Amino acid nutrition of dairy cows: invited review

Improving nitrogen (N) efficiency in dairy production systems is important due to environmental concerns and the economic cost of dietary protein. A substantial proportion of dietary N consumed by lactating dairy cows is not converted into milk protein and is instead excreted in urine and consequently manure. Advances in protein nutrition have shifted feeding strategies from crude protein (CP)-based approaches toward metabolizable protein and, more recently, amino acid (AA)-based systems designed to better match absorbed AA supply with animal needs. This review synthesizes current knowledge on AA nutrition in lactating dairy cows, with emphasis on the biological regulation of milk protein synthesis, identification of limiting AA, and opportunities to improve N utilization efficiency through more precise diet formulation. Absorbed AA originate primarily from microbial protein synthesized in the rumen and rumen-undegraded feed protein digested postruminally, and the relative contribution of these sources influences both the quantity and profile of AA available for metabolism. Milk protein synthesis is regulated not only by AA supply but also by energy availability, endocrine signaling, and tissue nutrient partitioning. Among individual AA, methionine, lysine, and histidine are most consistently associated with limitations to milk protein synthesis in dairy cows under common feeding conditions. Evidence indicates that balancing diets for these AA can increase milk protein yield, improve feed N conversion into milk N, and allows reductions in dietary CP without compromising production. However, practical implementation of AA-based feeding strategies remains constrained by limitations in predicting microbial protein synthesis and postruminal AA supply. Nutritional models provide useful tools for evaluating AA adequacy, but prediction errors and biological variability require that model outputs be interpreted alongside production responses and metabolic context. Overall, improved alignment of AA supply with metabolic demand represents a key strategy for enhancing productivity while reducing environmental N losses in dairy production systems.