178 Programing the rumen microbiome to optimize microbial efficiency in high forage diets

Abstract As the majority of energy and protein supplied to cattle arises as a result of ruminal fermentation, the rumen microbiome has an integral role in determining host feed efficiency. Counterintuitively, current evidence suggests that a less diverse rumen microbiome is associated with improved feed efficiency, possibly as a result of greater metabolic precision and avoidance of energy spilling fermentative pathways. The composition of the rumen microbiome is mainly determined by diet, but host traits such as rumen volume, rate of passage, rumination and immunity also have influence. Although less microbial diversity may improve feed efficiency in cattle fed a specific diet, reduced diversity may impair the ability of cattle to adapt to frequent changes in diet and the environment. Hydrogen exchange and capture is the energetic foundation of the rumen microbiome and considerable capital has been invested to develop additives that redirect hydrogen flow away from the reduction of CO2 to CH4 towards alternative sinks. These additives have been shown to reduce enteric CH4 emissions by 30 to 80%, but improvements in feed efficiency have been less than stoichiometric predictions. Approaches to improve the feed efficiency of cattle need to be multifaceted with consideration for host genetics, functional efficiency of the rumen microbiome, and the structure and composition of feed. Likewise, reductions in carbon emissions need to be broader than just CH4, with an appreciation of the role that cattle have within a circular bioeconomy to promote upcycling of nutrients and reductions in emissions from farming systems. Strategies to improve the efficiency of cattle production are a prerequisite for the sustainable intensification needed to ensure that the social license for milk and meat production from cattle is retained.