Nutritional Value of Urea Versus Preformed Protein for Ruminants. II. Nitrogen Utilization by Dairy Cows Fed Corn Based Diets Containing Supplemental Nitrogen from Urea and/or Soybean Meal

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Abstract
Holstein cows (15) at various stages of lactation, fed corn-based diets that contained 9-14.5% crude protein, were used in total collection digestibility and N balance trials to investigate the efficiency of N utilization from urea and soybean meal. Digestibility coefficients for dry matter, organic matter, crude protein and N-free extract [NEE], and total digestible nutrient content of diets that contained 13-14.5% crude protein, were 3-8 percentage units greater than for diets that contained 9-12% crude protein. The concentration of ammonia-N in rumen fluid of cows fed diets that contained 9-12% crude protein was < 5 mg/100 ml and may have decreased digestibility and synthesis of microbial protein in the rumen. When the diet contained 11-12% crude protein, N supplied as urea or soybean meal was used with equal efficiency. More milk and milk protein were produced when soybean meal was fed to the cows. The increased production from feeding soybean meal resulted in N from soybean meal being utilized more efficiently for milk production than N supplied as urea when the diet contained 13-14.5% crude protein.