Effect of supplemental protein on nitrogen recycling in beef cattle consuming low-quality forage

Four studies were conducted to measure urea kinetics and microbial incorporation of recycled urea N in steers fed low-quality forage. The first experiment evaluated experimental procedures. Supplemental protein was provided as soybean meal to 3 steers, and 2 steers received no supplemental protein. From this study it was determined that urine samples reach a plateau in isotopic enrichment after 48 h of 15N15N-urea infusion, whereas duodenal and bacteria samples plateau in isotopic enrichment after 72 and 48 h. Our methods were satisfactory for describing both urea kinetics and microbial incorporation of recycled urea N. Experiment 2 determined the effect of increasing degradable intake protein (DIP) on forage utilization, urea kinetics, and ruminal microbial incorporation of recycled N. Supplemental DIP increased forage and N intake (linear, P<0.01). Urea production and gut entry of urea increased linearly (P<0.01) with increasing DIP. Urea entry into the gut accounted for 96 to 99% of urea production, which increased with increasing DIP production, indicating the remarkable ability of cattle to conserve N through urea recycling. Across treatments, recycled urea N accounted for 28% of microbial N. Experiment 3 measured responses to undegradable intake protein (UIP). Provision of UIP increased forage and N intake (quadratic, P<0.02). Urea production and gut entry of urea increased linearly (P<0.01) with increasing UIP. The percentage of microbial N from recycled urea N increased from 31% when no UIP was supplemented to 58% at zenith. Experiment 4 evaluated the impact of level and frequency of DIP provision. Treatments were casein dosed ruminally at 61 or 183 mg N/kg BW daily or every third day and 549 mg of N/kg BW every third day. Intake of forage was not affected by frequency of supplementation but increased (linear, P=0.02) with increasing DIP. When 549 mg of N/kg BW was provided every third day, steers were more dependent on urea recycling than when the same amount was provided daily (183 mg of N/kg BW daily). These studies demonstrate the importance of urea recycling in meeting ruminal N requirements in cattle fed low-quality forage.