Nitrogen and phosphorus balance in cattle fed different degradable intake protein/undegradable intake protein ratios and different levels of dietary protein

Twenty-seven crossbred steers (average BW = 353.2 ± 8.4 kg) were used in a metabolism trial with three collection periods (beginning at approximately 35, 95, and 155 d on feed) to evaluate the effects of dietary crude protein (CP) source and concentration on nitrogen and phosphorus balance by steers. Steers were allotted to one of three blocks corresponding to a heavy, intermediate, and light weight group. Treatments were arranged in a factorial arrangement and consisted of three dietary CP concentrations (11.5, 13.0, and 14.5% of dietary dry matter) and three supplemental urea:cottonseed meal (CSM) ratios (100:0, 50:50, and 0:100 of supplemental N). During each nutrient collection period steers were housed in individual metabolism stalls and urine and feces excreted were collected and frozen. Collection periods consisted of a 2- to 5-d adaptation period followed by a 5-d collection period. In the first collection period, dietary CP concentration and source had a quadratic effect (P = 0.05 and P =0.03, respectively) on feed:gain, reaching minimums with the 14.5% CP and the 0:100 treatments, respectively. Treatments did not affect (P > 0.10) digestibility of DM, OM, or NDF Serum urea nitrogen as steers entered the stalls (SUN-in) increased linearly (P = 0.001) with increasing CP concentration. Urine urea nitrogen (UUN; mg/dl) increased linearly (P < 0.05) with increasing CP concentration, and steers in the 50:50 treatment tended (P = 0.10) to have higher UUN than steers in the 0:100 treatment. Total N excretion increased linearly (P = 0.002) with increasing CP concentration. Nitrogen absorbed (g/d) and N retained (g/d) linearly increased (P < 0.0001 and P = 0.01, respectively) with increasing CP concentration. Phosphorus intake linearly (P = 0.03) increased as dietary CP concentration increased. In the second collection period, daily gain tended (P = 0.06) to increase linearly with increasing CP concentration; while daily gain decreased linearly (P = 0.04) as the urea:CSM ratio decreased. Serum urea nitrogen-in and SUN as steers exited the stalls (SUN-out) increased linearly (P = 0.03 and P = 0.009, respectively) when dietary CP increased from 11.5 to 14.5%. Concentration of SUN-out tended (P = 0.08) to respond quadratically to decreasing ratio of urea:CSM, with steers in the 0:100 treatment having lowest SUN-in concentrations. Urine urea N linearly (P < 0.0001) increased with increasing CP Total N excretion linearly increased (P < 0.0001) with increasing CP concentration. Nitrogen absorbed (g/d) and N retained linearly increased (P < 0.0001 and P = 0.001, respectively) when CP increased from 11.5 to 14.5%. Nitrogen retained (% of absorbed) tended (P = 0.07) to respond quadratically to increased dietary CP concentration, with steers in the 11.5% CP treatment retaining the most N. Daily P intake increased linearly (P < 0.0001) with increasing dietary CP concentration. In the third collection period, daily gain and feed:gain linearly improved (P = 0.02) as CP concentration increased. Organic matter digestibility tended (P = 0.07) to linearly decrease with increasing dietary CP level. Serum urea nitrogen-in and SUN-out linearly increased (P = 0.005 and P = 0.003, respectively) with increasing CP concentration. Urine output increased linearly (P = 0.009) when CP level increased from 11.5 to 14.5%. Increasing CP concentration produced a linear increase (P = 0.04) in UUN (mg/dL). Urine urea N (% of UN) decreased linearly (P = 0.007) with increasing CP Dry matter digestibility decreased linearly (P < 0.05) with increasing dietary CP Fecal N excretion increased linearly (P < 0.0001) with increasing CP Nitrogen absorbed (g/d) decreased linearly (P = 0.03) with decreasing urea:CSM ratio. Nitrogen absorbed (% of intake) increased quadratically (P = 0.05) with decreasing urea:CSM ratio. Nitrogen retained (% of absorbed) decreased linearly (P = 0.009) as CP in the diet increased from 11.5 to 13%. As days on feed increased, total N excretion (P < 0.02), urinary urea N (P < 0.0001), and urinary P (P = 0.02) increased as well. Based on our observations, feeding growing steers diets containing 11.5 to 13.0% CP and supplemented with higher proportions of degradable protein may potentially optimize N utilization and potentially reduces N losses to the environment without hindering production efficiency. Also, the amount and degradability of dietary protein affect urea metabolism by feedlot steers, as evidenced by changes in serum and urinary urea nitrogen concentrations.