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

Abstract

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