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 pro tected Lys from effects of ruminally protected Met on the performance of lactating dairy cows fed a ration calculated to be first-limiting i n intestinally delivered Lys and second-limiting in intestinally deliv ered Met. Thirty multiparous Holstein cows were examined in a 20-wk st udy that started on wk 5 postpartum Rations contained timothy silage, corn silage, barley, corn, corn gluten meal, and soybean meal. Treatme nts were 1) no supplemental amino acids, 2) 21 g/d of intestinally ava ilable Lys, and 3) 22 g/d of intestinally available Lys and 6 g/d of i ntestinally available Met. Post-experimental calculations suggested th at, in contrast to the objective, the unsupplemented ration was colimi ting 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 virtual ly identical performance of cows fed the unsupplemented ration and cow s fed the ration supplemented with ruminally protected Lys demonstrate d that dairy cows did not respond to enhanced intestinal supplies of L ys when Lys was not calculated to be the first-limiting nutrient. In c ontrast, for cows fed rations supplemented with both ruminally protect ed Lys and ruminally protected Met, the production of both milk protei n (40 g/d) and fat (40 g/d) was numerically increased to an extent tha t was consistent with earlier reported studies, although calculations did not indicate that performance was limited by intestinal supplies o f Lys or Met. This result, which may be disputed because of a lack of statistical significance, suggests that Met, apparently unlike Lys, ma y enhance the production of milk components beyond an enhancement expe cted because of its role as a limiting amino acid.