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Effects of Dietary Crude Protein and Supplemental Urea Levels on Nitrogen and Phosphorus Utilization by Feedlot Cattle

J. T. Vasconcelos^*,, N. A. Cole, K. W. McBride^*,, A. Gueye^*, M. L. Galyean^*, C. R. Richardson^* and L. W. Greene

^* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409 , USDA-ARS Conservation and Production Research Laboratory, Bushland, TX 79012 Texas Agricultural Experiment Station, Amarillo 79016

jvasconcelos2{at}unl.edu

Abstract

Three dietary CP concentrations (11.5, 13.0, and 14.5% of DM) and 3 supplemental urea levels (100, 50, and 0% of supplemental N) were used in a completely randomized block design experiment conducted at 2 locations to determine N and P balance and serum urea N (SUN) concentrations of feedlot cattle. Crossbred steers (British and British x Continental; initial BW = 315.0 ± 3.2 kg at Location 1 [n = 27] and initial BW = 353.2 ± 8.4 kg at Location 2 [n = 27]) were used in 3 nutrient balance sampling periods (SP) at the beginning, middle, and end of the feeding period (154 d in Location 1 and 159 d in Location 2). Fecal N (g/d; P = 0.03), urinary N (g/d; P < 0.01), urinary urea-N (UUN; g/d; P < 0.01), apparent N absorption (g/d; P < 0.01), and SUN concentration (mg/dL; P < 0.01) increased linearly as dietary CP concentration increased. Nitrogen retention (g/d) was not affected (P = 0.61) by dietary CP concentration. Phosphorus intake (g/d; P = 0.02), fecal P (g/d; P = 0.04), and urinary P (g/d; P = 0.01) increased linearly as dietary CP increased, reflecting changes in diet composition with increasing CP concentrations. As dietary urea levels increased, urinary N (g/d; P = 0.04), UUN (g/d; P = 0.01), and apparent N absorption (g/d; P = 0.04) increased linearly, but P intake (g/d; P = 0.10) and urinary P (g/d; P = 0.02) decreased linearly. No interactions were observed between SP and dietary treatments for most variables. Evaluation of SP means, however, showed that as days on feed increased, fecal N (g/d; P = 0.01), urinary N (g/d; P < 0.01), UUN (g/d; P < 0.01), apparent absorption of N (g/d; P < 0.01), SUN (mg/dL; P < 0.01), and urinary P (g/d; P < 0.01) increased linearly; however, retained N (g/d) decreased linearly (P < 0.01) with increasing days on feed. These data suggest that changes in dietary CP and urea levels, as well as stage of the feeding period, markedly alter N and P utilization by feedlot cattle.

Key Words: beef cattle • crude protein • feedlot • nitrogen • phosphorus • urea