Lactation response to ruminally protected methionine and lysine at two amounts of ruminally available nitrogen

Sixty multiparous Holsteins were used in a 200-d continuous lactation trial designed to measure the response to a mixture of ruminally protected Met and Lys. Response to AA was determined at two dietary concentrations of degraded protein designed to provide 85 and 100% of NRC recommended amounts. This difference in degraded protein content was achieved by addition of urea. Diets without urea supported lactation as well as diets with added urea in early lactation. In midlactation, urea addition was detrimental to yields of milk and milk protein; addition of AA prevented this negative effect. In early lactation, AA addition raised milk protein concentration and yield by 1 g/kg of milk of 37 g/d and did not interact with urea. Increases in protein concentration in response to AA addition were similar in early and late lactation and corresponded to changes in the casein fraction of milk. These results support previous work showing the importance of adequate absorbable Lys and Met to maximize the protein content of milk. The results suggest that interactions may occur between ruminal ammonia production and AA supply.