The Effects of Dietary Urea on Microbial Populations in the Rumen of Sheep |
J. Kanjanapruthipong, R. A. Leng |
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Abstract |
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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