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

• Effect of level and source of DIP on nitrogen metabolism in cattle were evaluated.
• Increased DIP intake increased diet digestibility and nitrogen excretion.
• Diet digestibility was greater for urea than Optigen at recommended DIP levels.
• Correcting a deficiency in DIP intake did not increase N retention for urea.
• Increasing DIP intake above requirements increased N retention for Optigen.

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.