Published November 25, 2014
The effect of pasture pregrazing herbage mass on methane
emissions, ruminal fermentation, and average daily gain of grazing beef heifers1
T. M. Boland,*2 C. Quinlan,* K. M. Pierce,* M. B. Lynch,* D. A. Kenny,† A. K. Kelly,* and P. J. Purcell*
*School of Agriculture and Food Science, College of Agriculture, Food Science and Veterinary Medicine,
University College Dublin Lyons Research Farm, Newcastle, Co. Dublin, Ireland; and †Animal and Bioscience
Research Department, Animal and Grassland Research and Innovation Centre, Teagasc, Dunsany, Co. Meath, Ireland
ABSTRACT: The objective of this study was to determine the effect of pregrazing pasture herbage mass
(HM) on CH4 emissions, ruminal fermentation, and
ADG of grazing beef heifers at 2 stages of the grazing
season. Thirty Limousin cross heifers were allocated
to 1 of 2 target pregrazing HM treatments [a low HM
(LHM) or high HM (HHM) treatment] for 126 d in a
randomized block design experiment. Pasture herbage and heifer rumen fluid samples were collected,
and enteric CH4 emissions were determined using
an SF6 tracer technique during two 5-d measurement
periods [MP; MP 1 (25 to 29 May) and MP 2 (6 to
10 September)]. Both DMI and GE intake (GEI) were
measured during MP 2, and ADG of the heifers was
measured every 14 d throughout the 126-d grazing
period. Mean HM for the LHM and HHM treatments
were 1,300 and 2,000 kg DM/ha, respectively, during
MP 1 and 2,800 and 3,200 kg DM/ha, respectively,
during MP 2. The CP concentration of the offered
herbage was greater (P < 0.01) for the LHM treatment
during MP 1 and tended (P < 0.1) to be greater for the
LHM herbage during MP 2. No difference (P > 0.10)
in the NDF concentration of the herbage was found
between the HM treatments during MP 1 or 2. There
was no effect (P > 0.10) of HM treatment on total CH4
emissions (g/d) for either MP [mean value across HM
treatments of 121 (SED 5.4) g/d during MP 1 and 132
(8.8) g/d during MP 2], but CH4 emissions (g) per kilogram of ADG were reduced (P < 0.05) from heifers fed
the LHM treatment during MP 1 and 2 [mean values
for LHM and HHM of 135 and 163 (SED 9.5) g/kg,
respectively, during MP 1 and corresponding values of
150 and 194 (9.9) g/kg during MP 2]. Heifers fed the
LHM treatment had greater (P < 0.001) ADG throughout the grazing period [mean value across the 126-d
grazing period of 0.88 (SEM 0.032) kg/d] than those
fed the HHM treatment [corresponding value of 0.73
(0.034)]. For MP 2, CH4 emissions per kilogram of
DMI (g CH4/kg DMI) and per megajoule of GEI (MJ
CH4/MJ GEI) tended (P ≤ 0.08) to be less for heifers fed the LHM [19.3 (0.08) g/kg and 0.056 (0.0020)
MJ/MJ, respectively] than for the HHM (21.1 g/kg and
0.061 MJ/MJ) treatment, and there were no differences (P > 0.10) in DMI or GEI of the heifers between
the HM treatments. The results of this study suggest
that offering a low pregrazing HM sward will reduce
enteric CH4 emissions relative to ADG throughout the
grazing season because of increased ADG.
Key words: average daily gain, beef heifers, dry matter intake, herbage mass, methane emissions
© 2013 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2013.91:3867–3874
doi:10.2527/jas2013-5900
INTRODUCTION
Agriculture accounts for 0.305 of total green1Funding for this research was provided under the National Development
Plan through the Research Stimulus Fund, administered by the Department
of Agriculture, Food and the Marine, Ireland RSF 06361.
2Corresponding author: [email protected]
Received September 25, 2012.
Accepted April 14, 2013.
house gas (GHG) emissions in Ireland (Environmental
Protection Agency, 2012), approximately one-half
of which is methane (CH4) produced during enteric fermentation in ruminants (Duffy et al., 2011).
Approximately 0.9 of total agricultural land in Ireland
is grassland, and grazed grass is the lowest-cost feed
available for ruminant livestock production systems
(O’Riordan and O’Kiely, 1996; Finneran et al., 2012).
Thus, grass-based enteric CH4 mitigation strategies will
3867
3868
Boland et al.
form a key component in Irish agriculture meeting its
GHG reduction targets. Within the predominantly grassbased animal production systems operated in Ireland and
many other temperate countries, maintaining a lower pregrazing herbage mass (HM; kg DM/ha) sward may result
in greater OM digestibility (OMD) of offered herbage
compared with a high HM sward because of less accumulation of stem and dead material and a greater proportion
of more digestible leaf in the herbage offered (Holmes et
al., 1992; Wims et al., 2010). Such an increase in grass
OMD can result in greater DMI by ruminants (Hart et al.,
2009), which increases the rate of feed passage through
the rumen. Additionally, increased OMD results in less
enteric CH4 production per unit of feed digested due to
a greater proportion of propionic acid [which is an alternative hydrogen (H2) sink to methanogenesis] in the
total rumen VFA produced (Janssen, 2010). Furthermore,
maintaining a low HM sward may increase forage DMI,
which promotes greater animal performance, such as increased daily BW gain of grazing ruminants (Redmon et
al., 1995). Thus, it is feasible that maintaining a lower
pregrazing HM sward of generally high digestibility will
result in decreased enteric CH4 output and increased
ADG of grazing heifers because of greater herbage DMI.
Considering this, it is hypothesized that lower pregrazing
herbage mass will reduce enteric CH4 emissions from
grazing beef heifers. The objective was to determine the
effect of pregrazing pasture HM on CH4 emissions, ruminal fermentation, and ADG of beef heifers at 2 stages
of the grazing season.
Materials and Methods
All animal procedures used in this study were conducted under experimental license from the Irish Department
of Health and Children in accordance with the Cruelty
to Animals Act 1876 and the European Communities
(Amendment of Cruelty to Animals Act 1876) Regulations
2002 and 2005. The study was conducted at University
College Dublin (UCD) Lyons Research Farm (Newcastle,
Co. Dublin, Ireland; 53°17’56”N, 6°32’18”W). The grass
pastures used were Lolium perenne (L.) dominant permanent grassland swards.
Treatments and Experimental Design
The study was conducted as a randomized block design, where the experimental heifers were subjected to
1 of 2 pregrazing HM treatments [a low HM (LHM) or
high HM (HHM) treatment] The heifers were grazed on
the experimental paddocks throughout a 126-d grazing
period that commenced on May 14 (2008). During this
grazing period, sward herbage and rumen fluid samples
from the heifers were collected, and enteric CH4 mea-
surements of the heifers were made during 2 separate
5-d measurement periods (MP), defined as MP 1 (May
25 to 29) and MP 2 (September 6 to 10). The LHM and
HHM treatments examined were 1,300 and 2,000 kg
DM/ha, respectively, during MP 1 and 2,800 and 3,200
kg DM/ha, respectively, during MP 2. In addition, feces
samples were taken during MP 2 for the estimation of
DMI and GE intake (GEI), and the BW of the heifers
was measured every 14 d throughout the grazing period
for the determination of ADG.
Heifers and Grazing Management
Before the commencement of the study, 30 Limousin
cross heifers aged between 14 and 18 mo were blocked
on the basis of BW [mean BW of the heifers was 346
(SD = 34) kg] and were randomly allocated to either the
LHM or HHM treatment. Before the commencement of
the 126-d grazing period, differences in pasture pregrazing HM were produced by altering sward regrowth intervals between grazing periods after an initial grazing
of the entire experimental area (2.01 ha) on March 18.
Regrowth intervals between grazing periods were subsequently altered appropriately to generate the required
differences in HM during both MP and throughout the
grazing period. The experimental heifers were turned
out to pasture on April 30, 15 d before the beginning
of the grazing period, and were allocated fresh herbage
on a daily basis at the rate of 8 kg DM∙heifer-1∙d-1 using
strip grazing. Temporary fencing was erected each day
to prevent the heifers returning to the area grazed the
previous day. No P or K was applied during the duration
of the study. A total of approximately 130 kg N/ha was
applied to each HM treatment throughout the experimental period. Fertilizer was applied subsequent to each
grazing for both treatments.
Pregrazing Herbage Mass Determination
Pasture sward height above 40 mm was monitored
daily using a folding pasture plate meter with a steel
plate (plate diameter of 355 mm and area density of
3.2 kg/m2; Jenquip, Fielding, New Zealand), and the
pregrazing HM (kg DM/ha) of the experimental pastures
were subsequently estimated using the linear equation of
O’Riordan et al. (1997):
HM (kg DM/ha) = [242 × sward height (cm)] – 804.
Sward Herbage Sampling
Sward herbage samples representative of the herbage
grazed by the heifers were taken daily (at 0700 h) at ran-
Effect of Herbage Mass on Heifer Methane Emissions
dom during both MP using Gardena hand shears by cutting
10 strips (0.1 × 0.3 m) of the sward to a height of 40 mm
from the grazing horizon. Once collected, herbage samples were stored at -18°C and subsequently thawed at 4°C
for 24 h, after which each sample was thoroughly mixed
and dried at 55°C to a constant weight to determine DM
content. Samples were then milled (Christy and Norris
hammer mill; Christy and Norris Process Engineers Ltd.,
Chelmsford, UK) through a sieve with 1-mm apertures
before chemical composition analysis.
Enteric Methane Measurement
Daily enteric CH4 emissions from the heifers were
determined using the emissions from ruminants using
a calibrated tracer (ERUCT) technique, with sulfur
hexafluoride (SF6) as the tracer gas as described by
Johnson et al. (2007). The equipment was modified for
grazing beef cattle as described by Hart et al. (2009).
Permeation tubes containing SF6 were blocked by SF6
emission rate [mean rate 7.22 (SD = 0.50) mg/d] and
randomly allocated to both HM treatments and heifers
within HM treatment. Gas expired and eructated by the
heifers, in addition to ambient air, was drawn into an
evacuated gas collection canister at a rate of 0.5 mL gas/
min via passage through stainless steel capillary tubing
(50 mm in length, 12.7-μm i.d.; Valco Instruments Co.
Inc., Houston, TX), calibrated using a digital flowmeter
(Cole-Parmer Instrument Co., Vernon Hills, IL). During
each MP, canisters containing gas were removed from
heifers at 0800 h daily and replaced with preevacuated
canisters, after which the pressure in each removed canister was immediately measured using a Qwik-Connect
pressure gauge (Swagelok, Bray, Co. Wicklow, Ireland).
Where negative pressure did not occur in a canister, the
entire collection apparatus was replaced before the attachment of a preevacuated canister. In addition, 5 preevacuated canisters were placed at various locations
(around the grazing areas) throughout the MP to determine background concentrations of CH4 and SF6. After
daily canister change, canisters were positively pressurized with nitrogen (N2) gas to approximately 1,250 hPa,
after which concentrations of CH4 and SF6 in the gaseous contents of each canister were determined by gas
chromatography using a Varian 3800 gas chromatograph (Varian BC, Middelburg, the Netherlands) as described by Lovett et al. (2003).
Animal Measurements
Average Daily Gain. Heifers were individually
weighed (Tru-Test digital weighing scales; Tru-Test Ltd.,
Auckland, New Zealand) on 2 consecutive days at 14 d
intervals throughout the grazing period. The ADG of the
3869
heifers was then calculated using a linear regression of
BW (kg) with time (d).
Dry Matter Intake. The DMI (kg DM∙heifer-1∙d-1)
of the individual heifers was determined during MP 2
only using the n-alkane technique of Mayes et al. (1986),
as modified according to Dillon and Stakelum (1989).
Beginning 7 d before MP 2, all heifers were dosed twice
daily (at 0800 and 1700 h) for 12 consecutive d with a
paper pellet (Carl Roth, GmbH and Co. KG, Karlesruhe,
Germany) containing 500 mg of n-dotriacontane (C32
alkane). From d 7 to 12 of dosing (i.e., throughout
MP 2), fecal grab samples were collected from each
heifer before each dosing, after which all samples were
stored at -20°C. When required for n-alkane analysis,
the fecal grab samples were thawed at 4°C for 24 h, after
which the samples from each heifer were bulked (10 g
of each sample collected) by heifer, resulting in 1 fecal grab sample per heifer per MP. All samples were
then dried at 48°C for 48 h in a ventilated oven with
forced-air circulation, milled (Christy and Norris hammer mill; Christy and Norris Process Engineers Ltd.)
through a screen with 1-mm apertures, and stored until
required for n-alkane analysis. Pasture herbage samples
(manually collected using Gardena hand sheers; Accu
60, Gardena International GmbH, Ulm, Germany) representative of that grazed by the heifers were collected
twice daily for each treatment and pooled per treatment
during MP 2 for n-alkane analysis in accordance with
the method of Dillon and Stakelum (1989) and stored at
-20°C before n-alkane analysis. Alkane concentrations
were determined in duplicate on pooled daily samples
of herbage and feces using the extraction method of
Dove and Mayes (2006), with the modification that nheptane was substituted for n-dodecane as the solvent
used to rehydrate the extract before injection onto the
gas chromatograph. Samples were analyzed for concentrations of n-alkanes by gas chromatography using a
Varian 3800 GCL (Varian Inc., Palo Alto, CA) fitted with
a 30-m capillary column with a 0.53-mm i.d. and coated with 0.5 μm dimethyl polysiloxane (SGE Analytical
Science Pty Ltd., Ringwood, Victoria, Australia). The
ratio of herbage C33 (n-tritriacontane) to dosed C32 was
used to estimate pasture herbage DMI of the heifers as
described previously in Mulligan et al. (2004).
Rumen Fluid Sampling. On the final day of each MP,
rumen fluid samples were taken from each heifer using a
FLORA rumen scoop (Guelph, Ontario, Canada). Each
sample was then strained through 2 layers of cheesecloth
to remove feed particles, after which 10 mL were transferred into vials containing 0.5 mL of a 4 mM HgCl2
solution and vials containing 0.25 mL of a 0.5 M H2SO4
solution for subsequent VFA and ammonia-N (NH3-N)
analysis, respectively. These samples were then frozen
at −20°C before analysis.
3870
Boland et al.
Herbage Chemical Composition Analyses
The CP concentration of the herbage samples was
determined using a Leco FP 528 N analyzer (Leco
Instruments UK Ltd., Cheshire, UK) according to the
method of Dumas (AOAC, 1990). The NDF [assayed
without a heat-stable α-amylase (Ankom Technology,
Fairport, NY) or sodium sulfite; expressed inclusive
of residual ash] and ADF (expressed inclusive of residual ash) concentrations were determined using the
filter bag technique (Ankom, 2006a,b). The ADF analysis was performed sequentially on the residue from the
NDF assay. The ADL concentrations were determined
on the residue from the ADF assay using the sulfuric
acid method (Robertson and Van Soest, 1981). Ash concentration was determined by complete combustion in a
muffle furnace at 550°C for 4 h. The GE concentration
of the offered herbage was determined by bomb calorimetry (Parr 1281 Bomb Calorimeter, Parr Instrument
Company, Moline, IL).
Volatile Fatty Acid and Ammonia-Nitrogen Analyses
The concentrations of individual VFA (acetic, propionic, butyric, and valeric acids) in the rumen fluid of the
heifers were determined by gas chromatography (Varian
3800) using a CP-wax 58 capillary column (25 m ×
0.53 mm; Varian BC), according to the method of Porter
and Murray (2001). The concentrations of NH3-N in the
rumen fluid were determined using the microdiffusion
technique of Conway (1957).
Statistical Analysis
Data were checked for normality and homogeneity of variance using histograms, QQ plots, and formal
statistical tests as part of the UNIVARIATE procedure
(SAS Inst. Inc., Cary, NC). Data that were not normally
distributed were transformed by raising the variable to
the power of lambda. The appropriate lambda value
was obtained by conducting a Box-Cox transformation
analysis in the TRANSREG procedure of SAS. The
proportion of valeric acid required a transformation and
was raised to the power of 0.25. All herbage data were
analyzed for each MP separately using a mixed-model ANOVA (in the PROC MIXED procedure of SAS).
Similarly, all animal variables were analyzed for each
MP using a mixed-model ANOVA. The statistical model
used included the fixed effect of HM treatment. Heifer
BW at the start of test was included in the model as a
linear covariate. The statistical model used was
Yi = µ + Ti + blLi + ei,
where µ is the overall HM treatment mean, Ti is the fixed
effect of HM treatment (i = 1 to 2), b1Li is initial BW,
and ei is the residual error term. Differences between HM
treatments were determined by F tests using type III sums
of squares. The PDIFF option and the Tukey test were applied as appropriate to evaluate pairwise comparisons between HM treatment group means Treatment differences
with a P-value less than or equal to 0.05 are described as
significantly different, values greater than 0.05 and less
than or equal to 0.10 are described as a tendency and values greater than 0.10 are described as non significant.
RESULTS
Herbage Chemical Composition
There was no difference (P > 0.10) in the DM content (g/kg fresh weight) of the herbage between the
HHM and the LHM treatments for either MP (Table 1).
The CP concentration (g/kg DM) was greater (P < 0.01)
for the LHM compared with the HHM treatment herbage during MP 1 and tended (P < 0.1) to be greater for
LHM during MP 2. There were no differences (P > 0.10)
in NDF concentration (g/kg DM) between the HM treatments for either MP. The ADF concentration (g/kg DM)
was greater (P < 0.01) for the HHM herbage during MP 1
but did not differ (P > 0.10 between HM treatments during MP 2. The LHM treatment herbage had greater (P <
0.05) ADL and ash concentrations (g/kg DM) compared
with the HHM herbage during both MP. No differences
(P > 0.10) in GE concentration (MJ/kg DM) of the herbage were found during either MP.
Methane Emissions, Dry Matter Intake,
and Average Daily Gain
Overall, HM treatment had no effect (P > 0.10) on
total CH4 emissions (g/d) of the heifers during either MP
(Table 2). However, CH4 emissions per kilogram of ADG
(CH4/ADG) were decreased (P < 0.05) for heifers fed
the LHM treatment during both MP. For MP 2 only, CH4
output per kilogram of DMI (CH4/DMI) and the megajoules of CH4 per megajoule of GEI (CH4/GEI) tended
(P < 0.1) to be less for the heifers fed the LHM vs. the
HHM treatment herbage, and there were no differences
(P > 0.10) in the herbage DMI or GEI (MJ/d) of the heifers
between the HM treatments (Table 3). The overall ADG
(kg∙heifer-1∙d-1) of the heifers throughout the 126-d grazing
period was greater (P < 0.001) for the LHM treatment than
for the HHM treatment [a mean ADG of 0.88 (0.032 SEM)
kg∙heifer-1∙d-1 for heifers on the LHM treatment vs. 0.73
(0.034) kg∙heifer-1∙d-1 for those on the HHM treatment].
3871
Effect of Herbage Mass on Heifer Methane Emissions
Table 1. The effect of herbage mass (HM) treatment on the chemical composition of the herbage offered during 2
measurement periods (MP)
MP 11 (n = 5)
LHM3
Variables
HHM4
SED
DM, g/kg FW
190
212
11.2
CP, g/kg DM
232
151
16.1
NDF, g/kg DM
480
491
4.7
ADF, g/kg DM
225
237
2.4
ADL, g/kg DM
21.1
15.4
1.2
Ash, g/kg DM
9.42
7.61
0.28
GE, MJ/kg DM
19.3
19.2
0.14
1May 25 to 29.
2September 6 to 10.
3Low HM (1,300 and 2,800 kg DM/ha for MP 1 and 2, respectively).
4High HM (2,000 and 3,200 kg DM/ha for MP 1 and 2, respectively).
Ruminal Fermentation
The rumen fluid NH3-N concentration (mg N/L)
tended (P < 0.1) to be greater for heifers fed the HHM
vs. the LHM treatment during MP 1 but, conversely, was
greater (P < 0.001) for the LHM treatment during MP 2.
The total rumen fluid VFA (tVFA) concentration (mM)
was greater (P < 0.01) for heifers fed the LHM treatment
during MP 1 but greater (P < 0.05) for the HHM treatment during MP 2. For the proportions of individual VFA
in the tVFA concentration (mol/mol), acetic and propionic acids were greater and less than P < 0.001, respectively, for heifers fed the LHM vs. the HHM treatment
during MP 1. For MP 2, the proportion of acetic acid was
greater (P < 0.05) for the HHM treatment, whereas HM
treatment had no effect (P > 0.10) on propionic acid. The
proportion of butyric acid tended (P < 0.1) to be greater
for the HHM treatment during MP 1 and was greater (P <
P-value
0.20
0.005
0.14
0.007
0.008
0.001
0.72
MP 22 (n = 5)
HHM4
SED
179
10.2
161
11.1
575
8.9
284
7.0
25.9
1.9
7.82
0.34
19.1
0.10
LHM3
160
201
552
282
34.1
9.82
18.9
P-value
0.23
0.05
0.12
0.82
0.01
0.005
0.23
0.05) for the LHM treatment in MP 2. Herbage mass
treatment had no effect (P > 0.10) on the proportion of
valeric acid in the tVFA of rumen fluid of the heifers
during either MP. The acetic acid to propionic acid ratio
(A:P) was greater (P < 0.001) for the LHM treatment in
MP 1, with no effect (P > 0.05) of HM treatment on A:P
being found during MP 2 (Table 2).
DISCUSSION
Sward pregrazing HM is an important factor in efficient use of pasture and provision of grass herbage of
sufficiently high nutritional quality for grazing ruminants. In a zero-grazing study, Hart et al. (2009) found
that increasing the DM digestibility of perennial ryegrass
swards by reducing pregrazing HM reduced daily CH4
emissions from beef cattle. In the latter study, animals
Table 2. The effect of herbage mass (HM) treatment on enteric methane (CH4) emissions and ruminal fermentation
variables of beef heifers (n = 15/treatment) during 2 measurement periods (MP)
MP 11
Variables
LHM3
HHM4
SED
P-value
CH4, g/d
120
122
5.37
0.48
CH4/ADG,5 g/kg
135
163
9.54
0.02
NH3-N,6 mg N/L
88
106
7.0
0.06
Total VFA, mmol/L
105
90
3.7
0.008
VFA proportions, mol/mol total VFA
Acetic
0.73
0.71
0.004
<0.001
Propionic
0.14
0.16
0.003
<0.001
Butyric
0.09
0.10
0.003
0.06
Valeric
0.02
0.02
0.002
0.72
A:P7
5.21
4.42
0.120
<0.001
1May 25 to 29.
2September 6 to 10.
3Low HM (1,300 and 2,800 kg DM/ha for MP 1 and 2, respectively).
4High HM (2,000 and 3,200 kg DM/ha for MP 1 and 2, respectively).
5CH expressed relative to ADG recorded during the 2-wk period at the end of each MP.
4
6Ammonia-nitrogen.
7Acetic acid to propionic acid ratio.
MP 22
LHM3
127
150
132
80
HHM4
136
194
69
102
SED
8.76
9.87
5.8
3.6
P-value
0.40
0.03
<0.001
<0.001
0.70
0.16
0.10
0.01
4.57
0.71
0.16
0.09
0.01
4.56
0.003
0.005
0.003
0.001
0.140
0.03
0.63
0.04
0.78
0.56
3872
Boland et al.
Table 3. The effect of herbage mass (HM) treatment on
DMI, gross energy intake (GEI), and methane (CH4)
emissions of beef heifers (n = 15/treatment) during measurement period 2 (September 6 to 10)
Variables
LHM1
HHM2
-1
-1
DMI, kg∙heifer ∙d
6.50
6.44
GEI, MJ∙heifer-1∙d-1
123
123
CH4/DMI3
19.3
21.1
CH4/GEI4
0.056
0.061
1Low HM (2,800 kg DM/ha).
2High HM (3,200 kg DM/ha).
3Grams CH per kilogram DMI.
4
4Megajoules CH per megajoule GEI.
4
SED
0.080
1.4
0.68
0.0020
P-value
0.60
0.82
0.07
0.08
were accommodated indoors and offered freshly cut pasture herbage twice daily. On typical commercial beef
farms in Ireland and in many other temperate countries,
however, the majority of diets of ruminant animals are
perennial ryegrass–predominant herbage grazed at pasture, which is the cheapest feed available for ruminant
livestock production systems (Finneran et al., 2012).
Thus, considering the importance of grazed grass as a
feed for beef cattle, swards differing in pregrazing HM
were generated throughout the grazing season, and measurements were made at 2 stages of the grazing season,
in this study to determine the effect of pregrazing pasture
HM on CH4 emissions, ruminal fermentation, and ADG
of grazing beef heifers fed at pasture. The overall HM
treatments applied across both MP in this study (range
of mean HM across MP of 1,300 to 3,200 kg DM/ha)
are the extremes within what would be considered good
grazing management for grazing cattle in Ireland. Thus,
these considerable differences in the HM treatments applied between the MP means that the effect of pregrazing
HM on the aforementioned factors was examined under 2
broadly differing sets of experimental conditions.
Herbage Chemical Composition
Overall, the herbage offered during MP 1 was of generally greater nutritional quality than that offered during MP
2, as exemplified by its generally decreased concentrations
of cell wall components (the mean NDF, ADF, and ADL
concentration values across HM treatments during MP 1
were 0.86, 0.82, and 0.60, respectively, of the corresponding values for MP 2). This outcome most likely reflects a
reduced proportion of leaf and a greater accumulation of
stems and dead material (Hoogendoorn et al., 1992; Wims
et al., 2010), which are typically more highly lignified and
of greater fiber concentration than leaf, in the herbage offered during MP 2 because of the greater pregrazing HM
treatments applied. Furthermore, the herbage offered during MP 2 was most likely also of a more mature growth
stage at harvest than that offered during MP 1 because of
the longer regrowth intervals between grazing periods in
the strip-grazing system operated. Thus, the aforementioned findings for the fiber and ADL fractions of the offered herbage are also in accord with the increase in the
NDF, ADF, and ADL concentrations of perennial ryegrass
that occur as it progresses through successive stages of
physiological maturity (Čop et al., 2009; King et al., 2012).
Within the individual MP, the finding of greater CP
concentration for the LHM herbage during MP 1 and the
similar trend during MP 2, which is in agreement with
the findings of Wales et al. (1999), McEvoy et al. (2009),
and Purcell et al. (2011), most likely reflects a greater proportion of leaf (which is of generally greater CP concentration than the stem in perennial grasses; Mowat et al.,
1965) in the total LHM herbage offered. The latter finding
is in agreement with the decrease in the CP concentration
of perennial ryegrass that usually occurs as it advances in
maturity (Čop et al., 2009; King et al., 2012).
In contrast, the lack of differences in NDF concentration between the HM treatments for both MP, which is in
accord with the findings of and Owens et al. (2008), Wims
et al. (2010), and Purcell et al. (2011), was not congruent
with an expected increase in the NDF concentration of
the herbage with advancing maturity, as explained above.
However, the reduced ADF concentration for the LHM
herbage during MP 1 (May) was in line with expectation.
Considering the generally similar hemicellulose
(i.e., NDF minus ADF) concentrations in the LHM (255
g/kg DM) and HHM (254 g/kg DM) herbage during MP
1, this decreased ADF concentration for the LHM herbage during MP 1 appears to have been mainly due to a
generally reduced cellulose (i.e., ADF minus ADL) concentration for the LHM (mean LHM cellulose concentration of 204 g/kg DM) than the HHM treatment (corresponding value of 222 g/kg DM).
The GE concentration values for the herbage in this
study (range of mean values across HM treatments and
MP of 18.9 to 19.3 MJ/kg DM) were generally similar to
those reported by Hart et al. (2009; range of mean values
of 18.2 to 18.3 MJ/kg DM) and Smit et al. (2005; 18.3 to
18.7 MJ/kg DM) for perennial ryegrass, and the finding
of no difference in the GE concentrations of the offered
herbage between HM treatments during both MP is in
agreement with the similar finding reported for measurement 2 of Wims et al. (2010).
Methane Emissions, ADG, DMI, and GE Intake
The lack of a difference in total daily CH4 emissions
from the heifers between the HM treatments during both
MP in this study agrees with the findings for measurement
1 of Wims et al. (2010). This may have been due to the
lack of effects of HM treatment on NDF concentration
of the offered herbage for both MP, which most likely re-
Effect of Herbage Mass on Heifer Methane Emissions
sulted in similar DMI between the HM treatments (as occurred during MP 2 when DMI was measured) and thus
the similar daily CH4 emissions for both HM treatments.
This outcome reflects the close relationship between NDF
concentration and forage intake in ruminants (Van Soest,
1965; Waldo, 1986; Arelovich et al., 2008). The apparent
relationship between DMI and daily CH4 emissions in this
study is in general agreement with Pinares-Patiño et al.
(2007), who found total daily CH4 emission of heifers to
be consistently related to OM intake of pasture forage, and
Hart et al. (2009), who found greater daily CH4 emissions
(g/d) from heifers grazing grass swards due to greater DMI.
For MP 2, the lack of an effect of pregrazing HM
treatment on the DMI of the heifers is in general agreement with Curran et al. (2010), who found no effect of
pregrazing HM (1,600 vs. 2,400 kg DM/ha) on grass
DMI of dairy cows from April to October, and Owens
et al. (2008), who found no difference in beef steer DMI
(kg/d) between perennial ryegrass herbage after 28 (2,727
kg DM/ha) vs. 38 d (3,558 kg DM/ha) of sward regrowth.
This absence of an effect of HM treatment on total DMI,
in addition to the lack of an effect of HM treatment on the
GE concentration of the offered herbage, resulted in the
similar total GEI of the heifers for both HM treatments in
MP 2. Considering the absence of effects of HM treatment
on DMI, GEI, and daily CH4 emissions of the heifers, the
trends for decreased CH4 emissions for the LHM vs. the
HHM treatment during MP 2 when expressed relative to
DMI and GEI were unexpected. Despite this, the latter
findings are in partial agreement with Wims et al. (2010),
who found lower GEI lost as CH4 (measurements 1 and
2) and lower CH4 emissions per kilogram DMI (measurement 2) from dairy cows offered a decreased pregrazing
HM (1,000 vs. 2,200 kg DM/ha) sward. This tendency toward decreased CH4/DMI for the heifers subjected to the
LHM treatment despite the similar DMI of the heifers for
both HM treatments during MP 2 may reflect the greater
ADL concentration of the LHM herbage offered, as the
proportion of ADL in total DMI is negatively correlated
with daily CH4 emissions from ruminants (Hindrichsen et
al., 2004; Ellis et al., 2007).
Because the total daily CH4 emissions did not differ between HM treatments for either MP, the reduced
CH4/ADG for the LHM treatments appears to have been
mainly due to the greater ADG for the LHM treatment.
However, the reasons for the greater ADG for the heifers
on the LHM treatment are unclear.
The greater rumen NH3-N concentrations for the
heifers on the LHM treatment during MP 2 is in accord
with Owens et al. (2008) and most likely reflect the tendency toward a greater CP concentration of the offered
herbage, as dietary CP concentration is positively related to rumen NH3-N concentrations (Roffler and Satter,
1975; Pritchard and Males, 1985) because of dietary true
3873
protein and NPN being metabolized by rumen microbes
to ammonia (Bach et al., 2005). Considering this, the
finding that the NH3-N concentration tended to be greater for HHM during MP 1 despite there being a decreased
CP concentration in the herbage was surprising.
Conclusion
No difference in total daily CH4 emissions of the
grazing heifers was found between the HM treatments
examined in this study for MP 1 or 2 (i.e., during May or
September, respectively), most likely reflecting the similar total cell wall concentrations (i.e., NDF concentration) of the offered herbage during both MP. This resulted
in similar DMI for both HM treatments during MP 2.
However, when expressed relative to ADG (CH4/ADG),
CH4 emissions were decreased for the LHM treatment
during both MP, reflecting the greater ADG of the heifers
on the LHM treatment. During MP 2, CH4 output relative
to DMI (CH4/DMI) and GEI (CH4/GEI) tended to be less
for the LHM treatment, and there was no effect of HM
treatment on the DMI or GEI of the heifers. The results
of this study suggest that offering a low pregrazing HM
sward will reduce enteric CH4 emissions relative to ADG
throughout the grazing season because of increased ADG
and may also result in decreased CH4 emissions relative
to DMI and GEI for grazing beef heifers.
Literature cited
Ankom. 2006a. Acid detergent fiber in feeds. Filter bag technique
(for A2000, A2000I). Ankom Technology Method 8. Ankom
Technology Corp., Macedon, NY.
Ankom. 2006b. Neutral detergent fiber in feeds. Filter bag technique
(for A2000, A2000I). Ankom Technology Method 9. Ankom
Technology Corp., Macedon, NY.
AOAC. 1990. Official methods of analysis. 15th ed. Assoc. Off. Anal.
Chem, Arlington, VA.
Arelovich, H. M., C. S. Abney, J. A. Vizcarra, and M. L. Galyean.
2008. Effects of dietary neutral detergent fibber on intakes of
dry matter and net energy by dairy and beef cattle: Analysis of
published data. Prof. Anim. Sci. 24:375–383.
Bach, A., S. Calsamiglia, and M. D. Stern. 2005. Nitrogen metabolism in the rumen. J. Dairy Sci. 88:E9–E21.
Čop, J., A. Lavrenči, and K. Košmelj. 2009. Morphological development and nutritive value of herbage in five temperate grass species during primary growth: Analysis of time dynamics. Grass
Forage Sci. 64:122–131.
Conway, E. J. 1957. Microdiffusion analysis and volumetric error.
4th ed. Crosby, Lockwood, London.
Curran, J., L. Delaby, E. Kennedy, J. P. Murphy, T. M. Boland, and M.
O’Donovan. 2010. Sward characteristics, grass dry matter intake
and milk production performance are affected by pre-grazing
herbage mass and pasture allowance. Livest. Sci. 127:144–154.
Dillon, P., and G. Stakelum. 1989. Herbage and dosed alkanes as a
grass measurement technique for dairy cows. Ir. J. Agric. Res.
28:104. (Abstr.)
3874
Boland et al.
Dove, H., and R. W. Mayes. 2006. Protocol for the analysis of nalkanes and other plant-wax compounds and for their use as
markers for quantifying the nutrient supply of large mammalian
herbivores. Nat. Protoc. 1:1680–1697.
Duffy, P., B. Hyde, E. Hanley, C. Dore, P. O’Brien, E. Cotter,
and K. Black. 2011. Ireland national inventory report 2011.
Greenhouse gas emissions 1990–2009. Reported to the United
Nations Framework Convention on Climate Change. Environ.
Prot. Agency, Johnstown Castle, Co. Wexford, Ireland.
Ellis, J. L., E. Kebreab, N. Odongo, B. W. McBride, E. K. Okine, and
J. France. 2007. Prediction of methane production from dairy
and beef cattle. Can. J. Dairy Sci. 90:3456–3467.
Environmental Protection Agency. 2012. Ireland’s greenhouse gas
emissions in 2010. Environ. Prot. Agency, Johnstown Castle,
Co. Wexford, Ireland.
Finneran, E., P. Crosson, P. O’Kiely, L. Shalloo, D. Forristal, and M.
Wallace. 2012. Stochastic simulation of the cost of home-produced
feeds for ruminant livestock systems. J. Agric. Sci. 150:123–139.
Hart, K. J., P. G. Martin, P. A. Foley, D. A. Kenny, and T. M. Boland.
2009. Effect of sward dry matter digestibility on methane production, ruminal fermentation, and microbial populations of
zero-grazed beef cattle. J. Anim. Sci. 87:3342–3350.
Hindrichsen, I. K., H.-R. Wettstein, A. Machmüller, C. R. Soliva, K. E.
Bach Knudsen, J. Madsen, and M. Kreuzer. 2004. Effects of feed
carbohydrates with contrasting properties on rumen fermentation
and methane release in vitro. Can. J. Anim. Sci. 84:265–276.
Holmes, C. W., C. J. Hoogendoorn, M. P. Ryan, and A. C. P. Chu.
1992. Some effects of herbage composition, as influenced by
previous grazing management, on milk production on ryegrass/white clover pastures. 1. Milk production in early spring:
Effects of different regrowth intervals during the preceding winter period. Grass Forage Sci. 47:309–315.
Hoogendoorn, C. J., C. W. Holmes, and A. C. U. Chu. 1992. Some effects of herbage composition, as influenced by previous grazing
management, on milk production by cows grazing on ryegrass/
white clover pastures. 2. Milk production in late spring/summer:
Effects of grazing intensity during the preceding spring period.
Grass Forage Sci. 47:316–325.
Janssen, P. H. 2010. Influence of hydrogen on rumen methane formation
and fermentation balances through microbial growth kinetics and
fermentation thermodynamics. Anim. Feed Sci. Technol. 160:1–22.
Johnson, K. A., H. H. Westberg, J. J. Michal, and M. W. Cossalman.
2007. The SF6 tracer technique: Methane measurement from
ruminants. In: H. P. S. Makkar and P. E. Vercoe, editors,
Measuring methane production from ruminants. Int. At. Energy
Agency, Vienna. p. 33–67.
King, C., J. McEniry, M. Richardson, and P. O’Kiely. 2012. Yield and
chemical composition of five common grassland species in response to nitrogen fertiliser application and phenological growth
stage. Acta Agric. Scand., Sect. B, Soil Plant Sci. 62:644–658.
Lovett, D., S. Lovell, L. Stack, J. Callan, M. Finlay, J. Connolly, and
F. P. O’Mara. 2003. Effect of forage/concentrate ratio and dietary coconut oil level on methane output and performance of
finishing beef heifers. Livest. Prod. Sci. 84:135–146.
Mayes, R. W., C. S. Lamb, and P. M. Colgrove. 1986. The use of
dosed and herbage n-alkanes as markers for the determination
of herbage intake. J. Agric. Sci. 107:161–170.
McEvoy, M., M. O’Donovan, E. Kennedy, J. P. Murphy, L. Delaby,
and T. M. Boland. 2009. Effect of pregrazing herbage mass and
pasture allowance on the lactation performance of HolsteinFriesian dairy cows. J. Dairy Sci. 92:414–422.
Mowat, D. N., R. S. Fulkerson, W. E. Tossell, and J. E. Winch. 1965.
The in vitro digestibility of protein content of leaf and stem portions of forages. Can. J. Plant Sci. 45:321–331.
Mulligan, F. J., P. Dillon, J. J. Callan, M. Rath, and F. P. O’Mara.
2004. Supplementary concentrate type affects nitrogen excretion of grazing dairy cows. J. Dairy Sci. 87:3451–3460.
O’Riordan, E. G., S. Devaney, and P. French. 1997. Sward height as a
measure of pasture herbage supply. Ir. J. Agric. Food Res. 36:107.
O’Riordan, E., and P. O’Kiely. 1996. Potential for beef production systems based on grass. Ir. Grassl. Anim. Prod. Assoc. J. 30:185–217.
Owens, D., M. McGee, and T. M. Boland. 2008. Effect of grass regrowth interval on intake, rumen digestion and nutrient flow to
the omasum in beef cattle. Anim. Feed Sci. Technol. 146:21–41.
Pinares-Patiño, C. S., G. C. Waghorn, A. Machmüller, B. Vlaming, G.
Molano, A. Cavanagh, and H. Clark. 2007. Methane emissions
and digestive physiology of nonlactating dairy cows fed pasture
forage. Can. J. Anim. Sci. 87:601–613.
Porter, M. G., and R. S. Murray. 2001. The volatility of components
of grass silage on oven-drying and the interrelationship between
dry-matter content estimated by different analytical methods.
Grass Forage Sci. 56:405–411.
Pritchard, R. H., and J. R. Males. 1985. Effect of crude protein and
ruminal ammonia-N on digestibility and ruminal outflow in
beef cattle fed wheat straw. J. Anim. Sci. 60:822–831.
Purcell, P. J., M. O’Brien, T. M. Boland, M. O’Donovan, and P. O’Kiely.
2011. Impacts of herbage mass and sward allowance of perennial
ryegrass sampled throughout the growing season on in vitro rumen methane production. Anim. Feed Sci. Technol. 166:405–411.
Redmon, L. A., F. T. McCollum III, G. W. Horn, M. D. Cravey, S. A.
Gunter, P. A. Beck, J. M. Mieres, and R. S. Julian. 1995. Forage
intake by beef steers grazing winter wheat with varied herbage
allowances. J. Range Manage. 48:198–201.
Robertson, J. B., and P. J. Van Soest. 1981. The detergent system of
analysis. In: W. P. T. James and O. Theander, editors, The analysis of dietary fibre in food. Marcel Dekker, New York. p. 123–158.
Roffler, R. E., and L. D. Satter. 1975. Relationship between ruminal
ammonia and nonprotein nitrogen utilization by ruminants. 1.
Development of a model for predicting nonprotein utilization
by cattle. J. Dairy Sci. 58:1880–1888.
Smit, H. J., B. M. Tas, H. Z. Taweel, and A. Elgersma. 2005. Sward
characteristics important for intake in six Lolium perenne varieties. Grass Forage Sci. 60:128–135.
Van Soest, P. J. 1965. Symposium on factors influencing the voluntary
intake of herbage by ruminants: Voluntary intake in relation to
chemical composition and digestibility. J. Anim. Sci. 24:834–843.
Waldo, D. R. 1986. Symposium: Forage utilization by the lactating
cow. Effect of forage quality on intake and forage-concentrate
interactions. J. Dairy Sci. 69:617–632.
Wales, W. J., P. T. Doyle, C. R. Stockdale, and D. Dellow. 1999.
Effects of variations in herbage mass, allowance, and level of
supplement on nutrient intake and milk production of dairy
cows in spring and summer. Aust. J. Exp. Agric. 39:119–130.
Wims, C. M., M. H. Deighton, E. Lewis, B. O’Loughlin, L. Delaby,
T. M. Boland, and M. O’Donovan. 2010. Effect of pre-grazing
herbage mass on methane production, dry matter intake and
milk production of grazing dairy cows during the mid season
period. J. Dairy Sci. 93:4976–4985.