Response of cows fed a low crude protein diet to ruminally protected methionine and lysine

Ten cows (mean = 128 DIM) were utilized in a replicated 5 x 5 Latin square design and fed diets formulated to supply adequate (18% CP) or inadequate (14% CP) Met and Lys. A prototype supplement of ruminally protected Met and Lys was added to the 14% CP diet to provide 0, 50, 100, and 150% of the predicted deficiency of Met and Lys. The DMI; yields of milk, 3.5% FCM, total N, protein N, and whey N; plasma concentrations of Arg, Cit, His, Ile, Leu, Orn, Phe, Pro, Tyr, Val, and urea N were greater for cows fed the 18% CP diet. Supplementing ruminally protected Met and Lys to the 14% CP diet did not affect DMI or yields of milk, 3.5% FCM, milk CP, and milk SNF. Milk fat yield was affected quadratically because it was greater when 0 or 150% of the deficiency of Met and Lys was supplied. Percentages of CP and casein N in milk increased linearly as cows were fed increasing amounts of ruminally protected Met and Lys. Plasma concentrations of urea N, Met, and Lys increased when ruminally protected Met and Lys were fed; however, other nutrients probably were limiting for synthesis of milk and milk protein when cows were fed a 14% CP diet because yields of each were not increased by ruminally protected Met and Lys.