Plasma natural 15N abundance may predict both feed conversion efficiency and residual feed intake in beef cattle across different dietary conditions

Cheap and rapid methods to predict between-animal variability of feed efficiency (FE) are needed. We evaluated, by using a large dataset on beef cattle (n = 355) fed contrasting diets (grass vs corn based diets), the potential of plasma natural 15N abundance (δ15N) as a biomarker of between-animal variation in FE, measured either as feed conversion efficiency (FCE) or residual feed intake (RFI). Data were analyzed by mixed models, with diet and δ15N as a fixed effect and experimental farm, batch and pen as random effects. A negative and significant correlation was found between FCE and δ15N within each contemporary group, with a lower response (slope) however in grass vs corn diets (δ15N × diet interaction; P = 0.002). Likewise, we found a significant but positive relationship between δ15N and RFI, but without the influence of diet or any tested random effect. Our results indicate that δ15N is a promising biomarker of between-animal variation of feed efficiency (FCE and RFI) in growing beef cattle