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

Abstract

The objective of this study was to investigate the relationship between nitrogen isotopic fractionation (δ¹⁵N) 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 δ¹⁵N, 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 δ¹⁵N - feed δ¹⁵N and NUE: [NUE (%) = 58.9 - 10.17 × milk δ¹⁵N - feed δ¹⁵N (‰) (r² = 0.83, P < 0.001, SE = 1.67)] and between plasma δ¹⁵N - feed δ¹⁵N and NUE: [NUE (%) = 52.4 - 8.61 × plasma δ¹⁵N - feed δ¹⁵N (‰) (r² = 0.85, P < 0.001, SE = 1.56)]. This study confirmed the potential use of δ¹⁵N to predict NUE in cows consuming different levels of rumen degradable protein.