Invited review: Perspectives on nitrogen in dairy cattle nutrition

Nitrogen, as a key element in proteins and AA, is critically important in animal nutrition and influences production and product quality, animal health and reproduction, and environmental emissions of nitrogenous compounds. Because of its intricate interactions with feed intake, dietary energy, and animal performance, manipulation of dietary N to reduce feed costs, improve milk N efficiency, and decrease environmental emissions is challenging and carries a risk of loss of production, if the reduction in dietary N results in inadequate supply of digestible AA. This work offers a review of the aspects of N metabolism and its effects on ruminal fermentation, microbial protein synthesis, feed intake, AA nutrition of the lactating dairy cow, and interactions with energy and discusses feeding of low-protein diets as an opportunity to improve milk N efficiency and mitigate environmental emissions from dairy cattle operations. The advantages of NAN versus ammonia N in terms of microbial fermentation in the rumen are known and have to be considered when increased levels of NPN are included in dairy rations. Research has also revealed that soluble NPN fractions can escape ruminal fermentation and supply N postruminally, emphasizing shortcomings of current techniques for studying ruminal protein degradability. Also, it is important to consider interactions between energy and protein in predicting responses to changes in dietary nutrient supply because production responses to supplementary protein are derived to a large extent from increased energy supply from improved diet (fiber) digestibility and increased feed intake. Low-protein diets are feasible for reducing N emissions to the environment as long as feed intake, microbial fermentation, and fiber degradability in the rumen are not negatively affected, or there is a shortage of key EAA. There is clear evidence that lowering dietary N input has a large impact on reducing urinary N excretion and ammonia, nitrate, and nitrous oxide emissions. However, synergies and trade-offs with energy metabolism, animal health, reproduction, animal performance, enteric CH₄ 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.