The Utilization of Ammonia-Nitrogen
by the Sheep
I. W. M C D ONALD *
Summary
Ammonia plays a central role in the nitrogen metabolism of
the rumen; essentially, this is due, on the one hand, to its origin
from the deamination of proteins and other nitrogenous substances
and from the hydrolysis of urea, and on the other hand to its
use by ruminal bacteria as a source of nitrogen for growth.
Ammonia is absorbed from the rumen and converted in the liver
to urea, which in part return’s to the rumen in saliva and by
direct diffusion from the blood. These events and their implications
in the nutrition of the sheep are reviewed, and attention is drawn
to some deficiencies in present knowledge of this important aspect
of ruminant metabolism.
The fact that ammonia plays an important role in normal
rumen metabolism was first recognised by McDonald (1948 a, and
b), and has since been amply confirmed by other workers. The
dynamic nature of events in the rumen and the heterogeneity of
\ is contents make quantitative study extremely difficult and hence,
although the major qualitative features are now clear, much work
still remains to be done before the reactions within the rumen can
be fully related to the nutrition and tissue metabolism of the
ruminant.
ORIGIN
O
F
RUMEN AMMONIA
The main source of rumen ammonia is fodder protein. The
rumen contents are actively proteolytic (Schlottke, 1936; Sym,
1938; Pearson and Smith, 1943 ; Hoflund, Quin and Clark, 1948 ;
Chalmers et. al., 1954; Warner, 1956; Annison, 1956)) yet amino-acids
occur in the rumen in conspicuously low concentrations (el-Shazly,
1952 a; Lewis, 1955; Annison, 1956) ; the activity of rumen microbes
in deaminating amino-acids (el-Shazly, 1952 b; Sirotnak et. al., 1953 ;
Lewis, 1955; Warner, 1956) explains this finding and accounts fol
the occurrence of ammonia. Deamination of proteins from a variety
of sources has been demonstrated: casein, gelatin, ground-nut
meal, herring meal, meadow hay, preserved grass (frozen, dried
or ensiled) and fresh pasture (McDonald, 1952; Chalmers et al.,
1954; Chalmers and Synge, 1954 ; Annison et. al., 1954; Johns,
1955; Head and Rook, 1955 ; Briggs, Hogan and Reid, 1957). The
soluble proteins, e.g. in green leaves, peanut meal, or casein, are
readily degraded in the rumen while highly insoluble proteins, e.g.
zein, are very slowly attacked. The proteolytic activity is little
influenced by diet, but the capacity of rumen bacteria to deaminate
amino-acids is dependent on the presence in the diet of readily
attacked protein (el-Shazly, 1952 b; Warner, 1956).
Young growing plants contain significant amounts of aminoacids which contribute directly to the rumen ammonia. Some plants
contain large amounts of nitrate, which is readily reduced, via
nitrite, to ammonia (Sapiro et. al., 1949; Lewis, 1951). Other
*Division of Animal Health and Production, C.S.I.R.O. Sheep Biology
Laboratory, Prospect, New South Wales.
46
nitrogenous substances in plants (amides, amines, betaines, alkaloids,
purines and nucleic acids) will presumably also be degraded by
rumen organisms.
Urea is rapidly hydrolysed in the rumen (Lenkeit and Becker,
1938). Urea is present in the sheep’s saliva (McDonald, 1952;
Somers, 1957) and also reaches the rumen contents by diffusion
from the blood (Simonnet, Le Bars and Molle, 1957; McDonald,
unpublished data, 1957; Hogan, 1957)) and is thus an important
source of rumen ammonia.
THE CQNCENTRATION
OF AMMONIA IN THE RUMEN
The concentration of ammonia in the rumen depends on four
major factors: rate of formation within the rumen, rate of passage
to omasum, rate of absorption from rumen and rate of uptake
by bacteria. At present it is not possible to give precise values
for these rates in any given set of circumstances.
(i) Rate of Ammonia Formation in the Rumen.-This will be
influenced by feeding behaviour, e.g. pasture intake occurs over
many hours, while stall-fed sheep may consume a day’s ration
in an hour or so; this factor becomes very important in drought
feeding when small supplements can be rapidly consumed. Most of
the experimental work reported has been with animals fed once or
twice daily and more work is required on sheep at pasture. A
second factor is the ease of deamination of the nitrogenous compounds, especially proteins, in the feed. Thirdly, the level of
ammonia is augmented by urea secreted in saliva and diffusing
into the rumen from the blood. In mixed sheep saliva the urea
concentration is approximately 60 per cent. of that in whole blood
(McDonald, 1957) and the urea accounts for about 65-70 per cent.
of the total-N (Somers, 1957) ; the existing data for the volume
of saliva secreted by the sheep permit the calculation that some
0.5-I gm N can enter the rumen through the saliva (McDonald,
1948 b; Somers, 1957). No data are yet available to permit evaluation of the contribution from diffusion.
(ii) Rate of Passage to Omasum.-Since fluid is continuously
added to the rumen as saliva, and portions of the mixed contents
are continuously being passed through the reticula-omasal orifice,
the rate of passage of solutes out of the rumen is a logarithmic
function of the amount present at any given time and is best expressed in terms of a biological half-life. In recent experiments
on hay-fed sheep, the rate of passage of a soluble marker (polyethylene glycol-Sperber, Hyden and Ekman, 1953) varied considerably with an average half-life of 9 hours; for a sheep with
rumen volume of 6 litres and rumen ammonia an average of 15 mg
N/100 ml, this half-life would result in about 1.5 gm ammonia-N/day
passing to the omasum.
( i i i ) R a t e of Absoription from the Rumen.-Absorption o f
ammonia into the ruminal veins has been well established (McDonald,
1948 b; Bouckaert and Oyaert, 1952; Chalmers et. al., 1954) ; Lewis
et al. (1957) and Hogan (1957) found that the rate of absorption
is proportional to the concentration in the rumen and considered
that absorption is by simple diffusion, although the rate of absorption
may be influenced by pH or the concentration of other substances
in the rumen. Lewis et al. (1957)) using Schambye’s (1956) data
for portal blood flow in the conscious sheep, calculated that as
much as half the daily nitrogen intake may be absorbed as ammonia
from the rumen. Gray and Pilgrim (1956)) using N/lignin ratios,
concluded that when a high quality lucerne hay was fed to sheep,
some 60 per cent. of the fodder-N was absorbed from the rumen,
presumably chiefly as ammonia. An alternative procedure is to
compare the changes in concentration of ammonia and a soluble
marker ; in an experiment with this technique, the absorption of
ammonia, per hour, was found to be equivalent to the amount
47
contained in about 1 litre of rumen fluid; assuming a mea n con-centration of 15 mg ammonia-N/100 ml, this is equivalent to 3-4
gm N/day (McDonal d, unpublished data, 1957).
These calculations, imperfect though they are, do indicate the
importance of absorption of ammonia from the rumen.
(iv) Rate of Uptake of Ammonia by Micro-organisms of the
Rumen.-The present state of knowledge is very sketchy. The most
serious deficiency is that we do not know the concentration of
ammonia required for microbial growth, and. hence cannot determine,
by direct analysis, whether the ammonia level in the rumen is
adequate. It has been shown (McDonald, 1952; Annison et al., 1954)
that when adequate starch is present, the ammonia concentration
in the rumen may fall almost to zero- t h i s suggests that bacteria
may take up ammonia at very low concentrations, but gives no indication of the concentration at which N would be a limiting factor
for bacterial growth.
Again, it is not yet clear how the various carbohydrates compare in effectiveness as source of carbon and energy for bacterial
growth when ammonia is the N- source. Dietetic experiments strongly
suggest that cellulose is of little value in this direction but it is
not known whether this is due to the slowness of cellulolysis in
the rumen or whether, as seems more, probable, that the cellulolytic organisms have limited ability to use ammonia as a source
of nitrogen for growth. Examination of rumen ammonia concentrations rather suggests that sugars (glucose, sucrose) are as effective
in reducing ammonia as is starch, but the weight of dietetic evidence
indicates that starch is the more useful compound. Lewis and
McDonald (1957) found that xylan and levan reduced ammonia
concentration as much as starch, but no dietetic evidence on their
comparative value is available.
TOXICITY OF AMMONIA
McDonald (1948 b) found that the concentration of ammonia
in peripheral blood of sheep was extremely low, and concluded that
the ammonia absorbed from the rumen was wholly converted to
urea by the liver. Lewis et al. (1957) found that changes in the
rumen ammonia concentration of sheep fed various diets were
paralleled by changes in portal blood ammonia concentration, but
there was no increase in ammonia concentration in peripheral blood
unless the rumen ammonia concentration was raised above 80 mg
N/100 ml by adding ammonium acetate; similar results were
obtained by Hogan (1957). It is clear therefore that an hepatic
threshold is established by the rate at which the liver can remove
ammonia from the portal blood; Head and Rook (1956) observed
in cattle on spring pasture that ammonia absorption was sufficiently
great to cause an increase in concentration in the peripheral blood;
Johns (1955) did not record whether blood ammonia was elevated
in grazing sheep with very high rumen concentrations.
It has long been recognised that ammonium ion is toxic, and
interest in this topic has increased in recent years (see review by
Bessman, 1956). In the sheep the normal level of ammonia in
peripheral blood is less than 2pg NHs-N/ml (McDonald, 1948 b;
Repp et aZ., 1955; Lewis et al., 1957; Hogan, 1957). The symptoms
of ammonia intoxication have been described by Dinning et at.
(1948), Clark et al. (1951)) Repp et al. (1955) and Lewis et al.
(1957) ; toxicity occurs when blood concentration rises above 8-12
,ug NH3-N/ml (Repp et al., 1955; Lewis et ccl., 1957) Clark et al.
(1951)) Kaishio et al. (1952) and Hale and King (1955) have
doubted whether ammonia and urea poisoning are in fact due to
the toxicity of ammonium ion to tissues; the pathogenesis of these
syndromes has not been fully established, and the effects of ruminal
stasis and alkalosis, occurring concomitantly with accumulation of
ammonia in peripheral blood, are obscure. However, Payne (1955)
48
has found that ammonium hydroxide, chloride, acetate and carbamate all gave similar pharmacological effects when injected into dogs.
From the evidence available it seems justifiable to conclude
that any compound forming ammonia in the rumen will prove toxic
if the rumen ammonia level is sufficiently raised so that the rate
of ammonia absorption exceeds the conversion threshold of the
liver and thus leads to a rise in peripheral blood ammonia
to a level of more than 10 ,ug N/ml.
N O N-P ROTEIN -N ITROGEN XN RUMINANT N UTRITION
The capacity of N.P.N., especially urea, to replace part of the
protein in ruminant diets has been the subject of hundreds of
experiments (see reviews by Reid, 1953, and Hale, 1956). Although
there is no doubt that N.P.N. can, through the use of ammonia-N
for microbial growth in the rumen, provide protein to the ruminant,
it is noteworthy that the limitations of this activity have not been
ascertained. Emphasis has usually been placed on weight gain or
milk production as the criterion of utilization of N.P.N.; however,
under Australian conditions, this use of N.P.N. will probably find
greater application in low levels of feeding under drought conditions;
the work of Franklin, Briggs and McClymont (1955) leaves little
doubt that this will prove an economic procedure.
Urea is a normal constituent of the rumen contents and its
subsequent fate and contribution to the animal’s nitrogen metabolism
differs in no respect from that of ammonia derived from protein
or other nitrogenous substances. Obviously then, a clear understanding of the normal nitrogen metabolism in the rumen will
provide the background for prediction of the value of N.P.N. in
various ruminant rations. It is probable that, provided other factors
(e.g. palatability, toxicity) do not interfere, the value of any nonprotein-nitrogenous substance will depend on its conversion to
ammonia in the rumen and on the suitability of the rumen contents
as a medium for the growth of bacteria capable of using ammonia
as a N-source.
Recent experiments on sheep given urea with wheat or maizemeal indicate that the rate of hydrolysis of urea is extremely fast
and that ammonia leaves the rumen in quantity before there is
much fermentation of the starches; these observations suggest that
the N taken up by bacterial growth under these conditions may
be largely derived from urea returned to the rumen and not from
that of the ration.
Attention may be drawn to the relative amounts of protein-N
and ammonia-N in the rumen. The rumen contents normally have
an extremely high microbial population and this is reflected in a high
protein content. Boyne et al. (1956) report analyses of rumen contents
of sheep fed twice daily and receiving 34 gm N/day; at the period
12 hr. after feeding, the rumen-reticulum was found to contain
approximately 28 gm N, i.e. over 80 per cent. of the daily intake;
if one assumed a value of 20 mg ammonia-N/100 gm rumen contents
in these sheep, the NH3-N would then represent only 4-5 per cent.
of the total-N in the rumen. Comparable data are not available
for sheep fed rations with relatively high proportions of N.P.N.
but this example serves to indicate the need for information on the
composition and changes in composition of the rumen contents in
such cases.
B IOLOGICAL V ALUE
OF
F ODDER P ROTEINS
The complexities of nitrogen metabolism in the rumen have
important implications in nutrition. In the non-ruminant, the biological value of a food protein is chiefly determined by its amino-acid
composition. In the ruminant on the contrary, this would hold only
49
for that fraction of the food protein which escaped digestion in
the rumen and passed unchanged into the abomasum where it
could undergo peptic, and subsequently intestinal, digestion ; the
fraction of portein degraded in the rumen would have value to
the ruminant to the degree that it was converted into microbial
protein. Assuming that the nitrogen metabolism of ruminant tissues
is essentially the same as that of the non-ruminants, it is probable
that only that nitrogen absorbed in the form of amino-acid assemblies
can be utilized in the tissues; other forms of nitrogen absorbed
would be valueless since they would merely contribute to an excess
of N from non-essential amino-acids. Microbial activity will thus
be profitable to the ruminant only if it leads to an increase in
“quality” or quantity of amino-acids available to the host. A
consideration of the amino-acid composition of plant leaves suggests
that no marked improvement in amino-acid composition would
accrue by conversion to microbial protein (Weller, 1957) ; some seed
proteins could be markedly improved in this way. It is probably
safe to generalize as follows: on ordinary rations for maintenance
or production, the greater the resistance of the protein to deamination in the rumen, the more effectively will it be utilized provided
that subsequent digestion and absorption are not impaired. Such
a situation has been strikingly demonstrated by Chalmers et aZ.
(1954)) using casein as the main feed protein.
When, however, the diet contains high proportions of starch
a net gain of N can accrue due to the utilization of ammonia-N for
microbial growth. It is therefore clear that, for the ruminants,
biological value must be referred to the diet as a whole and not
to the proteins of the diet alone.
RUMEN AMMONIA
AND
BIDOD UREA C ONCENTRATION
Since ammonia is absorbed from the rumen and converted
by the liver into urea, it is obvious that this constitutes one factor
influencing the concentration of urea in the blood. The passage of
urea out of the blood stream into saliva and rumen similarly must
play a role in this connection. Lewis (1957). postulated that the
rumen ammonia level had a controlling influence on blood urea,
since there was usually a close correlation between the blood urea
concentration and the level of rumen ammonia at its maximum
about 4 hr after feeding ; he suggested that this finding might
form the basis of a supplementary test for assessing the value of
protein in ruminant rations. The amount of urea-N in the blood
and tissues is much in excess of the amount of ammonia-N in / the
rumen (e.g. in a sheep of body weight 40 kg, rumen volume 5 litres,
blood urea-N 15 mg/lOO ml, rumen NHs-N 12 mg/lOO ml, body
water 60 per cent. B.W., then urea-N : NHS-N :: 6 : 1). This explains
why blood urea concentration does not reflect the marked diurnal
changes in rumen ammonia (Lewis, 1957). Little is known of the
control of urea excretion rate by the ruminant kidney and thus
it is not yet possible to calculate the direct loss of rumen ammonia
via urea formation and urinary excretion.
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