Seventy-six Angus steers chosen from breeding lines divergently selected for residual feed intake (RFI) were studied to quantify the relationship between RFI and the daily rate of methane production (MPR). A 70-d feeding test using a barley-based ration was conducted in which the voluntary DMI, feeding characteristics, and BW of steers were monitored. The estimated breeding value (EBV) for RFI (RFI(EBV)) for each steer had been calculated from 70-d RFI tests conducted on its parents. Methane production rate (g/d) was measured on each steer using SF(6) as a tracer gas in a series of 10-d measurement periods. Daily DMI of steers was lower during the methane measurement period than when methane was not being measured (11.18 vs. 11.88 kg; P = 0.001). A significant relationship existed between MPR and RFI when RFI (RFI(15d)) was estimated over the 15 d when steers were harnessed for methane collection (MPR = 13.3 x RFI(15d) + 179; r(2) = 0.12; P = 0.01). Animals expressing lower RFI had lower daily MPR. The relationship established between MPR and RFI(15d) was used to calculate a reduction in daily methane emission of 13.38 g accompanied a 1 kg/d reduction in RFI(EBV) in cattle consuming ad libitum a diet of 12.1 MJ of ME/kg. The magnitude of this emission reduction was between that predicted on the basis of intake reduction alone (18 g x d(-1) x kg of DMI(-1)) and that predicted by a model incorporating steer midtest BW and level of intake relative to maintenance (5 g x d(-1) x kg of DMI(-1)). Comparison of data from steers exhibiting the greatest (n = 10) and lowest (n = 10) RFI(15d) showed the low RFI(15d) group to not only have lower MPR (P = 0.017) but also reduced methane cost of growth (by 41.2 g of CH(4)/kg of ADG; P = 0.09). Although the opportunity to abate livestock MPR by selection against RFI seems great, RFI explained only a small proportion of the observed variation in MPR. A genotype x nutrition interaction can be anticipated, and the MPR:RFI(EBV) relationship will need to be defined over a range of diet types to account for this.

Abstract. The methods for estimating methane emissions from cattle as used in the Australian national inventory are based on older data that have now been superseded by a large amount of more recent data. Recent data suggested that the current inventory emissions estimates can be improved. To address this issue, a total of 1034 individual animal records of daily methane production (MP) was used to reassess the relationship between MP and each of dry matter intake (DMI) and gross energy intake (GEI). Data were restricted to trials conducted in the past 10 years using open-circuit respiration chambers, with cattle fed forage-based diets (forage >70%). Results from diets considered to inhibit methanogenesis were omitted from the dataset. Records were obtained from dairy cattle fed temperate forages (220 records), beef cattle fed temperate forages (680 records) and beef cattle fed tropical forages (133 records). Relationships were very similar for all three production categories and single relationships for MP on a DMI or GEI basis were proposed for national inventory purposes. These relationships were MP (g/day) = 20.7 (AE0.28) · DMI (kg/day) (R 2 = 0.92, P < 0.001) and MP (MJ/day) = 0.063 (AE0.008) · GEI (MJ/day) (R 2 = 0.93, P < 0.001). If the revised MP (g/day) approach is used to calculate Australia's national inventory, it will reduce estimates of emissions of forage-fed cattle by 24%. Assuming a global warming potential of 25 for methane, this represents a 12.6 Mt CO 2 -e reduction in calculated annual emissions from Australian cattle.

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 CH 4 /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 (P¼ 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 (P¼ 0·27) or urinary allantoin excretion (P¼0·89). 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 (P¼ 0·10). These findings indicate that the selection of ruminants for low MY may have important consequences for an animal's nutritional physiology.Key words: Greenhouse gas abatement: Enteric methane: Rumen retention time Australia and other countries are devoting considerable resources to the abatement of enteric methane production by livestock. In the predominantly extensive pastoral production systems of Australia, the most practicable strategy may be that of exploiting observed differences in methane production within the ruminant populations (1 -3) through selective breeding. Lower methane yields (MY; g CH 4 /kg DM intake (DMI)) may arise due to one or more of the following factors: fermentation of less amounts of organic matter in the rumen; a shift in volatile fatty acid production towards alternative H þ -utilising (propionate or reductive acetogenesis) pathways; an increase in the relative yield of microbial cells produced by fermentation (4) , which may potentially be affected by host-derived differences in rumen morphology and function.Variation in the mean retention time (MRT) of rumen digesta affects the extent of degradation of organic matter in the rumen and the flow of undegraded microbial matter postruminally (5) , and MRT has been implicated as a basis for between-animal differences in wool production (6) . It has also been demonstrated that alterations in retention time can cause marked differences in the efficiency of microbial synthesis in vitro (7) , while more recently, in vivo studies have suggested that up to 40 % of the observed variation in methane production in sheep could be attributed to differences in mean rumen outflow (8) . As such, we hypothesised that MRT may contribute to between-animal differences in methane production among animals fed a constant diet. To test the hypothesis that differences in MY would be reflected in measurable differences in the rumen environment, MY together with the ...

Methanogens living on and within rumen ciliate protozoa may be responsible for up to 37% of the rumen methane emissions. In the absence of protozoa, rumen methane emissions are reduced by an average of 13% but this varies with diet. Decreased methane emissions from the protozoa-free rumen may be a consequence of: (1) reduced ruminal dry matter digestion; (2) a decreased methanogen population; (3) an altered pattern of volatile fatty acid production and hydrogen availability; or (4) increased partial pressure of oxygen in the rumen. The decline in methanogenesis associated with removal of protozoa is greatest on high concentrate diets and this is in keeping with protozoa being relatively more important sources of hydrogen on starch diets, because many starch-fermenting bacteria do not produce H2. Because protozoa also decrease the supply of protein available to the host animal, their elimination offers benefits in both decreasing greenhouse gas emissions and potentially increasing livestock production. Strategies for eliminating protozoa are reviewed. None of the available techniques is considered practical for commercial application and this should be addressed.

The effects of dietary nitrate on DM digestion, rumen volatile fatty acid concentrations, microbial protein outflow, rumen water kinetics, and methane production were studied. Eight rumen-cannulated sheep were acclimated to a diet consisting of chaffed oaten hay supplemented with either 4% KNO3 or 0% KNO3 but made iso-nitrogenous by the addition of urea. Nitrate supplementation did not affect blood methaemoglobin concentration, DM intake, whole tract or ruminal DM digestibility and the sheep appeared healthy at all times throughout the acclimation and experimental periods. Nitrate did cause changes in rumen fermentation consistent with its acting as a high-affinity hydrogen acceptor, i.e. there was a tendency towards a lower molar percentage of propionate in the rumen volatile fatty acids, and higher molar ratio of acetate to propionate. Methane yield (MY, L methane/kg DM intake) was reduced by 23% in KNO3-supplemented sheep (P < 0.05) and these sheep tended to have a shorter mean fluid retention time in the rumen (MRT). There was a significant association between MRT and MY, such that a shorter MRT was associated with a lower MY. The results confirmed that the presence of nitrate in the diet lowers enteric methane production even though there was considerable between-animal variation in gut kinetics and methane production.