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.

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 ...

No abstract

Greenhouse gas (GHG) emission measurements from livestock excreta in Africa are limited. We measured CH 4 and N 2 O emissions from excreta of six Boran (Bos indicus) and six Friesian (Bos taurus) steers near Nairobi, Kenya. The steers were fed one of three diets (T1 [chaffed wheat straw], T2 [T1 + Calliandra calothyrsus Meissner -0.2% live weight per day], and T3 [T1 + calliandra -0.4% live weight every 2 d]). The T1 diet is similar in quality to typical diets in the region. Calliandra is a leguminous fodder tree promoted as a feed supplement. Fresh feces and urine were applied to grasslands and emissions measured using static chambers. Cumulative 28-d fecal emissions were 302 ± 52.4 and 95 ± 13.8 mg CH 4 -C kg -1 dry matter for Friesen and Boran steers, respectively, and 11.5 ± 4.26 and 24.7 ± 8.32 mg N 2 O-N kg -1 dry matter for Friesian and Boran steers, respectively. For urine from Friesian steers, the N 2 O emissions were 2.8 ± 0.64 mg N 2 O-N 100 mL urine In sub-Saharan Africa, livestock comprise a large proportion of total agricultural emissions, most of which is from enteric CH 4 production in ruminants (Valentini et al., 2014). However, between 7 and 15% of agricultural GHG emissions are associated with livestock manure (Smith et al., 2014;Tubiello et al., 2014). However, these emission rates from livestock manure in Africa are estimated using emission factors (EFs) from the International Panel on Climate Change (IPCC) that have been derived using measurements primarily from states within the Organization for Economic Cooperation and Development. These regions have livestock species, breeds, diets, management systems, and climatic conditions that often differ from those in tropical Africa (IPCC, 2006).In tropical and subtropical agricultural production systems, the climate is generally warmer than temperate systems, which could result in greater N 2 O and CH 4 emissions from excreta because emissions are often positively correlated with temperature (González-Avalos and Ruiz-Suárez, 2001;Rochette et al., 2014). However, the types of management systems used, the quality of the feeds, and the species of cattle raised may also affect emissions. The majority of African ruminants graze for much of their life (Schlecht et al., 2006), which results in over 40% of excreta Journal of Environmental Quality ATMOSPHERIC POLLUTANTS AND TRACE GASES TECHNICAL REPORTS Core Ideas• GHG emissions from African livestock excreta is lower than IPCC tier 1 emission factors.• Low-quality feeds with low protein content result in low N content of excreta.• Supplementation of cattle diet with calliandra reduced the methane emissions from cattle feces.• The species of cattle causes differences in GHG emissions from feces.