Conventional Feed-Grade or Slow-Release Coated Urea as Sources of Dietary Nitrogen for Fattening Lambs

SIMPLE SUMMARY: Reducing the environmental impact and the use of plant foods in animal feeding are current challenges of ruminant production systems. The use of non-protein nitrogen, especially urea, allows reducing the use of protein supplements of vegetable origin, which contributes to reducing both the competition for nutritional resources between humans and other animal species and the water and carbon footprints. This article provides new knowledge, comparing feed-grade conventional urea and slow-release urea under the conditions of intensive fattening of lambs, characterized by use of compound feed with a high content of starch-rich feeds. Our results suggest that replacing conventional feed-grade urea with slow-release urea in the diet of Assaf fattening lambs does not improve the feed efficiency, metabolic profile, or carcass or meat characteristics, and increases the feeding costs. ABSTRACT: Twenty-two Assaf male lambs (29.2 ± 0.9 kg live weight and 89 ± 0.2 days of age), distributed in two experimental groups, were used to evaluate the use of either feed-grade conventional urea (Control diet; n = 11) or slow-release urea (SRU diet; n = 11) as sources of dietary nitrogen on animal performance, ruminal fermentation, blood acid-base status, plasmatic metabolic profile, and carcass and meat quality. Animals were housed individually and fed ad libitum. At the end of the fattening period (day 70), the animals were slaughtered to compare the fermentation patterns in ruminal digesta and to evaluate the carcass and meat characteristics. No statistically significant differences (p > 0.05) were observed between treatments in the dry matter intake, final live weight, average daily gain, and feed conversion rate. Regarding the ruminal fermentation parameters, the molar proportion of propionic acid was higher (p < 0.05) and that of butyric acid was lower (p < 0.05) with the SRU than with the Control diet. There were no significant differences (p > 0.05) between experimental treatments in the blood acid-base status and biochemical profile, except for the concentration of urea in plasma, which was significantly (p < 0. 05) greater in SRU than in Control lambs. No statistically significant differences were observed between treatments (p > 0.05) in the carcass and meat characteristics. In conclusion, the use of slow-release urea as a replacement for feed-grade conventional urea in the diet of Assaf fattening lambs, under the experimental conditions of this study, did not improve animal performance and increased the feeding costs.