Crude Protein for High Milk Production

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Abstract
In each of two 4 .times. 4 Latin squares, multiparous Holstein cows were fed ad lib, blended, equicaloric, corn silage-concentrate diets containing 11.1-19.2 or 13.8-20.9% crude protein during four 36-day periods between 18 and 162 days postpartum. In trial 1, with low soluble N, yields of milk, protein, and nonfat solids increased as dietary protein increased from 11-16%; higher protein increased gross N balance rather than urinary excretion or mammary secretion. Mean daily milk, 46-159 days postpartum, was 34 kg when the diet contained 15.7% protein. Trial 2 was with higher protein, higher-producing cows, lower proportion (0.51 vs. 0.59) of concentrate, and higher nitrogen solubility (urea-treated silage) than in trial 1. Mean milk yields were high (40 kg) but not affected significantly by protein in the ration. With diets over 18% protein, cows increased water intake, urine volume and urine N. Between 13.7 and 19.2 protein in the ration, conversion of absorbed N to milk N in isonitrogenous diets was about equal between trials, suggesting that urea in silage was used efficiently. Effects on various rumen and blood metabolites are discussed. Under these conditions 14% protein in the ratio was adequate for high-producing cows during the first 23 wk of lactation.