Effects of urea supplementation on ruminal fermentation characteristics, nutrient intake, digestibility, and performance in sheep: A meta-analysis

BACKGROUND AND AIM: As a non-protein nitrogen source, urea is a popular, low cost, and easily obtained protein supplement. The objective of the present study was to perform a meta-analysis of the effects of urea supplementation on rumen fermentation and sheep performance. MATERIALS AND METHODS: A total of 32 experiments from 21 articles were compiled into a dataset. The levels of dietary urea varied from 0 to 31 g/kg of dry matter (DM). Parameters observed were rumen fermentation product, nutrient intake, nutrient digestibility, and sheep performance. This dataset was analyzed using a mixed model methodology, with urea supplementation levels as fixed effects and the different experiments as random effects. RESULTS: Increasing levels of urea were associated with increases (p=0.008) in rumen pH, butyrate (C(4)) production, and ammonia (NH(3)–N) concentration. Urea supplementation had minor effects on total volatile fatty acids (p=0.242), total protozoa (p=0.429), and the microbial N supply (p=0.619), but tended to increase methane production (CH(4); p<0.001). Supplementation of urea increased the intake of dry matter (DM; p=0.004) and crude protein (CP; p=0.001). Digestibility parameters, such as DM digestibility (DMD) and CP digestibility (CPD), also increased (p<0.01) as a result of urea supplementation. Retained N (p=0.042) and N intake (p<0.001) were higher with increasing levels of urea supplementation. In terms of animal performance, supplementation of urea increased average daily gain (ADG; p=0.024), but decreased the hot carcass weight percentage (p=0.017). CONCLUSION: This meta-analysis reports the positive effects of urea supplementation on rumen fermentation products (i.e., pH, C(4), and NH(3)–N), intake (DM, CP, and N), digestibility (DMD and CPD), and ADG in sheep.