There is increasing demand for technologies to measure CH4 production (DMP) of ruminant livestock in inventory and mitigation research. Studies of genetic variation among animals in DMP require these emissions to be measured on thousands of animals which cannot be managed using traditional chamber studies. However, short-term emission measures have been reported to correlate well with DMP. Our study was conducted to determine the usefulness of 1 and 2 h emission measures using chambers in predicting DMP. In our first experiment, Merino sheep (n = 13) were measured for DMP over 22 h using open circuit chambers. On two subsequent days, after overnight fasts, sheep were offered feed for 1 h before return to the chambers for 2 h and their CH4 production determined. DMP estimated from 22 h measurements correlated moderately well with 2 h emission measures (r² of 0.42-0.48) and 1 h measures (r² of 0.39-0.43). In our second experiment, portable static chambers were designed for 1 and 2 h CH4 emission measurements. The portable static chambers retained 98-99% of an injected tracer gas after 2 h showing that gas leakage from the chamber was very low. With a sheep inside a portable static chamber, CO₂ concentration reached 2.2 x 103 ppmv after 2 h, but sheep showed no discomfort and the partial pressure of O₂ in their blood remained above 97% (i.e., safe for animal health). Our third experiment was to validate use of short-term emission measures from portable static chambers as predictors of DMP. Crossbred Dorset and Border Leicester and Merino ewes (n = 40, LW 54 ± 10.9 kg) were measured for three 22 h sessions in open circuit chambers, after which they were measured for 1 h in portable static chambers. Open circuit chamber measurements had high repeatability (i.e., 0.88) and, excluding one sheep with inconsistent eating patterns, the correlation (r) between 1 h portable static chamber measurements and average emissions in the open circuit chambers was 0.71. We conclude that 1 and 2 h measures of CH4 output in portable static chambers are useful for determining genetic differences in CH4 production in groups of ruminants.

A total of 2,600 methane (CH4) and 1,847 CO2 measurements of sheep housed for 1 h in portable accumulation chambers (PAC) were recorded at 5 sites from the Australian Sheep CRC Information Nucleus, which was set up to test leading young industry sires for an extensive range of current and novel production traits. The final validated dataset had 2,455 methane records from 2,279 animals, which were the progeny of 187 sires and 1,653 dams with 7,690 animals in the pedigree file. The protocol involved rounding up animals from pasture into a holding paddock before the first measurement on each day and then measuring in groups of up to 16 sheep over the course of the day. Methane emissions declined linearly (with different slopes for each site) with time since the sheep were drafted into the holding area.

The daily methane output of sheep is strongly affected by the quantity and digestibility of feed consumed. There are few widely applicable technologies that reduce the methane output of grazing ruminants without limiting feed intake per head or animal numbers. In contrast, there are many opportunities to increase the amount of animal product generated per unit of feed eaten. These include improving growth and reproductive rates of livestock and will reduce methane emission per unit of product (called emissions intensity) for individual animals. Producer responses to such improvements through changes to stocking rate and total area grazed will have a major effect on the total emission and profitability of the enterprise. First mating of ewes as lambs (~7 months of age) rather than as hoggets (~19 months of age) reduces the emissions intensity of self-replacing flocks but not that of flocks for which replacement ewes are purchased. Selection of sheep for improved residual feed intake reduces emissions intensity at the individual animal level as well as at the enterprise level. At present, emissions policies that motivate farm managers to consider generating fewer emissions rather than more profit or product are lacking.

Daily methane production and feed intake were measured on 160 adult ewes, which were the progeny of 20 sires and 3 sire types (Merino, dual-purpose and terminal) from a genetically diverse flock. All animals were housed in individual pens and fed a 50/50 mix of chaffed lucerne and oaten hays at 20 g/kg liveweight (LW), with feed refusals measured for at least 10 days before the first of three 22-h measurements in respiration chambers (RC). Feed was withdrawn at 1600 h on the day before each RC test to encourage the ewes to eat the entire ration provided for them in the RC. After the first 1-day RC test, the sheep were returned to their pens for a day, then given a second 1-day RC test, followed by another day in their pens, then a third RC test. After all animals had been tested, they were ranked according to methane emissions adjusted for feed intake in the RC and on the previous day, enabling 10 low and 10 high methane animals to be chosen for repeat measurement. No variation between sires nor consistent effects of LW on feed eaten (%FE, expressed as per cent of feed offered) was evident in the 10 days before the first RC measurement. However, significant differences between sires (equivalent to an estimated heritability of 41%) were identified for %FE during the 2nd and 3rd days of RC testing (2 and 4 days after the initial RC test). The analysis of all data showed that methane emissions in the RC were related to feed intake on the day of testing and the two previous days (all P<0.0005). Before correcting for feed intake on previous days, there was some variation between sires in methane yield, equivalent to an estimated heritability of 9%. Correction for feed intake on the 2 previous days halved the residual variation, allowing other effects to be detected, including effects of LW, twins reared as singles, test batch, RC and test-day effects, but estimated sire variation fell to zero. In order to avoid potential biases, statistical models of methane emissions in the RC need to consider potential confounding factors, such as those identified as significant in this study.

This study was designed to screen a large number of sheep to identify individuals with high and low methane (CH4) production, and to estimate repeatability and heritability of CH₄ emissions in sheep, utilising portable accumulation chambers (PAC) designed for in-field use. Mature ewes (n = 710) selected from a research flock with known sires had their CH₄ production over 1 h measured in PAC [CH₄(g1h)]. Individuals with High (n = 103) or Low (n = 104) CH₄ (g1h), adjusted for liveweight (LW), were selected and re-measured on three occasions 1-4 months later, at another site with more abundant and better quality pasture. Mean of the selected (207) ewes CH₄ (g1h) emissions were ~50% higher than at the first measurement site (0.66 g vs 0.42 g).LW was a significant correlate of CH₄ production (r = 0.47). Correlations between CH₄ (g1h) for the three PAC measurements at Site 2, before adjusting for LW ranged from 0.44 to 0.55. After adjusting for the effect of LW, repeatability was 0.33 at the first and 0.43 at the second site. The correlation between estimates of an animal's emissions at the first and second sites, adjusted for LW, was 0.24. Initial CH₄ production of the selected High group was 32% greater than the Low group (P < 0.0001). On re-measurement there was still a significant difference (9-15%, P < 0.006) between Low and High groups. The initial estimate of heritability of CH₄ (g1h), based on variation between the ewes' sires (0.13), was not maintained across the two sites. This may be due to genotype x environment interactions. We postulate that aspects of rumen physiology, which modulate CH₄ production, could be expressed differently in different nutritional environments. Our results indicate that field use of PAC to screen sheep populations for CH₄ production is both robust and repeatable. However, further investigations are required into the relationship between CH₄ output of individual animals in PAC compared with the more controlled conditions in respiration chambers.

Daily methane production (DMP) and 1-hour methane production (1-h MP) were measured twice, 4 weeks apart, on Merino wethers fed 'ad libitum'. This study aimed to determine the 4-week repeatability of DMP and 1-h MP as well as determine how well 1-h MP predicts DMP. After a 4-week interval, the repeatability of DMP was 0.49, while the repeatability of 1-h MP was 0.24. The correlation between DMP and 1-h MP was 0.56 for the first measurement and 0.66 for the second. It was estimated that the mean of 3 independent 1-hour measurements would be at least as repeatable as the DMP measurement. A 1-h MP measurement is a moderate predictor of DMP when sheep are fed 'ad libitum', which may occur during generous grazing conditions, and thus using 1-h MP as tool to select animals for low methane production may be feasible.

Rumen cannulated sheep (n = 12) were tested at each of three feeding levels: 0.8, 1.24 and 1.6 times maintenance. The ration (lucerne chaff) was provided in eight equal portions to emulate morning and afternoon grazing. After at least 10 days adaptation to each feeding level, methane production was measured in an open circuit calorimeter for 22 h using the same feeding regime. During measurement, 10 rumen samples were taken for volatile fatty acid (VFA) analysis by an indwelling rumen probe with a sampling tube that passed outside the calorimeter. Feed intake was strongly correlated with daily methane production (DMP, r = 0.87). Both methane production and VFA concentrations showed bimodal patterns related to the feeding cycle, but feed intake had a much smaller effect on VFA concentrations than on methane production rate. Average VFA concentration was a poor predictor of DMP. The best predictor, propionate concentration, explained 26% of the variance in DMP. The weakness of the association between VFA concentrations and methane production could be a consequence of differences in rumen volume and differences in VFA absorption associated with feeding level, although the possibility of accumulation of alternate fermentation end products or re-fermentation of VFA cannot be excluded. It is concluded that none of the suite of VFA parameters assessed offers a useful tool to predict daily methane production in grazing sheep.

Emissions of 710 ewes at pasture were measured for 1 h (between 09:00-16:30 h) in batches of 15 sheep in portable accumulation chambers (PAC) after an overnight fast continuing until 2 h before measurement, when the sheep had access to baled hay for 1 h. The test was used to identify a group of 104 low emitters (I-Low) and a group of 103 high emitters (I-Hi) for methane emissions adjusted for liveweight (CH4awt). The 207 ewes selected at the initial study were remeasured in 5 repeat tests from 2009 through 2014 at another location. The first repeat used the original measurement protocol. Two modified protocols, each used in 2 yr, drafted unfasted sheep on the morning of the test into a yard or holding paddock until measurement. Emissions of the I-Hi sheep were higher (102-112%) than I-Low sheep in all subsequent PAC tests, with statistical significance (P < 0.05) in 3 tests. Tests without overnight fasting were simpler to conduct and had repeatabilities of 51 to 60% compared with 31 and 43% for the initial and first repeat tests, respectively. After habituation to a diet fed at 20 g/kg liveweight, 160 of the 207 sheep were measured in respiration chambers (RC); 10 high (Hi-10) and 10 low (Low-10) sheep were chosen, representing extremes (top and bottom 6.25%) for methane yield (MY; g CH4/kg DMI). The Hi-10 group emitted 14% more methane (adjusted for feed intake) in a follow-up RC test, but Low-10 and Hi-10 sheep differed in only 1 of the 5 PAC tests, when Hi-10 sheep emitted less CH4awt than Low-10 sheep (P = 0.002) and tended to eat less in the feeding opportunity (P = 0.085). Compared with their weight on good pasture, Low-10 sheep were proportionately lighter than Hi-10 sheep in the relatively poor pasture conditions of the initial test. Sheep identified as low emitters by PAC tests using the initial protocol did not produce less CH4 (mg/min) when fed a fixed level of intake in RC. Correlations between estimates of an animal's CH4awt measured in PAC and CH4 adjusted for feed intake in RC were quite low (r = 0-19%) and significant (P < 0.05) in only 1 test of unfasted sheep. With moderate repeatability over the 5 yr, PAC tests of CH4awt could be a viable way to select for reduced emissions of grazing sheep. As well as exploiting any variation in MY, selecting for reduced CH4awt in PAC could result in lower feed intake than expected for the animals' liveweight and might affect the diurnal feeding pattern. Further work is required on these issues.

In Australia and New Zealand, the significance of enteric methane (CH₄) production is greater than in many industrialised countries, due to large livestock numbers (AGO, 2005). Consequently, there is considerable interest in both countries in breeding livestock which produce relatively less CH₄ per unit of production, as a mitigation strategy. A necessary step in achieving this aim is acquiring the capacity to quickly and reliably estimate the daily methane production (DMP) of ruminants. Mechanistic models used to predict CH₄ production from animals (e.g. Blaxter and Clapperton, 1965) do not reflect differences that have been observed in individual animals on similar intakes (Goopy et al., 2006); 24 h calorimetry remains the only proven method for discerning these observed differences. Unfortunately, this is difficult, resource intensive, and impractical for screening large numbers of animals. Thus, there is considerable impetus to develop alternative methods which will facilitate estimation of OMP in large numbers of animals. This study assessed six (direct and derived) estimates of enteric methane production against 22 h open circuit calorimetry measurements (CHB₂₂) to determine if a reliable, but less demanding estimate of DMP could be found.

Short-term studies with grazing sheep have shown that the concentration of volatile fatty acids (VFA) in the rumen is correlated with VFA production rate (Leng et al., 1968), which is stoichiometrically linked with enteric methane production rate. These associations may present an opportunity to predict daily methane production of grazing sheep from the concentration of VFA in the rumen. Such a method could have application in genetic improvement of methane traits, but a better understanding of the repeatability of VFA concentrations is required to assess the usefulness of a single VFA measurement in predicting daily methane production. A study was undertaken to determine between-animal and between-day variation in VFA concentrations in grazing sheep.