Slow ammonia release from urea: rumen and metabolism studies

A new slow-release urea (SRU) made by coating prilled urea with a tung oil-linseed oil-talc-catalyst mixture was evaluated for ammonia-nitrogen release rate, animal acceptability, toxicity and effects on dry matter digestibility, diet intake and nitrogen retention. When added at a level equal to 1% urea in an 80% concentrate steer diet and fed twice daily, SRU gave a ruminal ammonia-nitrogen peak 1 hr postfeeding of 32 mg/dl compared to a peak from prilled urea of 53 mg/dl at 30 minutes. Bi-hourly feeding of prilled urea and SRU produced similar rumen ammonia-nitrogen levels and demonstrated that SRU was almost completely hydrolyzed in the rumen. Steers fasted for 26 hr and refed with supplements containing 10% urea from prilled urea had rumen ammonia levels of 120 ml/dl and showed muscle tremors 35 min after feeding. Rumen ammonia levels of steers fed equivalent urea from SRU remained below 35 ml/dl and exhibited no toxicity symptoms. Sheep fed ad libitum cottonseed hulls were offered a supplement containing 5% or 10% urea from urea or SRU once daily. Intake of SRU supplement was 7 and 17% greater, while cottonseed hull intakes were similar for sheep fed urea or SRU at the 5 and 10% levels. In a nitrogen balance trial, steers were fed ad libitum cottonseed hulls unsupplemented or supplemented with isonitrogenous amounts of SRU, prilled urea or soybean meal. Added nitrogen from all sources increased cottonseed hull intake. Steers fed SRU consumed more (P less than .05) cottonseed hulls than steers fed urea. Dry matter digestibility and nitrogen retention values tended to be highest for steers fed soybean meal supplement with little difference noted between prilled urea and SRU supplements.