Fat accretion measurements strengthen the relationship between feed conversion efficiency and Nitrogen isotopic discrimination while rumen microbial genes contribute little

The use of biomarkers for feed conversion efficiency (FCE), such as Nitrogen isotopic discrimination (Δ(15)N), facilitates easier measurement and may be useful in breeding strategies. However, we need to better understand the relationship between FCE and Δ(15)N, particularly the effects of differences in the composition of liveweight gain and rumen N metabolism. Alongside measurements of FCE and Δ(15)N, we estimated changes in body composition and used dietary treatments with and without nitrates, and rumen metagenomics to explore these effects. Nitrate fed steers had reduced FCE and higher Δ(15)N in plasma compared to steers offered non-nitrate containing diets. The negative relationship between FCE and Δ(15)N was strengthened with the inclusion of fat depth change at the 3(rd) lumbar vertebrae, but not with average daily gain. We identified 1,700 microbial genes with a relative abundance >0.01% of which, 26 were associated with Δ(15)N. These genes explained 69% of variation in Δ(15)N and showed clustering in two distinct functional networks. However, there was no clear relationship between their relative abundances and Δ(15)N, suggesting that rumen microbial genes contribute little to Δ(15)N. Conversely, we show that changes in the composition of gain (fat accretion) provide additional strength to the relationship between FCE and Δ(15)N.