Ruminal taxonomic composition of two beef groups fed under extreme diets
The ratio of forage to concentrate in cattle feeding has a major influence on the composition of the microbiota in the rumen and on the mass of methane produced. Using methane measurements and microbiota data from 26 cattle we aimed to investigate the relationships between microbial relative abundances and methane emissions, and identify potential biomarkers, in animals fed two extreme diets - a poor quality zero-grazed grass diet (GRASS) or a high concentrate total mixed ration (TMR). Direct comparisons of the effects of such extreme diets on the composition of rumen microbiota have rarely been studied.  Data were analysed considering their multivariate and compositional nature. Diet had a strong effect on methane yield (g/kg of dry matter intake) of +35% for GRASS with respect to TMR, and on 20 microbial genera abundances (p=0.05). When predicting methane yield based on the abundance of 28 and 15 selected microbial genera in GRASS and TMR, respectively, we achieved prediction accuracies of 66.5±9% and 85±8%. Only the abundance of Fibrobacter had a consistent negative association with methane yield in both diets, whereas most microbial genera were associated with methane yield in only one of the two diets (p=0.1). This study highlights the stark contrast in the microbiota controlling methane yield between animals fed a high concentrate diet, such as that found on intensive finishing units, and a low-quality grass forage that is often found in extensive grazing systems. This contrast must be taken into consideration when developing strategies to reduce methane emissions by manipulation of the rumen microbial composition.