Effect of dietary fat in isocaloric rations on the nitrogen content of milk from Holstein cows.

Effects of dietary fat on milk composition, particularly milk N, were evaluated using 12 lactating Holstein cows in a replicated 4 X 4 Latin-square design. Four complete mixed diets were formulated to contain either 0 or 3.5% added fat (grease) and either 1.6 or 1.7 Mcal NE1 (0, 1.6; 0, 1.7; 3.5, 1.6; and 3.5, 1.7). Diets were fed for ad libitum intake, and periods were 21 d. Adding fat to the diet depressed total milk N, casein N, and whey protein N concentration compared with no added fat at both energy densities. The proportion of total N in the casein fraction was not affected by dietary fat, whereas the proportion of N associated with the NPN fraction was increased with diets containing added fat. The high concentration diet (0, 1.7) contained the highest content of milk N and casein N. Added dietary fat increased the proportion of long-chain fatty acids (C18 and C18:1) in milk fat and decreased the proportion of short-chain fatty acids (C8 to C16). Daily milk yield was higher for the high energy fat diet (3.5, 1.7) than for the other three diets, which were similar. Blood concentrations of triglycerides, nonesterified fatty acids, and cholesterol were increased by diets containing added fat.