We evaluated the potential to use the naturally occurring isotopic N fractionation between plasma or milk of ruminants and their diet (Δ 15 N animal-diet ) to predict between-animal variation in N use efficiency (NUE), as well as assessed the main mechanisms by which Δ 15 N animal-diet is related to NUE under common feeding conditions. Individual values for NUE (n = 217) and Δ 15 N animal-diet (n = 274) from 11 published experiments were analyzed by meta- analysis. All dietary treatments were characterized according to the newly-updated French feeding system. The Δ 15 N animal-diet reflects between-animal variation in NUE and underlying mechanisms by which Δ 15 N animal-diet is related to NUE across diets are at both the metabolic and rumen levels.
