 2) mean that concentrations are
higher in the afternoon than in the morning, when the plant has had time to
photosynthesis. Orr et al. (2001) showed positive milk yield benefits from allocating
fresh grass after afternoon milking, rather than after morning milking, using a strip
grazing system. Dry matter intake was not different between animals offered fresh grass
morning or afternoon, but milk yield was 5% higher from animals offered fresh grass
after afternoon milking, probably as a result of increased DM and WSC concentrations at
this time (Orr et al., 2001). The authors did not report milk composition, so it is not
possible to calculate NUE from their results, although increased milk yields from similar
rates of DM intake indicate increased efficiencies.
8
Water soluble carbohydrate
concentration, g/kg DM
250
200
150
100
50
0
06.00
08.00
10.00
12.00
14.00
Time of day (h)
16.00
18.00
20.00
Figure 2. Diurnal changes in grass WSC concentration (from Miller et al., 2001b).
Effects of ensiling grass on nitrogen utilisation efficiency
As a result of proteolysis and deamination reactions that occur during ensiling, the
proportion of true protein in forage is reduced and the concentration of other nitrogenous
components, predominantly ammonia and free amino acids, is increased. The increased
ratio of non protein nitrogen (NPN) constituents increases the rumen degradability of
grass silage nitrogen compared to the fresh material (Keady and Murphy, 1998; Van
Straalen et al., 1993). However, this does not necessarily lead to differences in NUE,
particularly in non-lactating animals with low N intakes (Keady and Murphy, 1998)
which increases the importance of urea recycling (Van Soest, 1994). In the same study,
NUE declined from 21% for grazed herbage to 18% for ensiled herbage in lactating
animals (Keady and Murphy, 1998) while supplemental FC, in the form of sucrose added
to the grass silage diet, appeared to increase NUE and milk protein concentration in the
lactating cows. The authors concluded that this was a result of an improved microbial
protein supply to the small intestine.
The need for accurate and appropriate
supplementation of grass silage diets is therefore important if high levels of NUE are to
9
be achieved, predominantly because of the increased protein breakdown products in the
forage.
Effects of feed supplements on nitrogen utilisation efficiency
Dairy cows are fed a variety of feeds to supply protein and energy, with the objective of
providing the best diet to supply nutrients for good productivity. Increased supplies of
protein in the diet often result in increased milk protein production, although this
frequently occurs at the expense of NUE. In experiments in which grazing dairy cows
received a supplement of 2 kg per day of soya (Astigarraga et al., 1993; Delagarde et al.,
1997) NUE fell by up to 6 %, and this reduction in efficiency was predominantly a result
of increased urinary N excretion. Similarly, supplementation of grass silage-based diets
with different levels of protein, and protein of varying degradability, resulted in a
decrease in NUE as crude protein concentration and degradability increased, although
diet had no effect on milk or milk protein production despite an increase in the flow of
rumen undegradable protein (RUP) to the small intestine (Castillo et al., 2000a). A recent
review of the literature on the effect of RUP on dairy cow performance concluded that
increased concentrations of dietary RUP had little effect on performance, especially when
it results in a shortage of rumen degradable protein (RDP; Santos et al., 1998).
Improvements in the efficiency of microbial protein synthesis are integral to improving
NUE, and the microbial protein synthesised, in terms of its essential amino acid content
compared to milk protein, is as good as most supplements containing high concentrations
of RUP (Santos et al., 1998). On grass-based diets which are abundant in N, provision of
additional carbohydrates to the diet have been shown to improve NUE while maintaining
animal performance (Keady et al., 1998; Reynolds et al., 2001; Valk, 1994).
Action of supplements
The efficient capture of dietary nitrogen and the synthesis of microbial protein by the
rumen microbial population both require a balanced supply of both nitrogen and energy.
When supply becomes unbalanced, losses of nitrogen as ammonia occur across the rumen
wall and this is subsequently lost as urea in urine. This is primarily due to a shortage of
ATP obtained from the fermentation of carbohydrates, which is necessary for protein
synthesis from amino acids and ammonia. Limited supplies of carbohydrate cause rumen
10
bacteria to degrade amino acids for the production of ATP, which produces additional
ammonia (Nocek and Russell, 1988).
Although the need for a balance between energy and nitrogen is well recognised
(Dewhurst et al., 2000), some authors advocate the requirement for strict synchronicity in
the supply of both fractions (Sinclair et al., 1993; Sinclair et al., 1995). Although it has
been shown that strict synchronicity results in lower urea plasma concentrations, which
indicates a more efficient use of dietary nitrogen (Witt et al., 2000), in vivo and in vitro
experiments have shown that strict synchrony is not necessary for maximum microbial
protein synthesis (Newbold and Rust, 1992), or production (Casper et al., 1999;
Chamberlain and Choung, 1995; Newbold and Rust, 1992). Theoretically, a high degree
of synchrony should result in improvements in NUE by reducing the amount of ammonia
escaping the rumen. However, nitrogen balance experiments in which synchronous and
asynchronous diets based on grazed herbage have been compared, showed little
difference in NUE or milk production, despite the fact that blood urea concentrations
were elevated in cows offered the asynchronous diet (Kolver et al., 1998). Experiments
carried out by Sinclair et al. (1993; 1995) showed an increase in the production of
microbial protein by sheep offered synchronous diets, although this did not appear to
extend to benefits in terms of increased milk production (Witt et al., 2000). Newbold and
Rust (1992) demonstrated in vitro that microbial protein production was not altered by a
synchronous supply of energy and nitrogen compared to an asynchronous supply. This
was probably due to the ability of rumen microbes to respond to transient periods of
energy depravation by storing polysaccharides (Van Kessel and Russel, 1997), and then
drawing on these reserves for protein synthesis when ammonia concentrations are
elevated. However, the situation in vivo differs to that occurring in the artificial in vitro
environment in two ways. Firstly, the rumen microbes do not remain in the rumen, but
are continually passing out of the rumen and therefore taking their stored polysaccharides
with them. Although it could be argued that any stored polysaccharide would be
metabolised in futile non-protein producing, self maintaining cycles, these would cease in
the acidic environment found post ruminally. Secondly, ammonia not captured by the
rumen microbes crosses the rumen epithelium, and although can return as urea via saliva
or transfer back across the rumen wall, losses inevitably occur.
11
The questions are therefore, if the efficiency of capture of rumen ammonia is reduced on
asynchronous diets then why do animals perform as well as those receiving synchronous
diets and why are no differences in nitrogen balance observed between the two regimes?
The recycling of N back to the rumen via plasma urea as an explanation seems
inadequate, since the animal’s urea pool is under physiological control. Van Soest (1994)
suggested that at the homeostatic level, urea recycling is very efficient, and this is
demonstrated by the NUE results obtained by Keady et al. (1998) with dry cows
consuming small quantities of N. However, as rumen absorption of ammonia increases
as a response to increased dietary N, the efficiency of urea N recycling is reduced and
losses in urine increase proportionally. This would be an important factor on low protein
diets, although it appears therefore that some other factor is responsible for reducing
urinary N loss and maintaining animal performance on asynchronous diets.
One possible mode of action is that microbial storage polysaccharides synthesised in the
rumen are degraded to glucose and absorbed in the lower gut. This supply of glucose
above that produced endogenously from propionate in the liver has the potential to be
metabolised to -ketoglutarate via the tricarboxylic acid cycle. Alpha-ketoglutarate may
then be involved in amination reactions with ammonia leading to the production of
glutamate, which itself is the precursor of numerous amino acids. This pathway may
serve as an alternative for detoxification of absorbed ammonia, and unlike the ornithine
cycle, theoretically does not consume amino acids or result in an increase in output of
urea in urine. This hypothesis is supported by the findings of Obitsu et al. (2000) who
demonstrated that glucose supplied post-ruminally resulted in lower plasma urea
concentrations. Similarly, Reynolds et al. (2001) found that post-rumen glucose infusions
reduced urinary nitrogen excretion and partitioned the retained N into tissue, rather than
increasing milk protein output, which may have been an effect of insulin secretion in
response to blood glucose concentrations. Absorption of glucose from the gut may also
improve NUE by reducing the deamination of glucogenic amino acids for the production
of energy, a degradative pathway that produces urea as a by-product. In this way,
increased glucose supplies allow milk yields to be maintained or improved on diets of
12
lower CP content. In a similar but different area, supplementation of diets with feeds that
supply otherwise limiting amino acids can help improve animal performance and NUE.
Although high concentrations of ammonia in the rumen can lead to losses of N in urine, a
number of in vivo and in vitro studies (Annison, 1970; Hume et al., 1970; Mehrez et al.,
1977; Mercer and Annison, 1976; Rooke et al., 1987; Satter and Slyter, 1974) have
suggested that if the rumen ammonia concentration falls below a concentration of about
8 mM (470 mg/litre), microbial protein synthesis can be limited. Kanjanapruthipong and
Leng (1998) found that the efficiency of microbial protein synthesis increased by up to
47 % when ammonia concentration was greater than 200 mg of N/litre. Although
minimum rumen ammonia concentration is important because microbial protein
production contributes a highly significant proportion of total protein supply to the
animal, defining a concentration that is optimal in terms of efficiency is difficult. This is
because of the close relationship between N and energy availability within the rumen,
which governs the ability of the microbial population to convert N into microbial protein.
The evidence suggests, therefore, that NUE in the dairy cow can be improved by correct
supplementation of grazed or ensiled forage with either a source of starch with a low
degradability in the rumen, which acts post-ruminally, or FCs that closely mirror the
degradative pattern of the nitrogenous components in the forage. Alternatively increasing
the WSC content of the forage through grass breeding or management manipulation can
result in improvements in NUE.
Effective supplements for improving nitrogen utilisation efficiency
There is considerable experimental evidence to suggest that there are two major means of
improving the NUE in dairy cows offered forage based diets: by improving the synchrony
and/or balance in supply of energy and nitrogen in the rumen, or by increasing the post
ruminal supply of glucose.
Synchrony may be improved by providing additional rumen FC to diets containing high
levels of soluble N or by reducing the N content or N solubility of the forage. The latter
two may be achieved by reducing the level of fertiliser N application, and the first may
13
be achieved by treating conserved forages with formic acid or an effective bacterial
inoculant.
A multiple regression analysis of published data (Astigarraga et al., 1993; Valk and
Hobbelink, 1992; Valk et al., 1990; Van Vuuren, 1993; Valk, 1994; Petit and Tremblay,
1995; Delagarde et al., 1997; Peyraud et al., 1997; Keady et al., 1998; Keady and
Murphy, 1998; Moorby and Theobald, 1999; Castillo et al., 2000b; Reynolds et al.,
2001; Castillo et al., 2000a; Aston et al., 1998; Kolver et al., 1998; Kebreab et al., 2000;
Miller et al., 2001a; Miller et al., 2001b; Kebreab et al., 2001; Moorby et al., 2001)
explored the relationship between NUE, N intake and N excretion in urine. Even though
it may be assumed that NUE and N intake would be closely correlated (Figure 3)
because N intake is used in the calculation of NUE, the most significant relationship is
between NUE and N excretion in urine (Figure 4). The regression equation was found to
be NUE% = 37.1 + 0.00048  N intake [g/d, P = 0.95] – 0.058  urine N excretion [g/d,
P < 0.001]; s.e. obs. = 3.98; r2 = 0.59. The intercept figure of approximately 40%
indicates the mean theoretical maximum NUE for the animals used in the calculation, the
remaining 60% representing obligate losses through protein turnover and other
maintenance requirements. The effect of N intake on NUE was found to be nonsignificant while the effect of the coefficient for urine N excretion was highly significant.
This suggests that the major effects on NUE are those occurring within the animal which
lead to losses of N in urine essentially regardless of the level of N intake. Many of these
effects act at the rumen level, and appropriate nutritional manipulation of the animal can
help to alleviate this problem.
14
Nitrogen utilisation efficiency, %
45
40
35
30
25
20
15
10
5
0
100
200
300
400
500
600
700
800
N intake, g/d
Figure 3. Relationship between N intake and nitrogen utilisation efficiency in the dairy
cow. References to data sources are given in the text.
Nitrogen utilisation efficiency, %
45
40
35
30
25
20
15
10
5
0
0
100
200
300
400
500
Urine N excretion, g/d
Figure 4. Relationship between excretion of N in urine and nitrogen utilisation
efficiency in the dairy cow. References to data sources are given in the text.
15
Experimentally, supplying glucose post ruminally has been achieved by intra gastric
infusions of glucogenic precursors or glucose. In practice, however, this appears to be
best achieved by increasing the supply of starch or by providing low rumen degradable
starch. The supply of starch to the small intestine is affected by a number of factors,
including source, maturity of forage and extent of processing of the grain supplement.
Resistance to rumen degradation of starch present in maize silage has been shown to
decrease with plant maturity, which was shown to alter N partitioning with an increase in
milk N and a decrease in urinary N (Cammell et al., 2000a). Cammell et al. (2000b)
showed non-ammnonia N flow to the duodenum was unaffected, and that the response
found was possibly mediated through a sparing of amino acids as discussed previously.
Hameelers (1998) found no effect on NUE or milk protein production of dairy cows
offered whole crop maize or wheat starch, but noted that neither crop was in the optimal
condition, the maize was low in starch and the wheat was too mature. Sutton et al. (1998)
and Abdalla et al. (1999) found that feeding whole crop wheat resulted in an increase in
supply of starch to the duodenum, but insufficient processing during ensilage meant that
this was largely due to the presence of whole grains in the digesta. In this situation the
effect of starch by-passing the rumen would be limited because it is not digested, and
although increases in protein concentration were obtained there was no reduction in
urinary N excretion (Abdalla et al., 1999; Sutton et al., 1998), which is in contrast to
results obtained by supplying glucose in a highly absorbable form via intra gastric
infusions (Obitsu et al., 2000; Reynolds et al., 2001).
It can be concluded that if
imbalance or asynchrony in the rumen leads to a large uptake of ammonia by the animal,
a supply of glucose from the gut can help to reduce further urea production by sparing
amino acids or even utilising some of the ammonia as a source of N for transamination
processes. If sufficient glucose is not available, the ammonia-N will mostly be lost in
urine.
Milk protein output and nitrogen utilisation efficiency
The process of milk protein synthesis in the mammary gland is another source of
inefficiency associated with nitrogen utilisation. For this reason an understanding of the
processes involved, and the possibilities of manipulating them are central to improving
NUE in the dairy cow. In order to put into context the importance of improving the
16
efficiency of mammary protein synthesis on whole animal NUE, a summary of reported
literature on the use of encapsulated ruminally-protected methionine and lysine showed
that supplementation had a consistent positive effect on N excretion in milk and on NUE
(Robinson et al., 1995). Lysine and methionine tend to be the two first limiting amino
acids on maize-based diets. Inefficiencies are due to the fact that the ratio between
mammary protein synthesis and protein excreted in the milk is in the region of 1.5-2.6 : 1.
This results in a continual cycling of proteins and amino acids through degradative and
synthetic pathways, that not only results in the loss of non-recyclable nitrogenous
products, but also incurs a high energetic cost in terms of ATP requirements (Bequette et
al., 1998). The cellular mechanisms regulating amino acid uptake and protein synthesis
by the mammary gland are complex, and results from experiments are often confounded
which makes it difficult to derive specific conclusions. This stems from the fact that
factors influencing the regulation of amino acid uptake are numerous, and include stage
of lactation, hormonal status, source of amino acids, and the profile of amino acids
absorbed by the mammary gland. However, discussion on all these interactions are
outside the scope of this review and the reader is referred to Bequette et al. (1998) for
further information.
Despite the complexity of interactions between numerous factors regulating milk protein
production, a number conclusions can be drawn and are important in terms of improving
the NUE of the dairy cow. One of the main benefits of improving the pattern of amino
acid supply to mammary gland, in terms of its homology with that of milk protein, is the
ability to reduce the total nitrogen intake of the dairy cow. Bach et al. (2000) showed that
by provision of an improved amino acid supply, dairy cows could perform as well on a
diet containing 15% CP as they did on a diet containing 18% CP, and as a result NUE
was increased from 35 to 45%. Bach et al. (2000) also showed that as the CP content of
the diet decreased, the amino acid profile of the diet became more important in order to
maintain production. Post ruminal infusions of methionine elicited a linear response in
milk casein production when a sufficient supply of lysine (7.3% of the total amino acids
absorbed in the small intestine) was provided (Pisulewski et al., 1996). A concomitant
decrease in plasma urea concentration was also observed, as were small increases in
NUE, which suggests that improvements in amino acid supply have the potential to
17
improve NUE. Robinson et al. (1995) showed a 2% improvement in NUE (30.6% to
32.6%) by addition of rumen protected methionine and lysine to the diet. A lack of
response to infusions of methionine alone has also been reported (Polan et al., 1991;
Younge et al., 2001), indicating the importance of a number of amino acids and the basal
diet offered to the animals.
On grass silage-based diets the data is more confused, with a number of experiments
showing that amino acids other than lysine and methionine, predominantly histidine, are
the most important. Vanhatalo et al. (1999) fed grass silage-based diets supplemented
with abomasal infusions of either histidine alone or in combination with methionine and
lysine.
They found that milk yield and protein production was increased by
supplementary infusions of histidine, and further increases associated with the addition of
the other amino acids were additive to the histidine response.
On the other hand,
Varvikko et al. (1999) demonstrated that supplementary methionine and lysine had no
effect on milk or milk protein production in cows fed grass silage diets, although it did
result in increases in fat production.
However, Younge et al. (2001) showed that
methionine and lysine supplementation, either in a rumen-protected form or supplied by
raw materials containing high concentrations of these amino acids, was effective at
increasing milk protein concentration and milk protein yield. Utilising rumen protected
amino acids however, resulted in better utilisation efficiency of dietary nitrogen, than
when lysine was derived from raw materials, the latter increased the urea concentration of
both plasma and milk, which predisposes to losses of N in urine. Iwanska et al. (1999)
also reported that methionine was the limiting amino acid on grass silage-based diets, as
supplementation with it improved all lactation parameters. Similarly, for diets based on
whole-crop wheat, arginine (Ragland-Gray et al., 1997), methionine and lysine (Baker et
al., 1996) have been found to be first limiting.
Mode of action of supplementary amino acids
From the literature reviewed it appears that energy balance, protein supply, amino acid
supplement, amino acid profile (both absorbed from the small intestine and supplied to
the mammary gland), and stage of lactation can all affect NUE in response to
supplementary amino acids.
18
Energy balance
Increases in milk fat are more common in early lactation or cows in negative energy
balance receiving supplementary rumen-protected methionine than those in positive
energy balance or in mid to late lactation (Iwanska et al., 1999; Lindquist et al., 1983;
Robinson et al., 1995; Rulquin and Delaby, 1997). Lysine can also have an effect on
milk fat and protein concentrations (Guinard and Rulquin, 1994). When the cow is in
negative energy balance, amino acids are important in two major ways. The first is as
constituents of casein and other proteins, and the second is as precursors of other
molecules involved in the energy metabolism of the animal. Methionine is involved in
the synthesis of very low density lipoproteins (VLDL) in the liver from circulating nonesterified fatty acids (NEFA) and when the animal is in negative energy balance plasma
NEFA concentrations increase together with the hepatic demand for methionine.
Synthesised VLDL are taken up by the mammary gland and converted into milk fat.
Increased plasma concentrations of NEFA also increases the demand for lysine, which is
involved in hepatic metabolism of NEFA as a component of the transport molecule
carnitine.
Therefore, at times of negative energy balance, particularly in animals
consuming diets based on maize, competition for lysine and methionine between milk
proteins and other sinks for these amino acids can reduce the NUE of the animal.
Amino acid profile
Over supply of protein and consequently improper profile of the amino acids supplied to
the mammary gland are probably the main reason for a lack of response to supplementary
rumen protected amino acids. This may be a result of competition for amino acid
transport binding sites in the mammary gland epithelium (Piepenbrink et al., 1996); for
example arginine competes for the same binding site as lysine, and this can result in loss
of nitrogen due to catabolism of excess amino acids. In the majority of experiments
supplementation has led to increased arterial concentrations of amino acids, although this
does not necessarily lead to an increase in milk protein output because mammary blood
flow can be decreased leading to the same rate of delivery of amino acids (Guinard and
Rulquin, 1994). The importance of the biological value of a mixture of amino acids,
compared to overall quantity, has been demonstrated a number of times.
Nitrogen
utilisation efficiency is increased by abomasal infusions of casein or a mixture of amino
19
acids similar to that of casein (Bach et al., 2000; Cant et al., 1991; Cant et al., 1989;
Choung and Chamberlain, 1993a; Choung and Chamberlain, 1995; Choung and
Chamberlain, 1993b) and although this is not a practical method of increasing milk
protein output for a specific protein intake, it demonstrates that the biological value of the
mixture amino acids leaving the rumen needs to be considered.
Effects of dairy cow management on nitrogen utilisation efficiency
Milking frequency and bST
Work done at the University of Maryland demonstrated the potential effect of using
bovine somatotropin (bST), three times a day milking (3 milking) and long day photo
period simulation (16 hours light and 8 hours dark; LDPP) on NUE (Dunlap et al., 2000).
From N balance experiments conducted during the study it was predicted that the
application of these technologies had the potential to reduce environmental N loading by
up to 16%. The effects of the individual technologies on NUE efficiency are shown in
Table 1.
Table 1. Mean effects (per cow) of adopting bST, 3 times a day milking (3), long day
photoperiod (LDPP) alone, in combination or all three on nitrogen utilisation efficiency
(from Dunlap et al., 2000).
Treatment1
Control bST
LDPP BST
BST
BST
3
3
+3
+LDPP +LDPP
+3
+LDPP
Milk yield, kg/d
29.1
33.3
32.3
30.8
36.2
34.7
34.0
37.9
N intake, g/d
470
515
500
490
543
534
519
562
N excretion, g/d
313
337
329
324
352
347
339
362
N use efficiency2
33.4
34.6
34.2
33.9
35.3
35.0
34.7
35.7
1
Treatments were no treatment (Control), administration of bovine somatotropin (bST),
three times a day milking (3), or long daily photo period (LDPP). Treatment
combinations are indicated by +.
2
% = ((N intake – N excretion) / N intake) x 100
20
Similar results were found in a recent survey of 454 American dairy farms (Jonker et al.,
2002), in which average NUE for milk production was found to be 28.4%. Interestingly,
farmers fed on average 6.6% more protein than recommended by published guidelines
(NRC, 1989), leading to a 16% increase in urinary N excretion (Jonker et al., 2002), and
although increased frequency of ration formulation was associated with an increase in
milk production it was not associated with increased NUE.
Although the use of bST is not relevant in Europe because it is not licensed for use, other
management tools such as 3 milking and LDPP have potential benefits. Three times a
day milking increases milk yield, and increases NUE through a dilution of maintenance
requirements (Dunlap et al., 2000). Increasing milking frequency can also increase the
efficiency of utilisation of amino acids for milk production by reducing the turnover of
milk and constitutive proteins in the mammary gland (Bequette et al., 1998).
Milk urea nitrogen
Milk urea concentration has been advocated as a useful management tool which can be
used to monitor the efficiency of nitrogen utilisation in commercial herds (Godden et al.,
2001; Jonker et al., 1998). Milk urea concentration is correlated with rumen ammonia
concentrations and blood urea concentrations, which are in turn affected by dietary
concentrations of CP and the diet protein-to-energy ratio (Blauwiekel and Kincaid, 1986;
DePeters and Ferguson, 1992; Oltner et al., 1985; Oltner and Wiktorsson, 1983; Roseler
et al., 1993). However, the interpretation of milk urea concentrations in relation to
nutritional management is not easy, because some studies have shown a positive
relationship between milk urea concentrations and milk production (Oltner et al., 1985)
while others have demonstrated a negative relationship or no relationship (Baker et al.,
1995; Carroll et al., 1988). Despite this, Frank and Swensson (2002) found that cows
offered diets which resulted in the most efficient use of dietary N (up to about 43%) had
the lowest concentrations of milk urea.
Despite the difficulties in interpreting milk urea N concentrations, Jonker et al. (1999)
suggested producers should aim for a concentration somewhere between 10 to 16 mg/dl
(100 to 160 mg/l). Bulk tank values above about 160 mg/l indicate that either dietary
21
protein concentrations are too high, rumen fermentable energy supplies are too low, or a
combination of the two. Bulk tank values below about 100 mg/l indicate that crude
protein concentrations are too low, the balance of rumen degradable protein and rumen
undegradable protein is not correct, or rumen fermentable energy supplies are too high.
High milk urea N concentrations (above about 160 mg urea/l milk) therefore indicate
inefficient use of dietary protein, while low concentrations (below about 10 mg/l) can
indicate restricted milk and milk protein yields.
Effects of forage conservation on nitrogen utilisation efficiency
Grass silage
The protein fraction in grass silage is very vulnerable to breakdown during ensilage.
Extensive proteolysis and deamination by bacteria present in the ensiled forage can result
in a large proportion of the N in grass silage being present as NPN (mainly free amino
acids and ammonia). In poor quality silage the NPN component can constitute 80% of the
N fraction, 25% of which may be present as ammonia. In contrast, in well made silage in
which a pH of less than 4.0 has been rapidly established the NPN component may only
account for 50% of the total N, with ammonia only accounting for 10% of that (Davies et
al., 1998; Merry et al., 1995; Winters et al., 2000; Winters et al., 2001; Winters et al.,
2002). Although there is a difference in the proportion of NPN in the total N betweeen
differently quality silages, there tends to be an overall imbalance between the availability
of N and energy required for microbial protein synthesis (quickly degradable N and
slowly fermentable plant cell-walls). The deficient supply of energy from grass silage
organic matter is further accentuated by the fact that the organic matter fraction can
consist of up to 17% lactic acid and 26% of other fermentation acids (Harrison et al.,
1994). Siddons et al. (1985) found that in sheep fed grass silage, the yield of ATP from
the fermentation of organic acids and degradable nitrogenous compounds was insufficient
to optimise microbial protein synthesis. Despite these limitations, NUE of ruminants
offered grass silage only is not necessarily low. Nitrogen balance experiments at IGER
using wether lambs have found NUE of up to 44% on a range of silages (Table 2).
Management of the crop prior to ensiling, and methods used during ensiling, can affect
the efficiency of utilisation of the nitrogenous components, as discussed below.
22
Table 2. N balance data (g N/d) from experiments investigating the effect of inoculation
of high DM grass silages on NUE in wether lambs (MD Fraser, unpublished results).
Control and inoculated silages 1 had a mean DM content of 370 g/kg, and control and
inoculated silages 2 had a mean DM content of 514 g/kg.
Treatment
N intake
Faeces N
Urine N
N balance
NUE %1
Control 1
23.5
9.2
7.0
7.3
30.4
Inoculated 1
25.9
9.0
5.5
11.5
44.4
Significance
NS
NS
*
***
***
Control 2
20.3
6.2
5.5
8.54
41.1
Inoculated 2
22.1
6.6
5.9
9.57
43.1
Significance
NS
NS
NS
NS
NS
NUE = (N retained/N intake) × 100.
*, P < 0.05; ***, P < 0.001.
1
Crop maturity
McAllan et al. (1994) proposed that the efficiency of microbial protein synthesis is
reduced as crop maturity increases because lignification of plant fibres reduces
fermentability of plant tissues. This hypothesis was not confirmed by the study of Rinne
et al. (1997) who found that the efficiency of microbial protein synthesis in cattle offered
grass silages was unaffected by increasing grass maturity. However, higher rumen losses
are associated young highly digestible herbage, due mainly to the increased solubility of
the protein fractions of the grass caused by weaker associations with the fibre fraction of
the plant. Comparisons of parent herbage and silage produced, showed that nitrogen
present as the slowly degradable fractions were reduced as a result of herbage maturation,
and that the nitrogen was converted into the highly soluble fraction (Rinne et al., 1997).
However, NUE of cows offered grass silage is not heavily dependent on the efficiency of
microbial protein synthesis in the rumen expressed as a ratio of microbial protein
produced to the amount of organic matter apparently digested in the rumen (OMADR).
This is because it takes no account of the N losses that occur in the rumen, only the
amount of N captured by microbes per unit of OMADR. Therefore the results of Rinne et
al. (1997) do not indicate that grass should be harvested at a later stage of maturity, but
emphasises the necessity to provide effective supplementation, or to manage the ensilage
23
process to retain higher concentrations of FC, when feeding silage made from young
forage.
Wilting
It is generally accepted that wilting of fresh grass prior to ensilage restricts the ensiling
fermentation, resulting in a higher pH value, lower total VFA production and higher
residual WSC concentrations. In terms of protein quality the speed and extent of wilting
is very important, as it has been shown that proteolysis and deamination within the cut
grass increases as the wilting period is extended, resulting in an increase in silage
ammonia content (Gordon, 1981; Yan et al., 1996). A reduction in the wilting time taken
to achieve the desired forage dry matter (DM) content reduces the extent of proteolysis
and deamination (Yan et al., 1996). For this reason, spreading the cut crop during
wilting, rather than leaving the crop in a swath can help control protein degradation
(Wilkinson et al., 1999): compared with material left in the swath for 48 hours, material
spread out for the same length of time had lower ammonia and higher amino N content.
Despite the benefits of wilting in terms of increased intake (Dawson et al., 1999; Gordon
and Peoples, 1986), there seems to be little benefit in terms of NUE in ruminants offered
wilted rather than unwilted silage (Gordon, 1981; Kebreab et al., 2000; Kebreab et al.,
2001). Indeed, Gordon and Peoples (1986) found cows ate significantly more wilted
silage, but gave significantly less milk from it compared to unwilted silage, leading to a
significant reduction in NUE of milk production (25.3% for unwilted and 22.5% for
wilted silage). This may be due to losses of FCs during wilting, through the action of
microbial organisms on the cut crop, or through an increase in the instantly available
fraction of feed N (Gordon and Peoples, 1986). For this reason, it would appear that NUE
was higher in cows offered more mature herbage, probably due to the presence of an
improved balance between soluble N and FC, although this would not necessarily lead to
best levels of milk production.
Silage additives
Silage additives can be categorised into two main groups: those that either stimulate
fermentation, such as bacterial inoculants, molasses and enzymes, or those that inhibit
fermentation, such as formic or sulphuric acids. The effects of these two groups of
24
additives on the performance of ruminants fed silage treated with them has been the focus
of many experiments. In general terms the two types of additive produce silages with
different properties. Compared with untreated silage, bacterial inoculants tend to increase
lactic acid concentrations and tend to decrease water-soluble carbohydrates. Inoculants
containing freshly cultured bacteria (e.g. Lactobacillus plantarum) tend to lead to a faster
reduction in silage pH and therefore reduce N degradation, while freeze dried inoculants
tend to act more slowly and result insilages with a higher concentration of protein
breakdown products (Merry et al., 1995). Silage additives which inhibit fermentation
prevent or reduce the extent of lactic acid production, therefore increasing the amount of
residual sugars and reducing the concentration of lactic acid and protein breakdown
products.
In terms of animal production performance there is little difference between formic acid
and freeze dried bacterial inoculants as silage additives.
However, a number of
experiments have shown that despite inferior NUE and DM intakes by animals
consuming inoculated silage, their performance is equal to or superior to that of animals
consuming silage treated with formic acid or no additive (Mayne, 1993). It has been
proposed that the reason for this is metabolisable energy utilisation is improved with the
use of a bacterial inoculant (Chamberlain et al., 1992). Similarly Mayne (1990) showed
that despite the lower NUE of cows offered inoculated silage, milk production and milk
energy output was significantly higher than from cows offered control and formic acid
treated silage.
Silages made using freeze-dried inoculants tend to have lower concentrations of residual
WSC and greater quantities of protein breakdown products than formic acid treated
silages.
This leads to the freeze-dried inoculant treated silages having lower NUE
potential than formic acid treated silages, which have a better ratio of highly degradable
N to WSC. In contrast to formic acid treated silage, untreated and inoculated silage
contains high levels of lactic acid, which can be metabolised by rumen bacteria via the
acrylate pathway to form propionate (Martin et al., 1990) which has the potential to
improve glucose supply, and spares glucogenic amino acids from catabolism.
The
acrylate pathway in the rumen also consumes hydrogen, a by-product of acetate and
25
butyrate production, which can help to reduce methane emissions and result in an
improvement in energy utilisation.
Although acid treated silage may have the potential of leading to better NUE than
fermented silage, many farmers and contractors are reluctant to use acid because of the
health and safety issues, and increased wear on farm equipment. Recent experiments
using freshly cultured lactic acid bacteria have also shown that they are more effective
than their freeze-dried counterparts and formic acid at reducing protein degradation and
preserving WSC (Merry et al., 1995). Combined with the improved energy utilisation
efficiencies associated with higher lactic acid content this may result in greater
improvements in animal performance. Despite this, Winters et al. (2001) found no
difference between the growth rates or feed conversion efficiency of steers offered grass
silage treated with formic acid or a bacterial inoculant. In some situations, however,
inoculation of grass silage has shown to be beneficial in terms of NUE (Table 2; MD
Fraser unpublished data). A comparison of the effect of inoculation on N balance in
wether lambs offered high DM grass silage showed that treatment improved N retention
and NUE. Significant improvements in NUE were associated with significantly lower N
excretion in urine, suggesting that the improvements in NUE were associated with
improved rumen efficiency.
Legume silages
Legumes are important livestock forages because of their ability to fix atmospheric
nitrogen through their symbiotic relationship with rhizobacteria. In this manner, legumes
offer a mechanism to immediately improve the overall farm nitrogen utilisation efficiency
because of the reduced requirement for synthetic nitrogen fertilisers.
Red clover
(Trifolium pratense) and lucerne (Medicago sativa) are able to produce yields of up to 12
tonnes of forage DM annually (Fychan et al., 2002). A sward of hybrid ryegrass, which
is comparable to the legumes in terms of longevity, can yield nearly 16 tonnes of DM per
year, but this requires the application of 400 kg N/ha (NIAB, 2001). From 2004 the
amount of nitrogen that farmers will be able to apply in areas designated as NVZ will be
170 kg/ha, which means that grass yields will be significantly reduced. In countries such
as New Zealand where legumes constitute a significant proportion of the sown species,
26
annual N fixation has been estimated to be in the region of one million tonnes and only
10k – 20k tonnes of fertiliser N are used. In comparison, the UK has a similar farmed
area but uses 750k tonnes of fertiliser N, and as a result the estimated annual N fixation is
only 80k tonnes (Frame et al., 1998). Legumes therefore have a tremendous potential to
reduce the synthetic fertiliser N inputs and due to their high protein content, also have the
ability to reduce supplementary feed N inputs, and as a result have cost saving benefits.
However, it must not be forgotten that N fixation is still a route for N input into the
production system, and therefore the pollution potential still exists if utilisation efficiency
is sub optimal.
Nitrogen utilisation efficiency of ruminants fed forage and grain legumes
Comparative N balance data for a range of forage and grain legumes have been compiled
at IGER (Table 3). This dataset is of particular relevance in our attempts to improve the
NUE of dairy cows, as it provides us with data highlighting those forages that require
energy supplementation. The partitioning of N outputs between urine and faeces are
indicative of the nature of the losses. High urinary losses of N are associated with ruminal
loss of ammonia, whereas faecal N loss is associated with the proportion of indigestible N
in the forage or feed.
In the experiment of Fraser et al. (2000; Table 3, Experiment 1), for example, the data
shows that the nitrogen fraction in sainfoin (Onobrychis viciifolia) silage was extremely
indigestible, and a high proportion of N in the diet was excreted in the animals’ faeces.
In contrast, the N fraction in lucerne silage was apparently extremely degradable in the
rumen, leading to a high proportion of dietary N being excreted in urine as ammonia was
absorbed from the rumen rather than being captured as microbial protein. Fraser et al.
(2000) concluded that lotus (Lotus corniculatus) had the greatest potential for
conservation as a high protein forage. However, recent lotus variety evaluation
experiments at IGER (R. Fychan, personal communication) have demonstrated that
establishment and persistency under conservation regimens are unreliable, and this will
be a crucial factor in determining its adoption by livestock producers. This aside, tannincontaining legumes have the ability to reduce rumen N losses, and therefore have a
27
potential contribution to make to improving the NUE of the dairy cow, and in this context
are worth reviewing here.
Table 3. Nitrogen partitioning data (g N/d) compiled at IGER for a range of diets based
on forage legumes and grain legumes fed with grass silage. All N figures rounded to the
nearest whole number.
N intake
Faeces N
Urine N
N balance
NUE2
51
51
38
39
55
24
16
17
12
12
15
20
25
24
28
24
24
7
10
11
-2
3
16
-3
19.6
21.6
-5.3
7.9
29.1
-12.5
Experiment 2
Lucerne, baled, control
Lucerne, baled, inoculated
Luc, clamped
Red clover, baled, control
Red clover, baled, inoculated
Red clover, clamped
41
41
45
44
42
40
13
12
13
19
18
19
16
19
17
13
13
13
11
10
15
11
11
8
26.8
24.4
33.3
25.0
26.2
20.0
Experiment 3
Barley/pea, control
Barley/pea, inoculated
15
14
6
6
6
6
2
3
13.3
21.4
Experiment 4
Field bean silage, control
Field bean silage, inoculated
Forage pea silage, control
Forage pea silage, inoculated
Lupin whole-crop silage1
30
26
28
26
24
9
10
8
7
6
16
15
13
14
11
5
1
7
5
6
16.7
3.9
25.0
19.2
25.0
Diet
Experiment 1
Early cut red clover
Late cut red clover
Early cut lucerne
Late cut lucerne
Lotus
Sainfoin
Experiment 5
Zero-grazed grass
33
7
11
16
48.5
Zero-grazed lotus
46
13
17
16
34.8
Zero-grazed lucerne
50
10
21
20
40.0
Zero-grazed red clover
68
13
29
26
38.2
Zero-grazed white clover
69
12
29
27
39.1
1
R Fychan, unpublished data.
Experiment 1: Fraser et al., 2000; Experiment 2: R Fychan, unpublished data; Experiment
3: MD Fraser, unpublished data; Experiment 4: Fraser et al., 2001; Experiment 5: MD
Fraser, unpublished data.
†
NUE = (N balance/N intake) × 100
28
Tannin-containing legumes
The effects of condensed tannins (CT) on protein nutrition were reviewed by Waghorn et
al. (1990). In vitro studies have shown that CT precipitate dietary protein in neutral to
weakly acidic conditions similar to those found in the rumen. This precipitation protects
protein from microbial degradation and increases the supply of intact protein to the small
intestine. In the more acidic conditions found in the small intestine, the tannin-protein
complexes dissociate and the protein is then available for digestion as normal. Using an
in vitro limited substrate inhibitor procedure, Broderick and Albrecht (1997) found that
differences in the rate of protein degradation and rumen escape of protein of nine forage
legume species were proportional to tannin concentration. In vivo effects of CT in lotus
on the performance of ewes nursing twin lambs was investigated in two groups of ewes,
one receiving lotus herbage (control) and the other receiving lotus herbage supplemented
with polyethylene glycol, which inactivates CTs (Wang et al., 1996). They found that the
control ewes had lower rumen ammonia concentrations and increased rates of milk yield
and of lactose and protein secretion. It was concluded that CT improved the efficiency of
milk production, as the improvement in performance was obtained from statistically
identical nutrient intakes. Apart from reducing rumen ammonia and increasing the flow
of NAN to the small intestine, CT have also been shown to have positive affects on the
flow of essential amino acids (EAA) to the small intestine. Waghorn et al. (1990)
reported experiments with abomasally cannulated sheep fed either active CT diets or
inactivated CT diets, in which the flow of EAA was 50% higher in sheep offered the CT
active herbage. However, high levels of CT intake can be deleterious to animal
performance, as extensive protein precipitation in the rumen can result in reduced
microbial growth and a reduction in plant fibre degradation. Waghorn et al. (1990)
demonstrated this with sheep, whose performance was decreased when fed Lotus
pendunculatus containing 5.5 % CT in the DM, although performance was not decreased
in sheep for lotus containing 2.2 % CT in the DM. Barry and Manley (1984) proposed
that CT concentration should not exceed 4 % of plant DM, and that 2-3 % is sufficient to
reduce excess proteolysis in the rumen. The effect of high levels of CT may have been a
reason for the poor NUE of sheep fed sainfoin in the experiment of Fraser et al. (2000);
analysis of CT concentration in nine forage legume species (Broderick and Albrecht,
29
1997) showed that the average conentration of CT in sainfoin was more than four times
that of lotus.
A tannin-containing forage would be especially valuable in a mixed forage diet if its
protein protecting properties could be conveyed to forages such as grass, lucerne or white
clover silage, which contain proteins that are highly degradable in the rumen. There is
mixed evidence for this effect in the literature. Beever and Siddons (1986) reported that
mixing sainfoin with red clover had no interactive effects on duodenal amino acid supply
compared to red clover alone. In contrast, the review by Waghorn et al. (1990) of 10 diets
consisting of combinations of low and CT-free legumes with CT-containing legumes
showed a large response in N-retention and EAA absorption from the small intestine.
Non-tannin containing legumes
As already mentioned, agronomic factors such as low annual DM yield, slow
establishment and inferior competitiveness with weeds, make lotus and sainfoin of
limited value in the United Kingdom, although breeding programmes would help to
address these problems. As a consequence, red clover seems to have the best potential as
a high-protein legume crop for use by dairy cattle in the UK (Sheldrick et al., 1995;
Wilkins and Jones, 2000). Results of the recent EU-funded LEGSIL project (Wilkins,
2001) showed that forage legumes can increase profitability in milk production systems,
and that red clover and lucerne were the most suitable species for preserving as silage.
White clover silage offers good potential for milk production, although the NUE for milk
protein production of animals fed on this tends to be less than that of animals fed on other
forages, including ryegrass silage (Bertilsson and Murphy, in press).
The performance of lactating dairy cows offered red clover silage (RCS), lucerne silage
(LS) and white clover silage (WCS) has been the focus of numerous studies. Broderick
(1995) reviewed the desirable characteristics of forage legumes, concentrating mainly on
LS. He summarised experiments that demonstrated the poor efficiency of N utilisation
associated with feeding LS supplemented with RUP, which although improved animal
performance in terms of milk production, resulted in reductions in NUE. The addition of
protein to high forage diets is often used to correct sub-optimal performance on such
30
diets, and although it is often successful it tends to result in a reduction in NUE. Poor
NUE in animals offered LS originates from the high levels of NPN present in the forage,
resulting in high rumen ammonia concentrations and losses of N in urine (Broderick,
1995). In a complete lactation study, Dhiman and Satter (1994) showed that by increasing
the supply of rumen FC, by replacing a third of the LS with maize silage, total lactation
yields were increased by 577 kg per cow from a diet that was lower in CP concentration.
Addition of FC reduced rumen ammonia concentrations, which combined with a lower
CP concentration in the diet and improved performance (Dhiman and Satter, 1994)
suggests that this improved NUE. A similar response to the addition of FC on NUE and
the performance of dairy cows offered legume silages was shown by Castle et al. (1984).
They supplemented white clover silage with barley grain or soyabean meal, or a
combination of both and found that barley, compared to no supplement, improved milk
and milk protein production, as well as improving NUE from 18 % to 23 %. Castle et al.
(1983) reported similar results from the addition of barley grain to a white clover silage
diet. In contrast, the addition of soyabean meal alone to WCS led to a reduction in NUE,
but an increase in animal performance, which is consistent with the effect of adding
protein to legume based diets found by Broderick (1995). Supplementing WCS with both
barley and soyabean meal led to an increase in both animal performance and NUE (Castle
et al., 1984).
Unlike LS and WCS, RCS is recognised as being less susceptible to proteolysis both in
the clamp (Winters et al., 1999) and in the rumen (Broderick and Albrecht, 1997).
Broderick and Albrecht (1997) concluded that red clover displayed the characteristics of
legumes containing low levels of tannins, but does not contain tannins. Hatfield and
Muck (1999) proposed that this was due to the presence of polyphenol oxidases and
polyphenols that have the ability to inactivate hydrolytic enzymes responsible for
proteolysis and deamination during ensilage. This would account for the lower levels of
NPN found in RCS compared to LS, and lower rumen ammonia and milk urea
concentrations in animals offered RCS compared to LS (Broderick et al., 2001; Broderick
et al., 2000). However, further analysis of data comparing animal performance on RCS
and LS in the lactating dairy cow (Broderick et al., 2001; Broderick et al., 2000; Hoffman
et al., 1997) shows no difference in the correlation between N intake and NUE (Figure 5).
31
These data suggest that the lower levels of protein breakdown products in RCS have little
effect on improving NUE.
Nitrogen utilisation efficiency
35%
Lucerne
33%
Red clover
31%
29%
27%
25%
23%
21%
19%
17%
15%
400
450
500
550
600
650
700
750
Nitrogen intake, g/day
Figure 5. Relationship between nitrogen intake from silages made from red clover or
lucerne and nitrogen utilisation efficiency ([milk N output/N intake] × 100) in dairy
cows. There was no difference between forages. Data from Broderick et al. (2000),
Broderick et al. (2001) and Hoffman et al. (1997).
The effect however, may have been masked by the concentrates fed with the forages, as
data from N balance experiments conducted at IGER showed that wether lambs offered
RCS as the sole component of the forage achieved a significantly higher level of NUE
than those offered LS (means of 20.6% and 1.3% for RCS and LS respectively;
Experiment 1, Table 3), although in a subsequent experiment no differences were
observed (grand mean of 25.6 % NUE; Experiment 2, Table 3). The use of high levels of
concentrates in the diets evaluated during the LEGSIL project may also have been
responsible for the poor NUE obtained by dairy cows offered ensiled legumes (Bertilsson
and Murphy, in press; Wilkins, 2001). Reis and Combs (2000) showed that by supplying
a source of rumen FC to dairy cows grazing mixed red clover, lucerne and grass swards,
milk yield (21.38 to 30.4 kg/d), milk protein production (620 to 930 g/d), NUE (22 to
32
30%) and DM intake were increased, despite a reduction in the CP concentration of the
diet (195 to 155 g/kg). Reductions in rumen ammonia concentration in cows offered a FC
supplement highlight the importance of providing the appropriate supplement to diets
based on forage legumes (Reis and Combs, 2000), and indicates that the potential exists
to reduce dietary CP content without detrimental effects on performance.
Harvest management of legumes and nitrogen utilisation efficiency
Drying legumes for preservation as hay can result in high field losses due to the fragility
of the plants, which tend to shed leaves at higher DMs. For this reason legumes are best
conserved as silage, although their chemical composition needs to be taken into account
to enable effecting silage production. Low concentrations of WSC in the forage makes
them difficult to ensile, as there is limited substrate available for bacterial fermentation
and the production of lactic acid. When WSC availability is limited, the extent of protein
breakdown is increased, producing ammonia that buffers the drop in pH. Extending the
wilting period prior to ensiling RCS and LS can result in a decrease in silage ammonia-N
concentration (Fychan et al., 2002). The decline in the ammonia-N content of RCS
following wilting was not statistically significant, and is evidence for the role of
polyphenol oxidases in the inhibition of protein breakdown in RC (Hatfield and Muck,
1999; Jones et al., 1995). Waldo (1984) recognised the importance of correct preservation
technique to NUE. He showed that ensilage of low DM forage resulted in extensive
protein breakdown, lower intakes, reduced animal performance and low or negative N
retention.
The extent of protein degradation in legumes during conservation is of major concern due
to its effect on NUE. Vagnoni et al. (1997) showed that compared to no additive, the
application of formic acid or sulphuric acid increased the B protein fraction (ammonia
and free AA) of LS by 60 %, whereas application of trichloroacetic acid increased the B
fraction by 98%. This was thought to be due to the ability of acid additives to induce
acidic conditions below the optimal for the activity of plant proteases.
Nagel and
Broderick (1992) also showed that compared to no additive, applying formic acid reduced
both the ammonia-N and free amino acid-N content of LS. Reducing protein degradation
33
in the silo resulted in an increase in milk yield, protein content, protein yield, milk urea
and a reduction in plasma urea concentrations.
Table 4. Proportion of NPN in the total N content of a range of alternative forages, fresh
and ensiled with (inoculated) or without (control) the use a bacterial inoculant. Figures in
% NPN in total N content.
Crop
Fresh herbage Control silage Inoculated silage
Pea/barley bi-crop
20
49
45
Red clover, early maturity
9
29
26
Red clover, late maturity
10
29
25
Kale silage
16
39
36
First cut lotus silage
12
33
31
Second cut lotus silage
6
30
29
Lucerne/red clover, early maturity
13
33
31
Lucerne silage, early maturity
17
38
42
Lucerne silage, late maturity
15
42
42
Pea silage
7
38
35
Mean NPN, % of total N
13
36
34
Analysis of alternative forage silage quality data (in the DEFRA-funded Alternative
Forages programme, LS0803) showed that compared to control untreated silages, the
amount of N present as NPN (free amino acids and ammonia) was not statistically
different to those treated with a freshly cultured bacterial inoculant, although in most
cases NPN values for inoculated silages were numerically lower (Table 4).
Recommendations
From the literature review it is possible to draw certain conclusions and produce
recommendations which can be applied on farm to help improve NUE.
Forage
Grazed forages
In order to improve the NUE of the grazing dairy cow we must have a clear perspective
of the factors predisposing to poor NUE. The review has already highlighted the high
solubility of forage N, as a result of increased fertiliser N application and the imbalance
that exists between forage N and FC as being the main causes. An obvious
recommendation would be to reduce the level of N application. However, levels of
fertiliser application are related to the output expected per hectare, and the stock that the
land must carry. Therefore, although reducing N application rate may improve NUE,
34
many farmers may be reluctant to do so because of the resultant loss in grass yield.
However, some farmers may have to reduce fertiliser application rates if their land falls
within an NVZ, which should therefore lead to improvements in the NUE of their dairy
cattle. An alternative view would be to recognise the increased requirement for FC
associated with higher levels of N application, and for dairy cows grazing pastures with a
high white clover content. This will allow the rumen microbes to better utilise the
available N. Provision of FC may be achieved by providing supplements containing
sugars, starches or digestible fibre. Alternative options include turning cows out to fresh
grass in the afternoon when WSC is higher, and when renewing pastures, selecting grass
varieties that have been bred for higher WSC concentrations. Sources of low-degradable
starch can be a valuable supplement for grazing dairy cows, and supplementation of
animals with sources of DUP, which may improve performance of grazing dairy cows,
masks inefficiencies in rumen N utilisation.
Conserved forages
The problem of a high soluble N content is further exaggerated when forages are ensiled,
and therefore the recommendations of increasing the FC content of the diet also applies in
the case of ensiled forages. Special attention should be paid to the soluble N content of
legume forages such as white clover and lucerne, grass silage and poorly preserved
forages that may contain high levels of soluble N. The objective is to minimise the extent
of protein breakdown by promoting rapid acidification of the crop during ensilage. Main
recommendations include:

Maximising the WSC concentration of the forage through rapid wilting or selection
of forage species high in WSC, as these are essential for a homofermentative lactic
acid fermentation.

Use of an effective bacterial inoculant that stimulates a rapid lactic acid fermentation,
or a fermentation inhibitor such as formic acid that acidifies the forage thus
preventing protein degradation by bacterial proteases.
At present no measure of the soluble N content of forage is widely available in the UK.
As a guide, forage with a high soluble N content often arises from material that has been
35
harvested too soon after fertiliser application, material harvested after long periods of wet
weather, and legumes harvested below the optimal DM content (less than 25% DM).
Rumen environment
The rumen is the main source of N utilisation inefficiency, and therefore the use of
dietary components which are high in DUP tends to increase NUE. However, soluble
protein and non-protein N are the most easily accessible source of N for the rumen
microbes and a shortage can lead to a reduction in microbial activity, leading to a
reduction in VFA and microbial protein production. Improving the capture of dietary N
in the rumen therefore improves NUE, and improves the protein nutrition of the animal.
This requires accurate balancing of N and FC supply to the rumen, although this is made
difficult by current descriptions of the N and carbohydrate fractions in the diet.
Peripheral and mammary tissue metabolism
Provision of sufficient energy to the peripheral and productive tissues of the animals is of
the highest importance. Limited uptake of energy-yielding nutrients results in the
inefficient utilisation of amino acids as sources of energy, which produces ammonia as a
by-product. Reducing the length and severity of periods of negative energy balance by
providing diets of optimal energy density may have positive effects on NUE. Feeding
rumen-protected amino acids has the potential of reducing the CP content of the diet
without having a detrimental effect on production, while improving NUE. Results
obtained from use of such supplements, however, are variable due to the many interacting
factors that effect amino acid metabolism discussed in the review. However, a number of
conclusions can be drawn:

Improving the profile of absorbed amino acids with regards to the amino acid profile
of milk protein would allow the overall diet CP concentration to be reduced.

As dietary CP level is reduced, protein quality in terms of amino acid profile becomes
more important in order to maintain performance.

The demand for lysine and methionine is increased when animals are mobilising body
fat depots.
36

Simply increasing plasma concentrations of amino acids is not effective at increasing
milk protein yield and NUE as it results in reduced mammary uptake and greater
catabolism of amino acids.

Lysine and methionine tend to be the first limiting amino acids in animals offered
maize-based diets.

Responses to methionine are dependent on the availability of dietary lysine.
However, there are currently no methods available to accurately determine the whole
animal amino acid requirements or the microbial amino acid supply to the small intestine
in a farming situation.
This means that diet formulation remains the only way to
calculate and supply approximate requirements.
Dairy cow management
Feeding and milking management can also affect NUE. Positive effects of artificially
maintaining a constant photoperiod of 16 hours of light in every 24 hours, and the use of
three times a day milking have been found. However, not all dairy farmers are able or
willing to milk their cows more than twice a day, in which case milking at 12 hour
intervals would probably lead to the most efficient compromise.
Feeding a total mixed ration (TMR) offers the opportunity of accurately balancing dietary
N and energy sources, provided that the diet has been formulated with that aim, and that
the components of the diet are present in the TMR in their correct proportions. Accurate
feeding of individual dietary components is more difficult when animals are offered
separate rations of forage and concentrate feeds, making it more difficult to balance
protein and energy components of the total diet. Monitoring milk urea concentrations
may help with this. Many of the limitations of the UK’s ME and MP systems used for
diet formulation (AFRC, 1993) were addressed by the Feed into Milk project (Offer et al.,
2002), the results of which are available to dairy farmers as a ration formulation system
both as a computer program for use on farm and through the use of feed compounders
who were part of the Sustainable Livestock Production LINK consortium which funded
the work.
37
Appendix 1
This table summarises some of the data used in the review. Data were taken from
measurements of nitrogen partitioning in dairy cows, and nitrogen utilisation efficiency
(NUE) was calculated as the proportion of dietary N that was used for productive
purposes (growth and milk production).
Source
1
1
1
1
1
2
2
3
3
4
4
4
4
5
5
5
5
5
5
5
5
5
6
6
6
7
7
7
7
7
8
8
8
8
8
8
8
8
8
8
†
Diet*
GS + high CP, low deg. concentrate
GS + low CP concentrate
GS + high CP, medium deg. concentrate
GS + high CP, high deg. concentrate
GS + high CP concentrate
GS + concentrate + NH3 infusion
GS + concentrate + acetate infusion
TMR + starch infusion
TMR + water infusion
GS + high NDF concentrate
GS + soluble sugar concentrate
GS + low degradability starch
GS + high degradability starch
GS
GS + barley straw + concentrate
GS + concentrate
GS + prairie meal + concentrate
GS + barley straw + concentrate
GS + prairie meal + concentrate
GS + prairie meal
GS
GS + barley straw
GS + low CP concentrate
GS + medium CP concentrate
GS + high CP concentrate
GS + 44% maize silage
GS + 24% maize silage
GS + 45% maize silage
GS + 43% maize silage
GS
GG + high fibre concentrate
GS + high starch concentrate
GS + fibre and starch concentrate
GS + high starch concentrate
GS + high fibre concentrate
GS + maize silage
GS + sugar beet pulp
GS + high moisture ear corn silage
GS + high moisture ear corn silage
GS + ensiled sugar beet pulp
N
intake
465
422
469
472
516
523
470
532
531
354
365
341
369
569
548
485
553
485
490
314
261
174
391
492
555
417
469
534
510
726
386
424
460
455
485
467
476
468
477
468
N in
faeces
150
134
143
131
148
178
174
174
159
112
140
137
125
143
150
123
147
133
132
86
76
64
112
133
134
173
167
174
150
157
162
152
179
159
194
140
138
140
138
126
N in
urine
162
151
184
219
225
260
220
175
226
96
97
87
134
241
232
217
245
227
241
178
151
101
180
245
299
110
166
216
225
437
112
148
156
173
160
205
233
229
239
248
N in
milk
114
110
118
116
122
106
107
125
127
98
95
102
99
139
137
118
143
116
111
88
106
104
134
136
144
124
132
112
123
129
123
131
121
107
98
100
94
N
retained
40
26
26
5.20
21
-20
-31
58
19
48
34
15
10
46
29
27
19
10
5
50
34
9
11
3
16
NUE§
33
32
31
26
28
16
16
34
27
41
35
34
30
33
30
30
29
26
24
16
13
5
25
22
22
32
29
27
24
18
29
29
28
27
27
26
22
21
21
20
38
Source
9
9
10
10
11
11
12
12
13
13
13
13
13
14
14
14
14
14
15
15
15
15
15
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
17
17
17
18
18
18
†
Diet*
Energy and nitrogen synchronous diets
Energy and nitrogen asynchronous diets
GG (high sugar PRG) + concentrate
GG (low sugar PRG) + concentrate
GG (high sugar PRG) + concentrate
GG (low sugar PRG) + concentrate
GG (high sugar PRG) + concentrate
GG (low sugar PRG) + concentrate
GS + maize grain
GS + sugar beet + fish meal
GS + barley + oat grain
GS + sugar beet + soya
GS
GS (organic acids) + supplement
GS + maize grain
GS + barley grain
GS (NH4 tetraformate) + supplement
GS (bacterial inoculant) + supplement
GS + starch
GS – low fertiliser + starch & fibre
GS – medium fertiliser + starch & fibre
GS – high fertiliser + starch & fibre
GS + fibre
ZG + high starch concentrate
ZG + Maize silage + concentrate
ZG + sugar beet concentrate
GG + maize silage + concentrate
ZG + maize silage + concentrate
ZG + high starch concentrate
ZG + sugar beet concentrate
GG + maize silage
GG + maize silage + concentrate
GG + concentrate
GG + concentrate
ZG + corn cob mix
ZG + corn cob mix
ZG + ground ear maize
ZG + wet beet pulp
ZG + wet beet pulp
GG + concentrate
ZG + ground ear maize
ZG + dry beet pulp
ZG + dry beet pulp
GG – 0 kg N/ha
GG – 0 kg N/ha + 2 kg soya
GG – 250 kg N/ha + concentrate
GG – 60 kg N
GG – 300 kg N/ha
GG – 60 kg N/ha + 2 kg soya
N
intake
491
510
358
453
280
290
362
320
555
565
543
559
502
482
462
476
467
459
378
345
409
395
387
318
372
376
418
540
481
491
501
537
726
604
535
562
562
562
554
629
547
554
592
315
466
585
423
503
594
N in
faeces
114
125
133
132
110
121
137
123
191
201
179
190
177
167
164
161
157
163
149
133
157
143
140
165
166
150
177
161
111
87
151
140
157
149
162
130
130
139
149
164
157
162
127
102
119
118
110
113
132
N in
urine
149
150
64
121
71
100
75
87
211
234
216
187
216
145
140
154
153
144
94
87
105
118
113
38
51
98
106
238
330
285
223
214
437
347
261
271
271
281
294
363
272
272
291
115
220
348
204
281
350
N in
milk
121
125
107
111
83
68
136
119
112
112
107
109
93
146
137
138
135
130
129
115
131
122
116
122
139
135
139
141
123
124
124
120
132
109
93
95
95
95
93
102
87
83
88
98
125
119
109
109
123
N
retained
NUE§
25
25
30
25
30
23
38
37
20
20
20
19
19
30
30
29
29
28
34
33
32
31
30
38
37
36
33
26
26
25
25
22
18
18
17
17
17
17
17
16
16
15
15
31
27
20
26
22
21
39
Source
19
19
20
20
20
21
21
21
21
22
22
22
22
22
†
Diet*
ZG - 0 kg N/ha
ZG - 250 kg N/ha
GS + 209g starch
GS + 384g starch
GS + 50g starch
GS + sucrose
ZG
GS + sucrose + fishmeal
GS
GS + sugar beet pulp
GS + starch concentrate
GS + sugar beet & starch
GS + sugar beet pulp
GS + starch
N
intake
257
365
521
543
528
203
266
284
218
466
524
576
590
596
N in
faeces
91
91
157
152
159
65
93
83
63
N in
urine
76
181
243
269
242
108
104
148
100
N in
milk
91
93
112
111
106
N
retained
66
78
79
54
139
153.92
159.09
160.03
161
NUE§
35
25
21
20
20
33
29
28
25
30
30
28
27
27
* GS, grass silage; GG, grazed grass; ZG, zero-grazed grass; NDF = neutral detergent
fibre; deg., degradable; CP, crude protein.
†
Source references: 1. Castillo et al., 2000b; 2. Moorby and Theobald, 1999; 3. Reynolds
et al., 2001; 4. Castillo et al., 2000a; 5. Moorby et al., 2000; 6. Aston et al., 1998; 7. Valk,
1994; 8. Van Vuuren, 1993; 9. Kolver et al., 1998; 10. Miller et al., 2001a; 11. Miller et
al., 2001b; 12. Moorby et al., 2001; 13. Petit and Tremblay, 1995; 14. Kebreab et al.,
2000; 15. Kebreab et al., 2001; 16. Valk and Hobbelink, 1992; 17. Delagarde et al., 1997;
18. Astigarraga et al., 1993; 19. Peyraud et al., 1997; 20. Keady et al., 1998; 21. Keady
and Murphy, 1998; 22. Valk et al., 1990.
§
NUE = [(N retained + N in milk)/N intake]*100
40
References
Abdalla, A. L., J. D. Sutton, R. H. Phipps, and D. J. Humphries. 1999. Digestion in the
rumen of lactating dairy cows given mixtures of urea-treated wholecrop wheat
and grass silage. Animal Science. 69:203-212.
AFRC. 1993. Energy and Protein Requirements of Ruminants. An Advisory Manual
prepared by the AFRC Technical Committee on Responses to Nutrients. CAB
International, Wallingford, UK.
Annison, M. J. 1970. Nitrogen metabolism of ruminant micro-organisms. Pages 456-473
in Physiology of Digestion and Metabolism in the Ruminant. A. T. Philipson, ed.
Oriel Press, Newcastle on Tyne.
Astigarraga, L., J. L. Peyraud, and M. Le Bars. 1993. Effect of level of nitrogen
fertilization and protein supplementation on herbage utilization by grazing dairy
cows: II. Faecal and urine excretion. Annales De Zootechnie. 43:292.
Aston, K., W. J. Fisher, A. B. McAllan, M. S. Dhanoa, and R. J. Dewhurst. 1998.
Supplementation of grass silage-based diets with small quantities of concentrates:
strategies for allocating concentrate crude protein. Animal Science. 67:17-26.
Bach, A., G. B. Huntington, S. Calsamiglia, and M. D. Stern. 2000. Nitrogen metabolism
of early lactation cows fed diets with two different levels of protein and different
amino acid profiles. Journal of Dairy Science. 83:2585-2595.
Baker, L. D., J. D. Ferguson, and W. Chalupa. 1995. Responses in urea and true protein
of milk to different protein feeding schemes for dairy cows. Journal of Dairy
Science. 78:2424-2434.
Baker, M. J., H. E. Amos, A. Nelson, C. C. Williams, and M. A. Froetschel. 1996.
Undegraded intake protein: effects on milk production and amino acid utilization
by cows fed wheat silage. Canadian Journal of Animal Science. 76:367-376.
Barry, T. N. and T. R. Manley. 1984. The role of condensed tannins in the nutritional
value of Lotus pendunculatus for sheep. 4. Sites of carbohydrate and protein
digestion as influenced by dietary reactive tannin concentration. British Journal of
Nutrition. 55:123-137.
Beever, D. E. and R. C. Siddons. 1986. Digestion and metabolism in the grazing
ruminant. Control of Digestion and Metabolism in Ruminants. Proceedings of the
6th International Symposium on Ruminant Physiology., Banff, Canada:479-499.
Bequette, B. J., F. R. C. Backwell, and L. A. Crompton. 1998. Current concepts of amino
acid and protein metabolism in the mammary gland of the lactating ruminant.
Journal of Dairy Science. 81:2540-2559.
Bertilsson, J. and M. Murphy. in press. Milk production based on clover silage - effects
on feed intake, milk production, nitrogen partitioning and rumen characteristics.
Grass and Forage Science.
Blauwiekel, R. and R. L. Kincaid. 1986. Effect of crude protein and solubility on
performance and blood constituents of dairy cows. Journal of Dairy Science.
69:2091-2098.
Broderick, G. A. 1995. Desirable characteristics of forage legumes for improving protein
utilization in ruminants. Journal of Dairy Science. 73:2760-2773.
Broderick, G. A. and K. A. Albrecht. 1997. Ruminal in vitro degradation of protein in
tannin-free and tannin containing forage legume species. Crop Science. 37:18841891.
41
Broderick, G. A., R. P. Wagenbach, and S. Maignan. 2001. Production of lactating dairy
cows fed alfalfa or red clover silage at equal dry matter or crude protein contents
in the diet. Journal of Dairy Science. 84:1728-1737.
Broderick, G. A., R. P. Walgenbach, and E. Sterrenburg. 2000. Performance of lactating
dairy cows fed alfalfa or red clover silage as the sole forage. Journal of Dairy
Science. 83:1543-1551.
Bussink, D. W. and O. Oenema. 1998. Ammonia volatilization from dairy farming
systems in temperate areas: a review. Nutrient Cycling in Agroecosystems. 51:1933.
Cammell, S. B., J. D. Sutton, D. E. Beever, D. J. Humphries, and R. H. Phipps. 2000a.
The effect of crop maturity on the nutritional value of maize silage for lactating
dairy cows 1. Energy and nitrogen utilisation. Animal Science. 71:381-390.
Cammell, S. B., J. D. Sutton, D. E. Beever, D. J. Humphries, and R. H. Phipps. 2000b.
The effect of crop maturity on the nutritional value of maize silage for lactating
dairy cows 2. Ruminal and post ruminal digestion. Animal Science. 71:391-400.
Cant, J. P., E. J. DePeters, and R. L. Baldwin. 1989. Effect of dietary fat and postruminal
casein administration on nitrogen uptake and output by the udder. Journal of
Dairy Science. 72:Supplement 1, 311 (Abstract).
Cant, J. P., E. J. DePeters, and R. L. Baldwin. 1991. Effect of dietary fat and postruminal
casein administration on milk composition of lactating dairy cows. Journal of
Dairy Science. 74:211-219.
Carroll, D. J., B. A. Barton, G. W. Anderson, and R. D. Smith. 1988. Influence of protein
intake and feeding strategy on reproductive performance of dairy cattle. Journal of
Dairy Science. 71:3470-3481.
Casper, D. P., H. A. Maiga, M. J. Brouk, and D. J. Schingoethe. 1999. Synchronization of
carbohydrate and protein sources on fermentation and passage rates in dairy cows.
Journal of Dairy Science. 82:1779-1790.
Castillo, A. R., E. Kebreab, D. E. Beever, J. H. Barbi, J. D. Sutton, H. C. Kirby, and J.
France. 2000a. The effect of protein supplementation on nitrogen utilization in
lactating dairy cows fed grass silage based diets. Journal of Animal Science.
79:247-253.
Castillo, A. R., E. Kebreab, D. E. Beever, and J. France. 2000b. A review of efficiency of
nitrogen utilisation in lactating dairy cows and its relationship with environmental
pollution. Journal of Animal and Feed Sciences. 9:1-32.
Castle, M. E., D. Reid, and J. N. Watson. 1983. Silage and milk production - studies with
diets containing white clover silage. Grass and Forage Science. 38:193-200.
Castle, M. E., D. Reid, and J. N. Watson. 1984. Silage and milk production - a
comparison between supplements of barley and soyabean-meal offered with white
clover silage. Grass and Forage Science. 39:287-289.
Chamberlain, D. G. and J.-J. Choung. 1995. The importance of rate of ruminal
fermentation of energy source in diets for dairy cows. Pages 3-27 in Recent
Advances in Animal Nutrition. D. J. A. Cole, ed. Nottingham University Press,
Nottingham.
Chamberlain, D. G., P. A. Martin, S. Robertson, and E. A. Hunter. 1992. Effects of the
type of additive and the type of supplement on the utilization of grass silage for
milk production in dairy cows. Grass and Forage Science. 47:391-399.
42
Choung, J. J. and D. G. Chamberlain. 1993a. Effects of Addition of Lactic-Acid and PostRuminal Supplementation With Casein On the Nutritional-Value of Grass-Silage
For Milk- Production in Dairy-Cows. Grass and Forage Science. 48:380-386.
Choung, J.-J. and D. G. Chamberlain. 1993b. Effects on milk yield and composition of
intra-abomasal infusions of sodium caseinate, an enzymatic hydrolysate of casein
or soya-protein isolate in dairy cows. Journal of Dairy Research. 60:133-138.
Choung, J.-J. and D. G. Chamberlain. 1995. Effects of abomasal infusions of sodium
caseinate and of casein hydrolysates varying in the relative proportions of
peptides and free amino acids on milk production in dairy cows. Journal of Dairy
Research. 62.
Davies, D. R., R. J. Merry, A. P. Williams, E. L. Bakewell, D. K. Leemans, and J. K. S.
Tweed. 1998. Proteolysis during ensilage of forages varying in soluble sugar
content. Journal of Dairy Science. 81:444-453.
Dawson, L. E. R., C. P. Ferris, R. W. J. Steen, F. J. Gordon, and D. J. Kilpatrick. 1999.
The effects of wilting grass before ensiling on silage intake. Grass and Forage
Science. 54:237-247.
DEFRA. 2001. Subject: Implementing the Nitrates Directive.
http://www.defra.gov.uk/environment/consult/nitrate1201/10.htm.
Delagarde, R., J. L. Peyraud, and L. Delaby. 1997. The effect of nitrogen fertilization
level and protein supplementation on herbage intake, feeding behaviour and
digestion in grazing dairy cows. Animal Feed Science and Technology. 66:165180.
DePeters, E. J. and J. D. Ferguson. 1992. Nonprotein nitrogen and protein distribution in
the milk of cows. Journal of Dairy Science. 75:3192-3209.
Dewhurst, R. J., D. R. Davies, and R. J. Merry. 2000. Microbial protein supply from the
rumen. Animal Feed Science and Technology. 85:1-21.
Dhiman, T. R. and L. D. Satter. 1994. Milk yield and rumen fermentation measurements
in cows fed diets containing different proportions of alfalfa and corn silage.
Journal of Dairy Science. 77:Abstract 222.
Dou, Z., R. A. Kohn, J. D. Ferguson, R. C. Boston, and J. D. Newbold. 1996. Managing
nitrogen on dairy farms: an integrated approach 1. Model description. Journal of
Dairy Science. 79:2071-2080.
Dunlap, T. F., R. A. Kohn, G. E. Dahl, M. Varner, and R. A. Erdman. 2000. The impact
of somatotropin, milking frequency, and photoperiod on dairy farm nutrient flows.
Journal of Dairy Science. 83:968-976.
Frame, J., J. F. L. Charlton, and A. S. Laidlaw. 1998. Temperate forage legumes. CAB
International, Wallingford, Oxon, UK.
Frank, B. and C. Swensson. 2002. Relationship between content of crude protein in
rations for dairy cows and milk yield, concentration of urea in milk and ammonia
emissions. Journal of Dairy Science. 85:1829-1838.
Fraser, M. D., R. Fychan, and R. Jones. 2000. Voluntary intake, digestibility and nitrogen
utilization by sheep fed ensiled forage legumes. Grass and Forage Science.
55:271-279.
Fraser, M. D., R. Fychan, and R. Jones. 2001. The effect of harvest date and inoculation
on the yield, fermentation characteristics and feeding value of forage pea and field
bean silages. Grass and Forage Science. 56:218-230.
Fychan, R., J. E. Roberts, and V. J. Theobald. 2002. Yield and silage quality of red clover
and lucerne stands. XIII International Silage Conference, Auchincruive, Scotland.
43
Godden, S. M., K. D. Lissemore, D. F. Kelton, K. E. Leslie, J. S. Walton, and J. H.
Lumsden. 2001. Relationships between milk urea concentrations and nutritional
management, production, and economic variables in Ontario dairy herds. Journal
of Dairy Science. 84:1128-1139.
Gordon, F. J. 1981. The effect of wilting of herbage on silage composition and its feeding
value for milk production. Animal Production. 32:171-178.
Gordon, F. J. and A. C. Peoples. 1986. The utilization of wilted and unwilted silages by
lactating cows and the influence of changes in the protein and energy
concentration of the supplement offered. Animal Production. 43:355-366.
Guinard, J. and H. Rulquin. 1994. Effects of graded amounts of duodenal infusions of
lysine on the mammary uptake of major milk precursors in dairy cows. Journal of
Dairy Science. 12:3565-3576.
Hameleers, A. 1998. The effects of the inclusion of either maize silage, fermented whole
crop wheat or urea-treated whole crop wheat in a diet based on a high-quality
grass silage on the performance of dairy cows. Grass and Forage Science. 53:157163.
Harrison, J. H., R. Blauwiekel, and M. R. Stokes. 1994. Fermentation and utilization of
grass silage. Journal of Dairy Science. 77:3209-3235.
Hatfield, R. and R. E. Muck. 1999. Characterising proteolytic inhibition in red clover
silage. The 7th International Silage Conference, Uppsala, Sweden.
Hoffman, P. C., D. K. Combs, N. M. Brehm, and D. A. Welch. 1997. Performance of
lactating dairy cows fed red clover of alfalfa silage. Journal of Dairy Science.
80:3308-3315.
Hume, I. D., R. J. Moir, and M. Somers. 1970. Synthesis of microbial protein in the
rumen. 1. Influence of the level of nitrogen intake. Australian Journal of
Agricultural Research. 21:283-296.
Iwanska, S., D. Strusinska, and B. Pysera. 1999. Effect of rumen protected methionine
supplementation on early lactational responses of dairy cows fed a grass silage
and cereals diet. Acta Veterinaria Hungarica. 47:191-206.
Jarvis, S. C. 1994. The Pollution Potential and Flows of Nitrogen to Waters and
Atmosphere From Grassland Uder Grazing. Pages 227-239 in Pollution in
Livestock Production Systems. M. H. Omed, ed. CAB International, Wallingford
(UK).
Jones, B. A., R. E. Muck, and R. D. Hatfield. 1995. Red clover extracts inhibit legume
proteolysis. Journal of the Science of Food and Agriculture. 67:329-333.
Jonker, J. S., R. A. Kohn, and R. A. Erdman. 1998. Using milk urea nitrogen to predict
nitrogen excretion and utilization efficiency in lactating dairy cows. Journal of
Dairy Science. 81:2681-2692.
Jonker, J. S., R. A. Kohn, and R. A. Erdman. 1999. Milk urea nitrogen target
concentrations for lactating diary cows fed according to National Research
Council recommendations. Journal of Dairy Science. 82:1261-1273.
Jonker, J. S., R. A. Kohn, and J. High. 2002. Dairy herd management practices that
impact nitrogen utilization efficiency. Journal of Dairy Science. 85:1218-1226.
Kanjanapruthipong, J. and R. A. Leng. 1998. The Effects of Dietary Urea on Microbial
Populations in the Rumen of Sheep. Asian-Australasian Journal of Animal
Sciences. 11:661-672.
44
Keady, T. W. J., C. S. Mayne, and M. Marsden. 1998. The Effects of Concentrate Energy
Source on Silage Intake and Animal Performance With Lactating Dairy Cows
Offered a Range of Grass Silages. Animal Science. 66:21-33.
Keady, T. W. J. and J. J. Murphy. 1998. The Effects of Ensiling and Supplementation
with Sucrose and Fishmeal on Forage Intake and Milk Production of Lactating
Dairy Cows Offered a Range of Grass Silages. Animal Science. 66:9-20.
Kebreab, E., A. R. Castillo, D. E. Beever, D. J. Humphries, and J. France. 2000. Effects
of Management Practices Prior to and During Ensilage and Concentrate Type on
Nitrogen Utilization in Dairy Cows. Journal of Dairy Science. 83:1274-1285.
Kebreab, E., J. France, D. E. Beever, and A. R. Castillo. 2001. Nitrogen pollution by
dairy cows and its mitigation by dietary manipulation. Nutrient Cycling in
Agroecosystems. 60:275-285.
Kolver, E. S., L. D. Muller, G. A. Varga, and T. J. Cassidy. 1998. Synchronisation of
ruminal degradation of supplemental carbohydrate with pasture nitrogen in
lactating dairy cows. Journal of Dairy Science. 81:2017-2028.
Korevaar, H. 1992. The Nitrogen Balance on Intensive Dutch Dairy Farms: A Review.
Livestock Production Science. 31:17-27.
Ledgard, S. F., J. R. Crush, and J. W. Penno. 1998. Environmental Impacts of Different
Nitrogen Inputs on Dairy Faarms and Implications for The Resource Management
Act of New Zealand. Environmental Pollution. 102:515-519.
Lee, M. R. F., A. E. Brooks, J. M. Moorby, M. O. Humphreys, M. K. Theodorou, J. C.
MacRae, and N. D. Scollan. 2002a. In vitro investigation into the nutritive value
of Lolium perenne bred for an elevated concentration of water-soluble
carbohydrate and the added effect of sample processing: Freeze-dried and ground
vs. frozen and thawed. Animal Research. 51:269-277.
Lee, M. R. F., L. J. Harris, J. M. Moorby, M. O. Humphreys, M. K. Theodorou, J. C.
MacRae, and N. D. Scollan. 2002b. Rumen metabolism and nitrogen flow to the
small intestine in steers offered Lolium perenne containing different levels of
water-soluble carbohydrate. Animal Science. 74:587-596.
Lindquist, R. G., J. G. Linn, and D. E. Otterby. 1983. Influence of Dietary Energy and
Protein on Yield and Composition of Milk from Cows Fed Methionine Hydroxy
Analog. Journal of Dairy Science. 66:475-491.
Martin, P. A., D. G. Chamberlin, and S. Robertson. 1990. Effects of Chemical
Composition of Silage and of Supplementary Feeds on Rumen Digestion in
Sheep. Proceedings of the Ninth International Silage Conference, University of
Newcastle-Upon-Tyne:46-47.
Mayne, C. S. 1990. An evaluation of an inoculant of Lactobacillus plantarum as an
additive for grass-silage for dairy cattle. Animal Production. 51:1-13.
Mayne, C. S. 1993. The effect of formic acid, sulphonic acid and a bacterial inoculant on
silage fermentation and the food intake and milk production of lactating dairy
cows. Animal Production. 56:29-42.
McAllan, A. B., J. D. Sutton, D. E. Beever, and D. J. Napper. 1994. Rumen Fermentation
Characteristics and Duodenal Nutrient Flow in Lactating Cows Receiving 2 Types
of Grass Silage with 2 Levels of Concentrates. Animal Feed Science and
Technology. 46:277.
Mehrez, A. Z., E. R. Ørskov, and I. Macdonald. 1977. Rates of Rumen Fermentationin
Relation to Ammonia Concentration. British Journal of Nutrition. 38:437-443.
45
Mercer, J. R. and E. F. Annison. 1976. Utilization of N in ruminants. The 1st
International Symposium on Protein Metaboloim and Nutrition:397-416.
Merry, R. J., M. S. Dhanoa, and M. K. Theodorou. 1995. Use of Freshly Cultured LacticAcid Bacteria as Silage Inoculants. Grass and Forage Science. 50:112-123.
Miller, L. A., D. H. Baker, M. K. Theodorou, J. C. MacRae, M. O. Humphreys, N. D.
Scollan, and J. M. Moorby. 2001a. Efficiency of Nitrogen Use by Dairy Cows
Offered Perennial Ryegrass with High Water Soluble Carbohydrate
Concentrations. 19th International Grassland Congress, Sao Paulo, Brazil.
Miller, L. A., J. M. Moorby, D. R. Davies, M. O. Humphreys, N. D. Scollan, J. C.
MacRae, and M. K. Theodorou. 2001b. Increased concentration of water-soluble
carbohydrate in perennial ryegrass (Lolium perenne L.): Milk production from
late-lactation dairy cows. Grass and Forage Science. 56:383-394.
Moorby, J. M., R. J. Dewhurst, J. K. S. Tweed, M. S. Dhanoa, and N. F. G. Beck. 2000.
Effects of Altering the Energy and Protein Supply to Dairy Cows During the Dry
Period. 2. Metabolic and Hormonal Responses. Journal of Dairy Science.
83:1795-1805+.
Moorby, J. M., L. A. Miller, R. T. Evans, N. D. Scollan, M. K. Theodorou, and J. C.
MacRae. 2001. Milk Production and N Partitioning in Early Lactation Dairy
Cows Offered Perennial Ryegrass Containing a High Concentration of Water
Soluble Carbohydrates. Proceedings of the British Society of Animal Science
Annual Meeting, York.
Moorby, J. M. and V. J. Theobald. 1999. Short Communication: The Effect of Duodenal
Ammonia Infusions on Milk Production and Nitrogen Balance of the Dairy Cow.
Journal of Dairy Science. 82:2440-2442.
Morard, V. 1999. Emmissions of Biogenic Greenhouse Gases From Agriculture: Political
Background and a European Perspective. Biogenic Emissions of Greenhouse
Gasese from Arable and Animal Agriculture - Processes, Inventories, Mitigation.
Nagel, S. A. and G. A. Broderick. 1992. Effect of Formic Acid or Formaldehyde
Treatment of Alfalfa Silage on Nutrient Utilization by Dairy Cows. Journal of
Dairy Science. 75:140-154.
Newbold, J. R. and S. R. Rust. 1992. Effect of Asynchronous Nitrogen and Energy
Supply on Growth of Ruminal Bacteria in Batch Culture. Journal of Animal
Science. 70:238-546.
NIAB. 2001. Recommended lists of grasses & herbage legumes 2001/2002. 1356-4978.
National Institute of Agricultural Botany, Cambridge.
Nocek, J. E. and J. B. Russell. 1988. Protein and energy as an integrated system.
Relationship of ruminal protein and carbohydrateavailability to microbial
synthesis and milk protein. Journal of Dairy Science. 71:2070-2107.
NRC, N. R. C. 1989. Nutrient requirements of dairy cattle. 6th revision ed, Washington,
DC.
Obitsu, T., D. Bremner, E. Milne, and G. E. Lobley. 2000. Effect of Glucose Infusion on
Alanine Metabolism and Urea Production in Sheep. British Journal of Nutrition.
84:157-163.
Offer, N. W., R. E. Agnew, B. R. Cottrill, D. I. Givens, T. W. J. Keady, C. S. Mayne, C.
Rymer, T. Yan, J. France, D. E. Beever, and C. Thomas. 2002. Feed into Milk An applied feeding model coupled with a new system of feed characterisation.
Pages 167-194 in Recent Advances in Animal Nutrition 2002. P. C. Garnsworthy
and J. Wiseman, eds. Nottingham University Press, Nottingham.
46
Oltner, R., M. Emanuelson, and H. Wiktorsson. 1985. Urea concentrations in milk in
relation to milk yield, live weight, lactation number and amount and composition
of feed given to dairy cows. Livestock Production Science. 12:401-407.
Oltner, R. and H. Wiktorsson. 1983. Urea concentrations in milk and blood as influenced
by feeding varying amounts of protein and energy to dairy cows. Livestock
Production Science. 10:457-467.
Orr, R. J., S. M. Rutter, P. D. Penning, and A. J. Rook. 2001. Matching grass supply to
grazing patterns for dairy cows. Grass and Forage Science. 56:352-361.
Petit, H. L. and G. F. Tremblay. 1995. Ruminal Fermentation and Digestion in Lactating
Cows Fed Grass Silage with Protein and Energy Supplements. Journal of Dairy
Science. 78:342-352.
Peyraud, J. L. and L. Astigarraga. 1998. Review of the effect of nitrogen fertilisation on
the chemical composition, intake, digestion and nutritive value of fresh herbage:
consequences on animal nutrition and N balance. Animal Feed Science and
Technology. 72:235-259.
Peyraud, J. L., L. Astigarraga, and P. Faverdin. 1997. Digestion of Fresh Perennial
Ryegrass Fertilized at Two Levels of Nitrogen by Lactating Dairy Cows. Animal
Feed Science and Technology. 64:155-171.
Piepenbrink, M. S., T. R. Overton, and J. H. Clark. 1996. Response of Cows Fed a Low
Crude Protein Diet to Ruminally Protected Methionine and Lysine. Journal of
Dairy Science. 79:1638-1646.
Pisulewski, P. M., H. Rulquin, J. L. Peyraud, and R. Verite. 1996. Lactational and
Systemic Responses of Dairy Cows to Postruminal Infusions of Increasing
Amounts of Methionine. Journal of Dairy Science. 79:1781-1791.
Polan, C. E., K. A. Cummins, C. J. Sniffen, T. V. Muscato, J. L. Vicini, B. A. Crooker, J.
H. Clark, D. G. Johnson, D. E. Otterby, B. Guillaume, L. D. Muller, G. A. Varga,
R. A. Murray, and S. B. Peirce-Sandner. 1991. Responses of Dairy Cows to
Supplemental Rumen-Protected Forms of Methionine and Lysine. Journal of
Dairy Science. 74:2997-3013.
Ragland-Gray, K. K., H. E. Amos, M. A. McCann, C. C. Williams, J. L. Sartin, C. R.
Barb, and F. M. Kautz. 1997. Nitrogen Metabolism and Hormonal Responses of
Steers Fed Wheat Silage and Infused with Amino Acids or Casein. Journal of
Animal Science. 75:3038-3045.
Reis, R. B. and D. K. Combs. 2000. Effects of Increasing Levels of Grain
Supplementation on Rumen Environment and Lactation Performance of Dairy
Cows Grazing Grass-Legume Pasture. Journal of Dairy Science. 83:2888-2898.
Reynolds, C. K., S. B. Cammell, D. J. Humphries, D. E. Beever, J. D. Sutton, and J. R.
Newbold. 2001. Effects of Postrumen Starch Infusion on Milk Production and
Energy Metabolism in Dairy Cows. Journal of Dairy Science. 84:2250-2259.
Rinne, M., S. Jaakkola, and P. Huhtanen. 1997. Grass Maturity Effects on Cattle Fed
Silage Based Diets. 1. Organic Matter Digestion, Rumen Fermentation and
Nitrogen Utilization. Animal Feed Science and Technology. 67:1-17.
Robinson, P. H., A. H. Fredeen, W. Chalupa, W. E. Julien, H. Sato, T. Fujieda, and H.
Suzuki. 1995. Ruminally Protected Lysine and Methionine for Lactating DairyCows Fed a Diet Designed to Meet Requirements for Microbial and Postruminal
Protein. Journal of Dairy Science. 78:582-594.
Rooke, J. A., N. H. Lee, and D. G. Armstrong. 1987. The effects of Intraruminal
Infusions of Urea, Casein, Glucose Syrup and a Mixture of Casein and Glucose
47
Syrup on Nitrogen Digestion in the Rumen of Cattle Receiving Grass Silage
Diets. British Journal of Nutrition. 57:89-98.
Roseler, D. K., J. D. Ferguson, C. J. Sniffen, and J. Herrema. 1993. Dietary protein
degradability effects on plasma and milk urea nitrogen and milk nonprotein
nitrogen in Holstein cows. Journal of Dairy Science. 76:525-534.
Rulquin, H. and L. Delaby. 1997. Effects of the Energy Balance of Dairy Cows on
Lactational Responses to Rumen-Protected Methionine. Journal of Dairy Science.
80:2513-2522.
Santos, F. A. P., J. E. P. Santos, C. B. Theurer, and J. T. Huber. 1998. Effects of RumenUndegradable Protein on Dairy Cow Performance: A 12 Year Literature Review.
Journal of Dairy Science. 81:3182-3213.
Satter, L. D. and L. L. Slyter. 1974. Effect of Ammonia Concentration on Rumen
Microbial Protein Production in vitro. British Journal of Nutrition. 32:199-208.
Sheldrick, R. D., G. Newman, and D. J. Roberts. 1995. Legumes for Milk and Meat.
Chalcombe Publications.
Siddons, R. C., J. V. Nolan, D. E. Beever, and P. R. Cornell. 1985. Nitrogen Digestion
and Metabolism in Sheep Consuming Diets Containing Contrating Forms and
Levels of Nitrogen. British Journal of Nutrition. 54:175-187.
Sinclair, L. A., P. C. Garnsworthy, J. R. Newbold, and P. J. Buttery. 1993. Effects of
Synchronising the Rate of Dietary Energy and Nitrogen Release on Rumen
Fermentation and Microbial Protein Synthesis in Sheep. Journal of Agricultural
Science. 120:251-263.
Sinclair, L. A., P. C. Garnsworthy, J. R. Newbold, and P. J. Buttery. 1995. Effects of
Synchronizing the Rate of Dietary Energy and Nitrogen Release in Diets with a
Similar Carbohydrate- Composition on Rumen Fermentation and Microbial
Protein- Synthesis in Sheep. Journal of Agricultural Science. 124:463-472.
Sutton, J. D., S. B. Cammell, D. E. Beever, D. J. Humphries, and R. H. Phipps. 1998.
Energy and Nitrogen Balance of Lactating Dairy Cows Given Mixtures of UreaTreated Whole-crop Wheat and Grass Silage. Animal Science. 67:203-212.
Vagnoni, D. B., G. A. Broderick, and R. E. Muck. 1997. Preservation of protein in wilted
lucerne using formic, sulphuric or trichloroacetic acid. Grass and Forage Science.
52:5-11.
Valk, H. 1994. Effects of Partial Replacement of Herbage by Maize Silage on NUtilisation and Milk Production of Dairy Cows. Livestock Production Science.
40:241-250.
Valk, H. and M. E. J. Hobbelink. 1992. Supplementation of grazing Dairy Cows to
Reduce Environmental Pollution. Proceedings of the 14th General Meeting of the
European Grassland Federation, Lathi, Finland:400-405.
Valk, H., H. W. Klein Poelhuis, and H. J. Wentink. 1990. Effect of Fibrous and Starchy
Carbohydrates in Concentrates as Supplements in a Herbage-Based Diet for HighYielding Dairy Cows. Netherlands Journal of Agricultural Science. 38:475-486.
Van Kessel, J. S. and J. B. Russel. 1997. The Endogenous Polysaccharide Utilization
Rate of Mixed Ruminal Bacteria and the Effect on Ruminal Fermentation Rates.
Journal of Dairy Science. 80:2442-2448.
Van Soest, P. J. 1994. Nutritional Ecology of the Ruminant. 2nd ed. Cornell University
Press, Ithaca, New York.
Van Straalen, W. M., F. M. H. Dooper, A. M. Antoniewicz, I. Kosmala, and A. M. Van
Vuuren. 1993. Intestinal Digestibility in Dairy Cows of Protein from Grass and
48
Clover Measured with Mobile Nylon Bag and Other Methods. Journal of Dairy
Science. 76:2970-2981.
van Vuuren, A. M., F. Krol-Kramer, R. A. van der Lee, and H. Corbijn. 1992. Protein
Digestion and Intestinal Amino Acids in Dairy Cows Fed Fresh Lolium perenne
with Different Nitrogen Contents. Journal of Dairy Science. 75:2215-2225.
Van Vuuren, A. M., Van Der Koelen, C.J., Valk, H. and De Visser, H. 1993. Effects of
Partial Replacement of RyeGrass by low Protein Feeds on Rumen Fermentation
and Notrogen Loss by Dairy Cows. Journal of Dairy Science. 76:2982-2993.
Vanhatalo, A., P. Huhtanen, V. Toivonen, and T. Varvikko, Vanhatalo, A., Jalava, T. and
Huhtanen, P. 1999. Response of dairy cows fed grass silage diets to abomasal
infusions of histidine alone or in combinations with methionine and lysine.
Journal of Dairy Science. 82:2674-2685.
Varvikko, T., A. Vanhatalo, T. Jalava, and P. Huhtanen. 1999. Lactation and Metabolic
Responses to Graded Abomasal Doses of Methionine or Lysine in Cows Fed
Grass Silage Based Diets. Journal of Dairy Science. 82:2659-2773.
Waghorn, G. C., W. T. Jones, I. D. Shelton, and W. C. McNabb. 1990. Condensed
Tannins and the Nutritive Value of Herbage. Proceedings of the New Zealand
Grasslands Association.:171-176.
Waldo, D. R. 1984. Nutritional Value of Legumes Preserved as Silage. Forage Legumes
for Energy Efficient Animal Production, Palmerston North, New Zealand.:220224.
Wang, Y., G. B. Douglas, G. C. Waghorn, T. N. Barry, and A. G. Foote. 1996. Effect of
condensed tannins in Lotus corniculatus upon lactation performance in ewes.
Journal of Agricultural Science. 126:353-362.
Weissbach, F. and P. Ernst. 1994. Nutrient Budgets and Farms Management to Reduce
Nutrient Emissions. 15th General Meeting European Grassland Federation,
Wageningen, The Netherlands:343-360.
Wilkins, R. J. 2001. Legume Silages for Animal Production.
Wilkins, R. J. and R. Jones. 2000. Alternative Home-Grown Protein Sources for
Ruminants in the United Kingdom. Animal Feed Science and Technology. 85:2332.
Wilkinson, J. M., J. Hill, and J. D. Leaver. 1999. Effect of Swath Treatment on Waterloss
During Field-Wilting and on Feeding Value of Perennial Ryegrass Silage. Grass
and Forage Science. 54:227-236.
Winters, A. L., J. E. Cockburn, M. S. Dhanoa, and R. J. Merry. 2000. Effects of lactic
acid bacteria in inoculants on changes in amino acid composition during ensilage
of sterile and nonsterile ryegrass. Journal of Applied Microbiology. 89:442-451.
Winters, A. L., R. Fychan, D. R. Davies, J. Lloyd, R. J. Merry, and R. Jones. 1999.
Protein Content of a Range of Ensiled Legumes. The 7th International Silage
Conference., Uppsala, Sweden.
Winters, A. L., R. Fychan, and R. Jones. 2001. Effect of formic acid and a bacterial
inoculant on the amino acid composition of grass silage and on animal
performance. Grass and Forage Science. 56:181-192.
Winters, A. L., J. D. Lloyd, R. Jones, and R. J. Merry. 2002. Evaluation of a rapid
method for estimating free amino acids in silages. Animal Feed Science and
Technology. 99:177-187.
Witt, M. W., L. A. Sinclair, R. G. Wilkinson, and P. J. Buttery. 2000. The effects of
synchronizing the rate of dietary energy and nitrogen supply to the rumen on the
49
production and metabolism of sheep: food characterization and growth and
metabolism of ewe lambs given food ad libitum. Animal Science. 69:223-235.
Woolford, M. K. 1984. The Silage Fermentation. Marcel Dekker, New York.
Yan, T., D. C. Patterson, F. J. Gordon, and M. G. Porter. 1996. The Effects of Wilting
Grass Prior to Ensiling on the Reponse to Bacterial Inoculation. 1. Silage
Fermentation and Nutrient Utilization Over Three Harvests. Animal Science.
62:405-417.
Younge, B. A., J. J. Murphy, M. Rath, and B. K. Sloan. 2001. Effect of Dietary
Absorbable Methionine and Lysine Concentrations on Milk Production and
Composition of Dairy Cows Offered Grass Silage Based Diets. Irish Journal of
Agricultural and Food Research. 40:1-11.
50