Invited review: Perspectives on nitrogen in dairy cattle nutrition

emission, and interactions between dietary N and nondietary intervention should be considered carefully when evaluating the potential of implementing mitigation strategies. In this review, we discuss temperate pasture-based systems as a special case because these systems often result in excess of dietary N supply relative to the cows' requirements. In pasture-based dairy systems, protein nutrition poses challenges related to the variability in chemical composition and the interlocked relationship between protein and energy-yielding substrates when grazed forage is the sole diet. Also, the dynamics of soluble OM need to be better characterized in high-moisture grazed forage diets to improve the ability of nutritional models to accurately predict microbial protein synthesis and AA supply in these diets. In models predicting milk protein yield responses, protein and energy should be integrated because responses to increased protein intake are closely related to an associated increase in energy intake. The potential to improve protein evaluation models, however, is rather limited before more accurate methods for determining ruminal protein degradability and intestinal digestibility of RUP are developed. Future research should be directed toward defining, and refining, EAA supply, from both microbial protein synthesis in the rumen and feed RUP, and requirements of high-producing dairy cows fed diets supplying MP and EAA below current requirements. Advancements in this area will facilitate reducing environmental N emissions from dairy cattle while minimizing potential negative effects on animal performance and health.