RESPONSE OF LACTATING COWS TO METHIONINE OR METHIONINE PLUS LYSINE ADDED TO HIGH-PROTEIN DIETS BASED ON ALFALFA AND HEATED SOYBEANS

Lactation diets based on wilted alfalfa silage and whole soybeans are common in the mid-western US. We examined the milk production response of multiparous Holstein cows to the addition of ruminally protected m ethionine at two percentages to a basal total mixed ration. An additio nal total mixed ration included both methionine and lysine supplementa tion. Sixteen Holstein cows in early lactation were used in a replicat ed 4 x 4 Latin square design with 21-d periods. Milk production, milk composition, and dry matter intake were determined for the last 5 d of each period. Milk production (41.5 kg/d), dry matter intake (25.9 kg/ d), and milli fat concentration (3.26%) were unaffected by the supplem entation of amino acids. The addition of methionine increased milk pro tein concentration and yield linearly. Each gram of methionine increas ed milk protein yield by 4 g, and milk protein concentration increased from 2.89 to 2.99% with the addition of 10.5 g/d of methionine. The p roportion of casein N in total milk N was unaffected by treatment. The addition of lysine did not elicit a response. Total mixed rations bas ed on alfalfa haylage, heated soybeans, and animal proteins were clear ly limited by their methionine content but were adequate in their lysi ne content.