EFFECTS OF SOURCE OF CARBOHYDRATE AND PROTEIN AND RUMEN-PROTECTED METHIONINE ON PERFORMANCE OF COWS

Multiparous Holstein cows were fed diets consisting of alfalfa silage, corn silage, and a concentrate mixture containing primarily ground sh elled corn or corn gluten feed; the diets were supplemented with 0 or 20 g/d of rumen-protected Met. The 183-d experimental period followed a 21-d covariate period beginning at calving. Data from early lactatio n (d 22 to 105) and the entire experiment were analyzed. Yields of mil k and milk crude protein were not affected by treatment. Corn gluten f eed increased the percentages of milk fat and total solids. The rumen- protected Met tended to increase both the crude protein and casein N c ontent of milk. Body weight, milk fat percentage, and yields of milk f at, 3.5% fat-corrected milk, and total solids were greater when ground shelled corn and 20 g/d of rumen-protected Met or corn gluten feed an d 0 g/d of rumen-protected Met were fed than when ground shelled corn and 0 g/d of rumen-protected Met or corn gl;ten feed and 20 g/d of rum en-protected Met were fed. A similar interaction for dry matter intake was significant only during d 22 to 204. Corn gluten feed increased p lasma concentrations of His, Ile, Leu, and Val. Rumen-protected Met in creased plasma concentrations of Met, decreased His, and tended to dec rease Arg, Lys, and Orn. These data suggest that the dietary source of carbohydrate and protein can modulate the response of cows to rumen-p rotected Met.