Influence of Dietary Protein Concentration on Milk Production by Dairy Cattle in Early Lactation

Abstract

In a 3 .times. 2 factorial experiment, 21 mature cows (3rd or later lactation) and 21 heifers in 1st lactation were fed complete rations containing 12.4, 15.1, or 17.7% crude protein in the dry matter. Rations contained in dry matter, 10% hay, 30% corn silage and a 60% mixture of high moisture corn, soybean meal and a mineral-vitamin premix. Average daily milk production of heifers was unaffected by protein in ration in periods 1 or 2 (5-6 or 12-13 wk, postpartum). For cows, milk production responded quadratically in period 1 and linearly in period 2. In period 2, response was linear for solids-corrected milk yield by cows, but not by heifers. Cows fed the 15.1% ration had greater prepartum s.c. fat measurements and lower dry matter intake (15.7 kg/day) during period 1 than cows fed 12.4 or 17.7% crude protein (18.0 and 19.9 kg/day). Increases in protein resulted in higher urea N and total protein concentrations in plasma, but rumen pH and total volatile fatty acids were unaffected. Apparent N digestibility increased linearly with protein. Although N balance was unaffected by protein concentration, conversion of N intake to milk N decreased linearly as protein intake increased.