PSX-B-6 - Effect of slow-release urea on gas production, fermentation characteristics, nutrient digestibility and microbial protein synthesis in an artificial rumen system

The objective of this study was to evaluate the effects of slow-release urea (SRU) on gas production, fermentation parameters, nutrient digestibility, and microbial protein synthesis using artificial rumen system. The experiment was a completely randomized design with four treatments and four replications of each treatment. The treatments were control diet (no SRU), control diet supplemented with 0.28% (U28), 0.56% SRU (U56) or control diet that was added by 1.5% more corn grain and substituted with 0.35% SRU for 1.85% soybean meal (U35-SBM). The diets were formulated to be isoenergetic (1.71 NEL Mcal/kg), but protein concentration was lower (15.48 vs. 16.24%) and starch concentration was higher (28.2 vs. 27.1%) with U35-SBM than other three treatments. Experimental period consisted of 8 d of adaptation and 7 d of sampling. Rumen inoculum was obtained from three ruminally fistulated Angus cows fed the same diet to the substrate incubated. Gas production (L/d) was lower (P = 0.02) with U35-SBM (1.51) than control (1.79) or U28 (1.92); however, methane production was not affected by treatments (average, 47 mg/d). Dry matter digestibility was greater (P < 0.01) with U35-SBM (78.5%) than other treatments (74.6%), which had no differences in DM digestibility. Total volatile fatty acid (VFA) production did not differ among treatments (average, 53.1 mmol/d), but increasing SRU supplementation linearly (P < 0.01) decreased molar proportion of propionate from 27.0 to 25.7% and linearly (P < 0.01) increased acetate to propionate ratio from 1.69 to 1.78. The ammonia nitrogen (N) linearly (P < 0.01) increased from 7.70 to 10.25 mmol/L with increasing SRU addition. Microbial protein synthesis was greater (P = 0.03) with SRU treatments (average, 83.9 mg N/d) than control (78.9 mg N/d). The present results demonstrated the benefits to add the SRU in dairy cow diet for improving ruminal digestibility and microbial protein synthesis.