The effects of degradable nitrogen level and slow release urea on nitrogen balance and urea kinetics in Holstein steers

The objective of this study was to compare nitrogen metabolism and urea kinetics between diets containing either rapidly degrading or slow degrading non-protein nitrogen (NPN) at varying levels of degradable intake protein (DIP). Treatments were slow release urea (Optigen®, Alltech, Inc.) fed at 1.01 and 1.14 and feed grade urea (UREA) fed at 0.89 and 1.00 of calculated DIP requirements. Eight Holstein steers (209  ± 15 kg) implanted with 28 mg estradiol + 200 mg trenbolone acetate (Synovex Plus, Fort Dodge Animal Health, Fort Dodge, IA) were used in a replicated 4 × 4 Latin square. Experimental periods were 27 days, with 19 day adaptation followed by 7 day of urine and fecal collection and 1 day of blood sampling. Continuous (78 h) intravenous infusion of 15N15N-urea allowed the estimation of systemic urea kinetics. Dry matter intake was not different between treatments (7.2 kg/day). Increasing DIP had a tendency to increase dry matter digestibility (DMD) for both Urea and Optigen®. Urea had higher DMD than Optigen®. Increasing DIP increased urinary N output for both UREA and Optigen®, and increased N-retention at 1.14 Optigen®. Increasing DIP increased urea-N entry rate (UER) and urinary urea-N excretion (UUE) for both Optigen® and UREA. Gastrointestinal entry of urea-N, urea-N lost to feces and urea-N apparently used for anabolism were not different between treatments. Plasma urea concentration was greater in higher DIP diets and higher for Urea than Optigen® at 1.00 DIP. Therefore increasing DIP level will increase N-excretion related to higher urea production and excretion in urine but may also increase diet digestibility. Most changes in N metabolism were driven by N intake.