Effect of increasing dietary protein with constant lysine:methionine ratio on production and omasal flow of nonammonia nitrogen in lactating dairy cows

Eight lactating cows were fed 4 diets in which dietary crude protein (CP) was increased steps of approximately 2 percentage units from 11 to 17% DM by replacing high-moisture corn with soybean meal supplemented rumen-protected Met maintain a Lys:Met ratio 3:1 metabolizable protein. Trial design replicated × Latin square; experimental periods lasted 28 d, data and sample collection being performed during wk 3 each period. Digesta samples collected the rumen as well omasum measure metabolite concentrations ruminal outflow N fractions using infusion 15N-enriched ammonia quantify microbial nonammonia (NAN) nonmicrobial NAN. Data analyzed MIXED procedure SAS (SAS Institute Inc.). There linear increases yields milk true concentration urea N, decrease efficiency, increasing CP. Apparent total-tract digestibility linearly CP, but estimated not affected. other macronutrients influenced diet. Ruminal ammonia, total AA peptides, branched-chain VFA also The 15N enrichment liquid- particle-associated microbes declined CP due decreasing pool. Although no effect on NAN flow detected, 525 g/d at 11% 637 increase 406 482 Under conditions this study, when adding Met, improved driven RUP supply