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 ...

The effects of dietary nitrate and of Propionibacterium acidipropionici (PA) on methane and nitrous oxide emissions, methaemoglobinaemia, volatile fatty acid (VFA) concentration and productivity of sheep were studied. It was hypothesised that PA supplementation would increase the rate of nitrite reduction to ammonia in the rumen and therefore reduce risks of methaemoglobinaemia. Fine-wool Merino wethers (n = 28; 31.8 ± 3.7 kg; 11 months of age) were acclimated to four isonitrogenous and isoenergetic diets based on oaten chaff (1.0 kg/day) supplemented with either urea (1.1% of DM; T1 and T2) or a nitrate source (2.0% of DM; T3 and T4) while T2 and T4 were also supplemented with PA (11.5 × 1010 CFU/day). Replacing urea with nitrate lowered methane production (g/day) by 19% and methane yield (g/kg DMI) by 15%, improved clean wool growth by 12% (P < 0.001) and tended to increase skin temperature (P < 0.1). Nitrate increased ruminal acetate to propionate ratio by 27%, increased plasma nitrite and nitrate concentrations and blood methaemoglobin (MetHb) level up to 45% of total haemoglobin. Nitrous oxide emission from sheep confined in respiration chambers was higher (P < 0.001) when nitrate was fed, lowering the net benefit of methane mitigation on global warming potential (CO2 equivalents/kg DMI) by 18%. In contrast, PA had little effect, decreasing total VFA concentration (P < 0.05), increasing rumen pH (P < 0.05) and clean wool growth (P < 0.05) of urea-fed sheep. This study confirmed the beneficial effects of nitrate on net greenhouse gas reduction and wool growth, but showed that methaemoglobinaemia risks may be higher when diets are fed at a restricted level and contain only low levels of readily fermented carbohydrate. PA supplementation was not effective in reducing methaemoglobinaemia, but did increase clean wool growth of urea-fed sheep.

Methane yield from ruminants is positively correlated with mean retention time (MRT) of digesta, which is known to be influenced by the hormone triiodothyronine (T3).We hypothesised that a decrease in the MRT in the rumen in response to administration of a T3 solution to sheep would reduce their methane yield. To test this hypothesis, 10 mature Merino wethers were injected with T3 (300 µg) on two different protocols (daily; n = 5 and every second day; n = 5) and the effect on daily methane yield, digesta MRT, DM digestibility, rumen volatile fatty acid concentrations, microbial protein output and plasma T3 concentrations studied. Compared with when injected with saline (control), injection of sheep with T3 every second day resulted in decreased methane yield (P < 0.05) and lower acetate (P < 0.001), butyrate (P < 0.001) and propionate (P < 0.01) concentrations in the rumen. MRT of digesta, derived from faecal excretion of CoEDTA and Cr-mordanted fibre, were reduced in the total tract (P < 0.001) and hindgut (P < 0.01) but not in the rumen (P > 0.05). DM digestibility was not affected by injection of T3 every second day but water intake (P < 0.05) and urine output (P < 0.01) were increased. When sheep were injected with T3 daily, changes were only observed in plasma T3 concentration (P < 0.001) and volume of CO2 produced (P < 0.05). The results indicate that increasing plasma concentration of the thyroid hormone T3 within physiological levels reduces digesta retention time, especially retention time in the hindgut and leads to a reduction in enteric methane yield. Further work is warranted to assess whether plasma T3 concentrations may be indicative of enteric methane yield.

Ruminant methane yield (MY) is positively correlated with mean retention time (MRT) of digesta. The hormone triiodothyronine (T3 ), which is negatively correlated with ambient temperature, is known to influence MRT. It was hypothesised that exposing sheep to low ambient temperatures would increase plasma T3 concentration and decrease MRT of digesta within the rumen of sheep, resulting in a reduction of MY. To test this hypothesis, six Merino sheep were exposed to two different ambient temperatures (cold treatment, 9 ± 1 °C; warm control 26 ± 1 °C). The effects on MY, digesta MRT, plasma T3 concentration, CO2 production, DM intake, DM digestibility, change in body weight (BW), rumen volatile fatty acid (VFA) concentrations, estimated microbial protein output, protozoa abundance, wool growth, water intake, urine output and rectal temperature were studied. Cold treatment resulted in a reduction in MY (p < 0.01); digesta MRT in rumen (p < 0.01), hindgut (p = 0.01) and total digestive tract (p < 0.01); protozoa abundance (p < 0.05); and water intake (p < 0.001). Exposure to cold temperature increased plasma T3 concentration (p < 0.05), CO2 production (p = 0.01), total VFA concentrations (p = 0.03) and estimated microbial output from the rumen (p = 0.03). The rate of wool growth increased (p < 0.01) due to cold treatment, but DM intake, DM digestibility and BW change were not affected. The results suggest that exposure of sheep to cold ambient temperatures reduces digesta retention time in the gastrointestinal tract, leading to a reduction in enteric methane yield. Further research is warranted to determine whether T3 could be used as an indirect selection tool for genetic selection of low enteric methane-producing ruminants.

Cysteamine is a biological compound produced in the gastrointestinal tract and hypothalamus of all animals that acts on the somatotrophic axis. Cysteamine is finding increasing application in human medicine and also as a natural, in-feed growth promotant for monogastric and ruminant livestock that increases feed conversion efficiency, growth rate and leanness. It improves nutrient digestion and absorption by increasing portal-drained viscera blood flow and net portal absorption, while also reducing gastroenteropancreatic, plasma and hypothalamus concentrations of the inhibitory hormone, somatostatin (SRIF). Dietary inclusion rates required to achieve growth responses are typically about 10 times higher in ruminants than those required for pigs, but it is unclear whether ruminal breakdown of cysteamine is contributing to this difference. While short-term stimulation of growth, milk production and improved feed use efficiency are apparent, studies over longer periods are required, especially in breeding animals, due to the process of SRIF depletion being reversible. This review provides an overview of cysteamine’s mode of action in improving nutrient utilisation and its application in human nutrition and health, as well as its potential use as a growth promotant in the livestock industries.