Level of urea in high grain diets: nutrient digestibility, microbial protein production, and rumen metabolism

Abstract

Four ruminally and duodenally fistulated steers (1228 lb) were used in a 4 × 4 Latin square design to evaluate the effects of dietary urea level on nutrient digestion, microbial protein production, and rumen metabolism of steers fed a rolled corn diet without urea or with .5, 1.0, or 1.5% urea (dry matter basis) and no other supplemental protein. Rumen digestibilities increased 33% for organic matter and 25% for starch with the first increment (.5%) of urea, but little or no improvement occurred with subsequent urea additions. Apparent rumen nitrogen digestibility decreased linearly, whereas total tract and true ruminal nitrogen digestibility increased linearly with increased urea. Duodenal nitrogen flow and microbial protein production were not affected by treatment. Rumen pH decreased and total volatile fatty acids increased as dietary level of urea increased. Molar proportions of propionate increased and butyrate decrease d linearly with the addition of urea, suggesting increased efficiency of rumen fermentation. Rumen NH3 increased 63% following 1.0% urea addition to the diet. Urea improved ruminal digestibility and increased efficiency of fermentation but did not increase metabolizable protein to the small intestine.