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

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

ABSTRACT

In a 3 x 2 factorial experiment, 21 mature cows (third or later lactation) and 21 heifers in first 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 not affected by protein in ration in Periods 1 or 2 (5 to 6 or 12 to 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 subcutaneous 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 nitrogen and total protein concentrations in plasma, but rumen pH and total volatile fatty acids were not affected.

Apparent nitrogen digestibility increased linearly with protein. Although nitrogen balance was not affected by protein concentration, conversion of nitrogen intake to milk nitrogen decreased linearly as protein intake increased.

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