ARS | Publication request: Effects of Crude Protein Concentration and Degradability on Performance, Carcass Characteristics, and Serum Urea Nitrogen Concentration in Finishing Beef Steers.

Submitted to: Journal Of Animal Science
Publication Acceptance Date: May 20, 2004
Publication Date: August 6, 2004
Citation: Gleghorn, J.F., Elam, N.A., Galyean, M.L., Duff, G.C., Cole, N.A., Rivera, J.D. 2004. Effects Of Crude Protein Concentration And Degradability On Performance, Carcass Characteristics, And Serum Urea Nitrogen Concentration In Finishing Beef Steers. Journal Of Animal Science. 82:2705-2717.

Interpretive Summary: The quantity and quality of protein fed to finishing beef cattle not only affects weight gain and production efficiency, it also affects the amount of nitrogen (and possibly phosphorus) that is excreted to the environment. An increase in nitrogen excretion can potentially lead to an increase in ammonia losses to the atmosphere. An increase in manure P decrease the nitrogen:phosphorus ratio of manure and thus makes it more difficult to use the manure as a fertilizer for crops. In order to determine the optimal quantity and quality of protein to feed finishing beef cattle we conducted two initial studies to determine the effects on animal performance and carcass characteristics. The two cooperative experiments were conducted at Texas Tech University and New Mexico State University. A total of 585 steers were fed one of nine diets in a factorial arrangement. The steam-flaked corn based diets contained three crude protein concentrations (11.5, 13, or 14.5% of DM) and three sources of supplemental crude protein (N basis): 100% urea; a 50:50 blend of urea and cottonseed meal; or 100% cottonseed meal. Animal performance and carcass data were pooled across locations for statistical analysis. Highest daily gains and best feed efficiencies were in cattle fed 13% crude protein. Increasing the concentration of supplemental urea linearly increased daily gain and improved feed efficiency. Backfat thickness, carcass grade, and yield grade were not significantly affected by dietary protein. Results indicate that with steam flaked corn based diets increasing dietary crude protein concentrations from 11.5 to 13% will increase daily weight gain as will increasing the proportion of urea in the diet. Crude protein concentrations above 13% seemed detrimental to animal performance.

Technical Abstract: Two experiments were conducted at two locations to determine the effects of dietary CP concentration and source on performance, carcass characteristics, and serum urea nitrogen (SUN) concentrations of finishing beef steers. British x Continental steers were blocked by BW (357 plus/minus 28 and 305 plus/minus 25 kg initial BW; n = 360 and 225; four and five pens/treatment in Exp. 1 and 2, respectively). Steam-flaked corn-based diets were arranged in a 3 x 3 factorial with three CP concentrations (11.5, 13, or 14.5% of DM) and three sources of supplemental CP (N basis): 100% urea (U); 50:50 blend of urea and cottonseed meal (B); or 100% cottonseed meal (C). Performance and carcass data were pooled across locations and analyzed with mixed model procedures using pen as the experimental unit. No CP concentration x source interactions were observed (P > 0.10). Crude protein concentration affected ADG (P < 0.05) and carcass-adjusted ADG quadratically (P < 0.10). Increasing the concentration of supplemental urea linearly increased carcass-adjusted ADG and G:F (P < 0.05) and carcass-adjusted G:F (P < 0.001). Dry matter intake was not affected (P > 0.10) by either CP concentration or source. Hot carcass weight (HCW), LMA, and dressing percent tended (P < 0.06) to increase linearly with increasing urea concentration, whereas increasing CP concentration quadratically affected HCW (P < 0.05), with a maximum at 13% CP. Differences in backfat thickness, marbling score, carcass grade, and yield grade were negligible across treatments. Serum urea nitrogen concentrations were affected by CP concentration and source (P < 0.05) over time. At all times measured, SUN concentrations increased (P < 0.05) with increasing CP concentration, but effects of CP source were small and variable across time. Results indicate that increasing CP concentrations from 11.5 to 13% slightly increased ADG, whereas increasing the proportion of supplemental urea increased ADG, and G:F, and tended to increase HCW, LMA, and dressing percent. A CP concentration above 13% seemed detrimental to ADG and HCW. Serum urea N increased over time, with CP concentration having a greater effect than source.