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

摘要

Three dietary CP levels (11.5, 13.0, and 14.5% of DM) and three levels of supplemental urea (100, 50, and 0% of supplemental CP from urea) were fed to determine performance, serum urea N (SUN), and N and P balance. Crossbred steers (n = 27; BW = 315 kg) were blocked by weight and individually fed the nine treatments in a completely randomized block design. A steam-flaked corn-based diet was fed, with supplemental CP supplied by either all urea, a 50:50 blend of urea and cottonseed meal (CSM), or all CSM. Steers were used in three nutrient balance collection periods (NBCP) at the beginning, middle, and end, of the finishing period. Venous jugular blood was obtained at the start and end of each NBCP. No CP level x CP source interactions (P 0.10) were observed. Steer DMI, ADG, and feed efficiency did not differ (P 0.05) among treatments. For each NBCP, urinary total N, urinary urea N (UUN), and SUN increased linearly (P 0.10) as CP level increased. For NBCP 1 and 3, fecal N output increased linearly (P 0.10) as the level of supplemental urea decreased. For NBCP 2 and 3, UUN decreased linearly (P 0.10) as urea level decreased. For NBCP 1, fecal and urine P excretion increased linearly (P 0.10), and P retained (% of intake) decreased linearly (P 0.10), as CP level increased. Phosphorus intake linearly increased (P 0.10) as urea level decreased for each NBCP. Fecal P excretion increased linearly (P 0.10) in all NBCP, and urinary P excretion in NBCP 1 and 2 increased linearly (P 0.10) as urea level decreased. Phosphorus retained (% of intake) decreased linearly (P 0.10) as urea level decreased for NBCP 3. Results suggest that as CP level increased, N retention decreased, and as level of supplemental CP supplied by urea decreased P balance decreased in steers. As days on feed increased, less N and P were retained, suggesting the potential to decrease N and P excretion by feeding less N and P as the finishing period progresses.