In the present study, following the measurement of methane emissions from 160 mature ewes three times, a subset of twenty ewes was selected for further emission and physiological studies. Ewes were selected on the basis of methane yield (MY: g CH₄/kg DM intake) being low (Low MY: >1 sd below the mean; n 10) or high (High MY: >1 sd above the mean; n 10) when fed a blended chaff ration at a fixed feeding level (1·2-fold maintenance energy requirements). The difference between the Low- and High-MY groups observed at the time of selection was maintained (P= 0·001) when remeasured 1-7 months later during digesta kinetics studies. Low MY was associated with a shorter mean retention time of particulate (P< 0·01) and liquid (P< 0·001) digesta, less amounts of rumen particulate contents (P< 0·01) and a smaller rumen volume (P< 0·05), but not apparent DM digestibility (P= 0.27) or urinary allantoin excretion (P= 0·89). Computer tomography scanning of the sheep's rumens after an overnight fast revealed a trend towards the Low-MY sheep having more clearly demarcated rumen gas and liquid phases (P= 0·10). These findings indicate that the selection of ruminants for low MY may have important consequences for an animal's nutritional physiology.

Increasing interest in reducing emission of the greenhouse gas methane from ruminants has lead to a need for reliable and accurate determination of the daily methane production (DMP) phenotype in ruminants. Twenty-four hour direct calorimetry is generally considered to give an accurate measurement of DMP. However, single measurements, while accurate on the day, does not account for day-to-day intra-animal variability in determination of DMP phenotype. This paper draws on the results of a large body of data (Blaxter and Clapperton, 1965) to estimate the number of measurements needed to discern real differences in the DMP phenotype with varying levels of confidence. Blaxter and Clapperton's (1965) results from 989 individual 24h CH₄ (sheep and cattle) determinations, indicated a day-to-day CV within animals of 7.2%. Using this value we calculated the number of measurements required to detect a varying degrees of difference at different confidence levels from a t-distribution. Figure 1 presents estimates of the number of measures required to detect an expected difference between individual animals at 3 levels of confidence. Increasing the number of DMP measurements will enable the detection of smaller differences in DMP between animals. ... Using a published CV of 7.2, it is clear that single measures of DMP are inadequate to characterise DMPs with a reasonable degree of confidence. We conclude that greater consideration needs to be given as to the number of measures required to adequately discern true differences in DMP phenotype.

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.

Since first proposed by Johnson et al. (1994), the sulphur hexafluoride (SF₆) tracer technique has been used to quantify ruminal methane emissions from both cattle and sheep. To make the SF₆ technique more robust, permeation tubes with higher release rates have been developed (Hegarty and Woodgate 2003). Although SF₆ is thought to be an inert and non-toxic marker gas, the higher release rates from the new capsules could invalidate this method of determining methane production if there is any effect on microbial metabolism. The intra-ruminal concentration of SF₆ after inserting capsules releasing 100-200 mg/d was from 1-35μl/l(Goopy et al. 2003). A study was made to determine whether SF₆ had any effect on microbial VFA production in the range of 1-100 μl/l.