Complete Rations for Dairy Cattle. VIII. Effect of Percent and Source of Protein on Milk Yield and Ration Digestibility

Abstract

Two experiments evaluated 13.5 and 16.3% protein (dry basis) for effects on milk yield, milk composition, feed intake, and digestibility of organic matter of the ration. The first experiment used cottonseed meal, and the second experiment had cottonseed, soybean, and peanut meals to substitute for corn to vary protein. Addition of fungal cellulases to rations of Experiment 1 had no effect on milk yield, feed intake, or digestibility of organic matter but did lower ammonia of rumen, increase disappearance of organic matter from rumen, decrease disappearance of organic matter from lower tract, and interact with protein to increase milk fat percent in 13.5% protein diet. Feed intake and milk yields of cows were higher from 16.3% than 13.5% protein diets. In Experiment 2, 13.5% protein with soybean meal appeared equal to 16.3% with cottonseed meal. Digestibilities of organic matter of ration in Experiment 2 were affected by both percent and source of protein; 16.3% diets were digested more than 13.5% diets, and soybean and peanut meal diets more than cottonseed meal. Digestibility of organic matter was affected less by protein percent in Holsteins than Jerseys.Data from 13 experiments with soybean meal in complete rations and comparing two or more percents of supplementation were used to show responses of milk yield and feed intake to varied protein percent of ration (third degree polynomial). Marginal returns and feed costs to incremental increases in ration protein percent were generated. For one economic situation, optimum protein percent with soybean meal was 14.0% of total dry matter of the ration.