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.

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.