THE EFFECT OF RUMINAL BYPASS LYSINE AND METHIONINE ON MILK-YIELD AND COMPOSITION OF LACTATING COWS

Fifty-six multiparous Holstein cows were assigned at 3 wk prepartum to rations based on grass silage with 1) corn distillers grains to provi de 86 and 90% of estimated required metabolizable Lys and Met, respect ively; 2) a blend of blood meal, fish meal, and meat and bone meal as amino acid (AA) sources to provide 112 and 103% of required metaboliza ble Lys and Met, respectively; 3) ruminally protected Lys and Met adde d as a top-dressing to ration 1 to provide 27 g/d of Lys and 8 g/d of Met as available AA at the duodenum postpartum; and 4) ruminally prote cted AA for 8 wk postpartum as a top-dressing to ration 1 to provide 4 0 g/d of Lys and 13 g/d of Met as available AA at the duodenum. Cows f ed rations 3 and 4 were offered 13.5 g/d of duodenally available Lys a nd 4 g/d of duodenally available Met for 3 wk prepartum. The total len gth of the study was 43 wk. Cows fed ration 4 consumed 3 to 4 kg more dry matter than did cows fed the other three rations, and milk yield a nd the percentage of milk protein and fat were significantly increased during the first 8 wk of lactation. In early lactation, cows fed rati on 3 had a greater milk fat percentage but similar dry matter intake, protein percentage, and yield of 4% fat-corrected milk compared with c ows fed ration 2. The concentrations of blood serum glutamic oxaloacet ic transaminase, serum glutamic pyruvic transaminase, triglyceride, an d nonesterified fatty acids were lower for cows fed ration 4 during th e first 8 wk of lactation than they were for cows fed the other three rations. The mammary arteriovenous difference of whole blood AA indica ted that Met along with His and Arg may be the most limiting AA for mi lk yield.