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, P0.01). Urea production and gut entry of urea increased linearly (P0.01) 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, P0.02). Urea production and gut entry of urea increased linearly (P0.01) 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.
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机译：进行了四项研究，以测量低质饲料饲喂的公牛中尿素动力学和回收尿素氮的微生物掺入。第一个实验评估了实验程序。补充蛋白作为豆粕提供给3头牛，其中2头没有补充蛋白。从这项研究中可以确定，尿液样本在15N15N-尿素输注48小时后达到了同位素富集的稳定水平，而十二指肠和细菌样本在72和48小时后达到了同位素富集的稳定水平。我们的方法对于描述尿素动力学和回收尿素N的微生物掺入均令人满意。实验2确定了可降解摄入蛋白（DIP）增加对饲草利用率，尿素动力学和瘤胃微生物掺入回收N的影响。补充DIP增加了草料和N摄入（线性，P <0.01）。随着DIP的增加，尿素产量和尿素的肠道进入量线性增加（P <0.01）。尿素进入肠道占尿素产量的96％至99％，随着DIP产量的增加而增加，这表明牛具有通过尿素回收来保存氮的显着能力。在所有处理中，回收的尿素氮占微生物氮的28％。实验3测量了对不可降解摄入蛋白（UIP）的反应。提供UIP可增加草料和氮的摄入量（二次方，P <0.02）。尿素产量和尿素肠道进入量随UIP的增加呈线性增加（P <0.01）。回收尿素氮中微生物氮的百分比从最高时未添加UIP时的31％增加到58％。实验4评估了DIP提供水平和频率的影响。酪蛋白以瘤胃剂量每天或每三天瘤胃剂量为61或183 mg N / kg BW，每三天为549 mg N / kg BW。饲草的摄入量不受补充频率的影响，但随着DIP的增加而增加（线性，P = 0.02）。每三天提供549 mg N / kg BW时，与每天提供相同量（每天183 mg N / kg BW）相比，ste牛皮对尿素回收的依赖性更大。这些研究表明，尿素回收对于满足低质饲草牛瘤胃中氮素的需求非常重要。
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