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Ruminant Nutrition and Forage Utilization
Asian-Australasian Journal of Animal Sciences 1998;11(6): 661-672.
https://doi.org/10.5713/ajas.1998.661    Published online December 1, 1998.
The Effects of Dietary Urea on Microbial Populations in the Rumen of Sheep
J. Kanjanapruthipong, R. A. Leng
Two experiments were conducted to examine the effects of a range of concentrations of ruminal fluid ammonia (NH3-N) on forage digestibility, microbial growth efficiency and the mix of microbial species. Urea was either continuously infused directly into the rumen of sheep fed 33.3 g/h of oaten chaff (Exp. I) or sprayed onto the oaten chaff (750 g/d) given once daily (Exp. 2). Concentrations of NH3-N increased with incremental addition of urea (p < 0.01). Volatile fatty acids (VFA) concentrations and 24 h in sacco organic matter digestibility in the rumen were higher when supplemental urea was given (p < 0.01). The (C2+C4) : C3 VFA ratio was lower (p < 0.05) when NH3-N was above 200mgN/l. The fungal sporangia appearing on oat leaf blades were significantly higher when urea was supplemented, indicating that NH3-N was a growth-limiting nutrient for fungi at levels of NH3-N below 30 mgN/l. The density of protozoa was highest when NH3-N concentrations were adjusted to 30 mgN/l for continuously fed (4.4횞105/ml) and to 168 mgN/l for once daily feeding (2.9횞105/ml). Thereafter increasing concentrations of NH3-N, were associated with a concomitant decline in protozoal densities. At the concentration of NH3-N above 200 mgN/l, the density of protozoa was similar to the density of protozoa in ruminal fluid of the control sheep (1.8횞105/ml). The efficiency of net microbial protein synthesis in the rumen calculated from purine excretion was 17-47% higher when the level of NH3-N was above 200mgN/l. The possibilities are that 1) there is less bacterial cell lysis in the rumen because of the concomitant decrease in the protozoal pool and/or 2) microbial growth per se in the rumen is more efficient with increasing NH3-N concentrations.
Keywords: Ammonia; Urea; Microbial Growth; Rumen; Purine Derivatives

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