Effect of Protein Source and Energy on Milk Composition in Midlactation Dairy Cows

Thirty multiparous Holsteins and 12 multiparous Jerseys (90 15 d in lactation) were assigned in a 3 x 2 factorial design. Diets containing O. 1, or 2 kg fish meal at 70 or 75% TDN were fed for 8 wk. Dry matter intake was greater on the high TDN diet Milk and FCM yields were not different. Percentage of milk fat, percentage of protein, and protein yield decreased with inclusion of fish meal in the diet. In Experiment 2, 30 primiparous Holsteins (106 24 d in lactation) were assigned one of two diets in an incomplete block design for two consecutive 32-<1 periods. Corn silage-based diets varied in CP and source as follows: 12% CP, soybean meal; 20% CP, soybean meal; 20% CP, fish meal; 20% CP, com gluten meal; 20% CPo gluten:soybean; and 20% CP, fish meal:soybean. Milk fat and milk lactose percentages and FCM yield (kg) were higher on corn gluten meal (3.13, 5.30,23.0) than on fish meal (2.11, 5.00. 20.5). Milk protein yield was affected by amount and source of protein. Rumen ammonia and plasma urea were greater on high (13.5, 16.2 mg/dl) than on low soybean meal (7.3, 6.4 mg/dl). Results indicat~protein source may affect milk composition in cows fed corn silage diets.