Technical note: Nitrogen isotopic fractionation can be used to predict nitrogen-use efficiency in dairy cows fed temperate pasture

The objective of this study was to investigate the relationship between nitrogen isotopic fractionation (delta N-15) and nitrogen-use efficiency (milk nitrogen/nitrogen intake; NUE) in pasture-fed dairy cows supplemented with increasing levels of urea to mimic high rumen degradable protein pastures in spring. Fifteen cows were randomly assigned to freshly cut pasture and either supplemented with 0, 250, or 336 g urea/d. Feed, milk, and plasma were analyzed for delta N-15, milk and plasma for urea nitrogen concentration, and plasma for ammonia concentration. Treatment effects were tested using ANOVA and relationships between variables were established by linear regression. Lower dry matter intake (P = 0.002) and milk yield (P = 0.002) occurred with the highest urea supplementation (336 g urea/d) compared with the other two treatments. There was a strong linear relationship between milk delta N-15 - feed delta N-15 and NUE: [NUE (%) = 58.9- 10.17 x milk delta N-15 - feed delta N-15 (%) (r(2) = 0.83, P < 0.001, SE = 1.67)] and between plasma delta N-15 - feed delta N-15 and NUE: [NUE (%) = 52.4 - 8.61 x plasma delta N-15 - feed delta N-15 (%) (r(2) = 0.85, P < 0.001, SE = 1.56)]. This study confirmed the potential use of delta N-15 to predict NUE in cows consuming different levels of rumen degradable protein.