Ruminally Protected Lysine or Lysine and Methionine for Lactating Dairy Cows Fed a Ration Designed to Meet Requirements for Microbial and Postruminal Protein

The purpose of this study was to separate the effects of ruminally protected Lys from effects of ruminally protected Met on the performance of lactating dairy cows fed a ration calculated to be first-limiting in intestinally delivered Lys and second-limiting in intestinally delivered Met. Thirty multiparous Holstein cows were examined in a 20-wk study that started on wk 5 postpartum. Rations contained timothy silage, corn silage, barley, corn, corn gluten meal, and soybean meal. Treatments were 1) no supplemental amino acids, 2) 21 g/d of intestinally available Lys, and 3) 22 g/d of intestinally available Lys and 6 g/d of intestinally available Met. Post-experimental calculations suggested that, in contrast to the objective, the unsupplemented ration was colimiting in intestinally available His (0.96 of requirement), followed by Lys (1.00), digestible ruminally undegraded protein (1.01), Ile (1.03), Arg (1.04), Val (1.10), and Met (1.14). In this context, the virtually identical performance of cows fed the unsupplemented ration and cows fed the ration supplemented with ruminally protected Lys demonstrated that dairy cows did not respond to enhanced intestinal supplies of Lys when Lys was not calculated to be the first-limiting nutrient. In contrast, for cows fed rations supplemented with both ruminally protected Lys and ruminally protected Met, the production of both milk protein (40 g/d) and fat (40 g/d) was numerically increased to an extent that was consistent with earlier reported studies, although calculations did not indicate that performance was limited by intestinal supplies of Lys or Met. This result, which may be disputed because of a lack of statistical significance, suggests that Met, apparently unlike Lys, may enhance the production of milk components beyond an enhancement expected because of its role as a limiting amino acid.