Nitrogen and phosphorus utilization by beef cattle fed three dietary crude protein levels with three supplemental urea levels

Abstract

Three dietary CP levels (11.5, 13.0, and 14.5% of DM) and three supplemental urea levels (100, 50, and 0% of supplemental CP from urea) were used in a 3 x 3 factorial arrangement of treatments to determine performance, blood urea N (BUN), and N and P balance. Crossbred steers (n = 27; average BW = 315 kg) were blocked by BW and individually fed the dietary treatments in a completely randomized block design. A steam-flaked corn-based diet was fed, with upplemental CP supplied by either urea, a 50:50 blend (N basis) of urea and cottonseed meal (CSM), or CSM (100, 50, and 0% of supplemental CP from urea, respectively). Steers were used in three nutrient balance collection periods (NBCP) at the beginning, middle, and end, of the feeding period. Venous jugular blood was obtained at the start and end of each NBCP. No dietary CP level x supplemental CP source interactions (P > 0.10) were observed for any variables. Steer DMI, ADG, and gain efficiency did not differ (P > 0.10) among treatments. For each NBCP, urinary total N (g/d), urinary urea N (UUN), and BUN increased linearly (P < 0.10) as dietary CP level increased. For NBCP 1 and 3, fecal N output increased linearly (P < 0.10) as supplemental CP from CSM increased. For NBCP 2 and 3, UUN decreased linearly (P < 0.10) as supplemental urea level decreased. For NBCP 1, fecal and urinary P excretion (g/d) increased linearly (P < 0.10), and P retained (% of intake) decreased linearly (P < 0.10), as dietary CP level increased. Phosphoms intake increased linearly (P < 0.10) as supplemental CP supplied by CSM increased for each NBCP. Fecal P output increased linearly (P < 0.10) in all NBCP, and urinary P excretion in NBCP 1 and 2 increased linearly (P < O.IO) as supplemental urea level decreased. Phosphoms retained (% of intake) decreased linearly (P < 0.10) as supplemental CP from urea decreased for NBCP 3. Results suggest that as dietary CP level increased, N retention (% of intake) decreased, and as supplemental CP supplied by urea decreased, P balance decreased in feedlot steers. As days on feed increased, less (P < 0.10) N and P were retained, suggesting the potential to decrease N and P excretion by feeding less N and P as the feeding period progresses.