Methane Production and Productivity Changes Associated with Defaunation in Ruminants

Abstract

With increasing world population, global demand for a secure and growing food supply challenges the livestock producers of today to increase output of milk and meat while reducing the environmental impact of animal production. This thesis reports a review of literature and targeted new research assessing the consequences of eliminating rumen protozoa (defaunation) on the performance, digestive function and emissions of the greenhouse gas methane, by livestock.<br/> • Comparative studies of rumen fermentation and animal growth were conducted in growing Merino lambs, crossbred sheep and Brahman cattle. In these studies ruminants were defaunated using coconut oil distillate to suppress protozoa then dosed with sodium 1-(2-sulfonatooxyethoxy) dodecane in a protocol that suppressed feed intake for an average of 10 days but had no detrimental effects on animal health.<br/> • Reflecting the diversity in published literature, these studies found inconsistent effects of defaunation on volatile fatty acid (VFA) concentrations and proportions. Averaged over all experiments conducted, defaunation was associated with a small (5%) reduction in total VFA concentration and an increase (5%) in the ratio of acetate to propionate in the rumen.<br/> • While effects on VFA were not consistent, an average 30% reduction in rumen ammonia concentration and a 16% increase in microbial crude protein outflow (estimated by allantoin excretion) were apparent, suggesting substantial differences in the ruminal degradation and outflow of protein due to defaunation. These changes were associated with an 18% increase in average daily gain (ADG), but surprisingly no increase in wool growth rate.<br/> • Defaunation was associated with a lower enteric methane emission (average 20% reduction) compared to faunated ruminants, with the first studies of daily methane production (DMP) ever made while grazing, made using GreenFeed Emission Monitoring (GEM) units, confirming a 3% lower DMP (non-significant; P > 0.05) and a 9% lower methane yield (MY; CH4/kg DMI; P = 0.06) in defaunated sheep.<br/> • Protozoa affected the rumen response to nitrate, with the nitrate induced reduction in MY being 29% greater in faunated compared to defaunated lambs.<br/> • With dietary coconut oil, no interaction with defaunation was apparent with both coconut oil and defaunation significantly reducing DMP and MY in cattle.<br/> • While defaunation tended to increase average daily gain and reduced enteric methane emissions in cattle by 10%, establishing defaunated cattle proved difficult and is a major constraint to expanding defaunation into commercial herds.<br/> • Assessment of the distribution of protozoa in the forestomaches showed that the number of entodiniomorph protozoa attached to the 'leaves' of the bovine omasum was at least as great as the number attached to the entire surface of the rumen, though all tissue-attached populations are far fewer than the population in the rumen fluid.<br/> • It is concluded that defaunation alone or in combination with dietary supplements of nitrate is effective in decreasing methane emissions, while increasing microbial protein supply and ADG. Commercial implementation of defaunation for cattle will not be able to rely on addition of surfactants to the rumen and it is suggested a bioactive compound distributed through the blood may be needed to remove protozoa residing in the omasum.