Influence of Levels of Supplemental Urea on Characteristics of Digestion and Growth Performance of Feedlot Steers Fed a Fat-Supplemented High-Energy Finishing Diet

Abstract

Four Holstein steers (382kg) with cannulas in the rumen and proximal duodenum were used in a 4×4 Latin square experiment. Treatments consisted of 90% concentrate (steam-flaked cornbased) diet supplemented with either 5.5% soybean meal (SBM) or .8, 1.2, or 1.6% urea (dry matter basis). All diets contained 4% tallow. There were no treatment effects (P>.10) on ruminal pH and ammonia concentrations. Ruminal digestion of organic matter, acid detergent fiber, and starch tended to increase with supplemental urea level. There was a quadratic component (P<.10) to urea level effects on total tract dry matter and organic matter digestibility, being maximal with 1.2% supplemental urea. Total tract acid detergent fiber and starch digestion increased (P<.05) with urea level. Passage of nonammonia nitrogen, microbial nitrogen, and feed nitrogen were similar (P>.10) across treatments. Nonammonia nitrogen entering the small intestine/nitrogen intake decreased linearly (P<.05) with increasing level of urea supplementation. Thirty-two Holstein steers were used in a 4×4 Latin square experiment to evaluate treatment effects on feedlot growth performance. Experimental periods were 35 d in duration. There were no treatment effects (P>.10) on daily weight gain, dry matter intake, and diet net energy. Observed:expected net energy increased (P<.10) with increasing urea level. Urea can be efficiently utilized as the sole source of supplemental nitrogen in fat-supplemented highenergy finishing diets for feedlot cattle. Furthermore, feedlot cattle growth performance may be enhanced by supplementing diets with urea at levels beyond that necessary for maximal microbial growth.