RUMINALLY PROTECTED LYSINE AND METHIONINE FOR LACTATING DAIRY-COWS FED A DIET DESIGNED TO MEET REQUIREMENTS FOR MICROBIAL AND POSTRUMINAL PROTEIN

Dairy cows, 20 at each of two sites, were used to determine responses to ruminally protected Lys and Met in a full lactation study. Cows wer e fed corn silage twice daily for ad libitum intake and a concentrate four times daily in proportion to milk production. At Truro, cows were fed 2.7 kg/d of alfalfa and timothy hay DM at 0600 and at 1500 h. At Fredericton, cows were fed 2.7 kg of timothy silage DM at 0600 h and 2 .7 kg of alfalfa hay DM at 1500 h. Diets were designed to meet, but no t to exceed, recommendations for ruminally degradable CP and intestina lly digestible protein. Ten cows at each site were fed ruminally prote cted L-Lys . HCl (19 g/d) and DL-Met (6.5 g/d). Cows fed AA at each si te produced more milk, lactose, protein, and fat; milk protein and fat percentages were also higher. No time x treatment interactions occurr ed for any production parameter. In spite of similar production respon ses between sites, cows fed AA consumed more DM at Truro, but those at Fredericton did not. Thus, gross efficiency of utilization of dietary N for milk N was increased with AA at Fredericton but not at Truro. H owever, considering the increased intake of CP by cows fed AA at Truro , an event that would have been expected to depress efficiency of util ization of dietary N, the lack of difference at Truro between treatmen ts can be interpreted as an improvement, relative to expectations, bec ause of AA feeding. High producing dairy cows fed a diet that was adeq uate in CP responded to ruminally protected Lys and Met primarily with increased production of milk protein and fat throughout the full lact ation.