Complete Rations for Dairy Cattle. IV. Comparison of Supplemental Nitrogen Sources by Metabolizable Protein Concept

Four types of 3 x 3 Latin squares differing in nonprotein nitrogen source and protein percent were used with 4-wk periods and 36 Holstein cows grouped in trios. Basal rations (9.0 to 10.6% crude protein) high in citrus pulp and pelleted cane bagasse, were equalized in estimated metabolizable protein, ignoring urea fermentation potential, by adding raw urea, extruded corn-urea, or soybean meal at three percents. Corresponding rations of higher protein (12.1 to 13.4%)contained a soybean meal increment partially replacing corn. Mean daily feed intake was lower for basal than soy-increment rations (18.0 versus 19.3 kg) as were milk and solids-corrected milk yields (16.0 to 18.3 kg and 12.8 to 14.8 kg). All-soy controls were superior in milk production by 1.35 and .92 kg. Liveweight changes and milk composition were little affected by treatments. Milk fat and protein percents averaged 2.71 and 2.92 overall. Molar porportions of rumen acetate in Period 3 were relatively low, but not acetate/propionate ratios. Increased rumen ammonia concentrations resulted from higher protein, higher nonprotein nitrogen, and urea exceeded extruded corn-urea exceeded soybean meal.