ARS | Publication request: Effects of dietary crude protein and supplemental urea concentrations on nitrogen and phosphorus utilization by feedlot cattle

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 6, 2008
Publication Date: February 1, 2009
Repository URL: http://jas.fass.org/cgi/content/full/87/3/1174
Citation: Vasconcelos, J.T., McBride, K., Gueye, A., Galyean, M., Cole, N.A., Richardson, C., Greene, L. 2009. Effects of dietary crude protein and supplemental urea concentrations on nitrogen and phosphorus utilization by feedlot cattle. Journal of Animal Science. 87:1174-1183.

Interpretive Summary: The concentration and form of protein in diets for feedlot cattle can affect animal performance and effects on the environment – such as ammonia emissions. Most studies on the protein requirements of feedlot cattle have been performed with diets based on dry–rolled corn: few studies are available using steam-flaked corn-based diets. These studies were conducted to determine the effects of dietary protein concentration (11.5, 13.0, and 14.5% of diet dry matter) and supplemental protein form (100, 50, and 0% of supplemental N from urea) on nitrogen and phosphorus metabolism of feedlot cattle at 3 times during the feeding period. Crossbred steers (initial BW = 335.0 kg [n = 54]) were used in 3 nutrient balance collection periods at the beginning, middle, and end of the feeding period (157 d). Crude protein concentration, supplemental crude protein source, and days on feed affected nitrogen and phosphorus metabolism. These results suggest that as days on feed increased, N retention decreased. Increasing supplemental CP supplied by urea decreased P excretion due to decreased P concentration in diets with higher concentrations of urea; however, potential ammonia losses were increased. These results indicate that dietary protein concentration and source can affect nutrient metabolism, potential ammonia losses, and potential phosphorus losses.

Technical Abstract: Three dietary crude protein (CP) levels (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 blood urea nitrogen (BUN) of feedlot cattle. Crossbred steers (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 collection periods (NBCP) at the beginning, middle, and end of the feeding period (154 d in Location 1 and 159 d in Location 2). Crude protein level x CP source interactions were observed for urine output (P < 0.01), urinary N (P < 0.01), urinary urea nitrogen (UUN; P = 0.04), retained N (P = 0.05), fecal (P = 0.06) and urinary P (P < 0.01), and P absorption (P = 0.03). Blood urea N (P < 0.01), fecal output (P = 0.02), fecal (P <0.01), and urinary N (P < 0.01), UUN (P <0.01), and N apparent absorption (P <0.01) increased linearly as dietary CP level increased. Nitrogen retained as a percentage of intake (P = 0.02) and as a percentage of N absorbed (P = 0.02) decreased linearly as dietary CP level increased. Phosphorus intake (P <0.01), fecal P (P < 0.01), and urinary P (P < 0.01) also increased linearly as dietary CP level increased, reflecting changes in diet composition with increasing CP. As a percentage of intake, apparent absorption of P (P = 0.03) and P retained (P < 0.01) decreased linearly as dietary CP level increased. Fecal output (P = 0.07), DMI (P = 0.08), urine N (P < 0.01), UUN (P <0.01), N apparent absorption (P <0.01), and N absorbed (percentage of intake; P < 0.01) increased linearly as supplemental CP supplied by urea increased, but fecal N (P < 0.01), P intake (P <0.01), urine P (P < 0.01), and P apparent absorption (P = 0.07) decreased linearly with increasing urea. Data suggest that as days on feed and dietary CP concentration increased, N retention decreased. Increasing supplemental CP supplied by urea decreased P excretion, reflecting decreased P concentration in diets with higher concentrations of urea.