LACTATION RESPONSE TO RUMINALLY PROTECTED METHIONINE AND LYSINE AT 2 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 prote cted Met and Lys. Response to AA was determined at two dietary concent rations of degraded protein designed to provide 85 and 100% of NRC rec ommended amounts. This difference in degraded protein content was achi eved by addition of urea. Diets without urea supported lactation as we ll 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 and 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 an d corresponded to changes in the casein fraction of milk. These result s support previous work showing the importance of adequate absorbable Lys and Met to maximize the protein content of milk. The results sugge st that interactions may occur between ruminal ammonia production and AA supply.