Ruminant nutrition and function: understanding methane mitigation routes and impacts

Abstract

Methane is a potent greenhouse gas with a global warming potential 21 times that of carbon dioxide. Globally, ruminants are the main anthropogenic contributors to methane release to the atmosphere. Methane is produced in the gastrointestinal tract of ruminants, mostly within the rumen by methanogenic archaea. However, methane production represents a loss of 2 to 12% of dietary gross energy for the animal, which could otherwise be available for growth or milk production. Therefore, mitigation of methane production by ruminants could produce both economic and environmental benefits, with more sustainable and energy efficient livestock, and offering a promising way of slowing global warming. Despite extensive research undertaken to find ways of reducing methane emissions from ruminants, progress has been relatively limited. Furthermore, there is still a lack of studies linking rumen microbiology and ruminant nutrition and production. The central purpose of this research was to investigate feed additives to reduce methane emissions and to understand associated changes that occur in the rumen microbiota. For the first experiment (Chapter 2), biochar was evaluated as an antimethanogenic compound for beef cattle. The in vitro gas production technique was used to study the effects of biochar on rumen fermentation and methane production. Overall, methane production was reduced by 5% by the addition of biochar compounds (10 g/kg of substrate). The observed reduction in methane produced was not associated with a change in volatile fatty acid profile suggesting biochar primarily inhibited fermentation. Ammonia concentration was significantly reduced with biochar inclusion. Because different biochars had different effects on methane production, further investigation of relationships between the physicochemical properties of biochars and antimethanogenic effects are necessary. However, due to the small reduction in methane production recorded, research with biochar was discontinued. Encapsulated nitrate was then explored as an antimethanogenic additive and as an alternative non-protein nitrogen source to urea (Chapter 3). The effect of using encapsulated nitrate as a replacement for urea or dietary protein, plus the addition of inorganic sulphur, on enteric methane emissions, nutrient digestibility, nitrogen utilization and microbial protein synthesis from crossbred beef steers were studied. In addition, nitrate toxicity and eating behaviour were investigated. The inclusion of encapsulated nitrate reduced methane production compared to urea and a true protein source, with no adverse effects on rumen fermentation or nitrogen metabolism and no effects with the inclusion of elemental sulphur. The level of addition of encapsulated nitrate (14.3 g nitrate /kg DM) and the time of adaptation chosen for this study (14 days) were adequate to avoid nitrate toxicity. Finally, the effects of adding nitrate inclusion to different basal diets on rumen microbial populations and relationships of these populations with methane production were investigated (Chapter 4). The V4 hypervariable regions of the bacterial and archaea 16S rRNA genes were amplified and sequenced. Effects on microbial population induced by nitrate were dependant on the basal diet but nitrate altered specific archaeal and bacterial OTUs consistently between studies. A direct and strong correlation between some archaea taxonomic groups and OTUs with methane production was observed.