Protein Levels in Grass Silage-Based Total Mixed Rations for Dairy Cows in Midlactation

Total mixed rations containing three levels of crude protein (10, 13 and 16%) were fed ad libitum to a total of 48 cows in midlactation in two experiments. Rations were composed of 60% wilted timothy silage and 40% concentrates on a dry matter basis. Supplemental protein was supplied as soybean meal. Daily nitrogen intake per cow averaged 263, 377 and 510 g in exp. 1, and 305, 435 and 509 g in exp. 2 for the three rations, respectively. Cows fed rations containing 10% crude protein consumed less (P < 0.05) total dry matter and lost more (P < 0.01) body weight than cows fed rations with either 13 or 16% crude protein. Milk production was not affected by protein level in exp. 1 but in exp. 2, 10% crude protein decreased (P < 0.01) actual milk and 4% fat corrected milk yields. Milk fat and protein percentages tended to increase with increasing dietary protein level. Rumen acetate tended to decline while propionate increased (P < 0.05) at higher crude protein levels. Rumen ammonia nitrogen and plasma urea nitrogen levels were closely related to nitrogen intake.