Crude Protein for High Milk Production

In each of two 4 x 4 Latin squares, multiparous Holstein cows were fed ad libitum, blended, equicaloric, corn silageconcentrate diets containing 11.1 to 19.2 or 13.8 to 20.9% crude protein during four 36-day periods between 18 and 162 days postpartum.In trial 1, with low soluble nitrogen, yields of milk, protein, and nonfat solids increased as dietary protein increased from 11 to 16%; higher protein increased gross nitrogen balance rather than urinary excretion or mammary secretion.Mean daily milk, 46 to 159 days postpartum, was 34 kg when the diet contained 15.7% protein.Trial 2 was with higher protein, higher-producing cows, lower proportion (.51 versus .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 nitrogen.Between 13.7 and 19.2% protein in the ration, conversion of absorbed nitrogen to milk nitrogen 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 ration was adequate for high-producing cows during the first 23 wk of lactation.