Effect of commercial slow-release urea product on in vitro rumen fermentation and ruminal microbial community using RUSITEC technique

BACKGROUND: The objectives of this study were to determine the effect of commercial slow-release urea (SRU) on in vitro fermentation characteristics, nutrient digestibility, gas production, microbial protein synthesis and bacterial community using a rumen simulation technique (RUSITEC). The experiment was a completely randomized design with four treatments and four replications of each treatment. Treatments were: control diet (no SRU addition), control diet plus 0.28% SRU (U28), or plus 0.56% SRU (U56), and control diet that was modified substituting a part of soybean meal equivalent to 0.35% SRU (MU35; dry matter [DM] basis). The experiment consisted of 8 d of adaptation and 7 d of data and sample collection. Rumen inoculum was obtained from three ruminally fistulated Angus cows fed the same diet to the substrate incubated. RESULTS: Digestibility of DM, organic matter (OM), crude protein (CP), fibre and starch was not affected, but daily production of gas (P < 0.07) and methane (P < 0.05) was quadratically increased with increasing SRU supplementation. The increase of SRU addition did not affect fermentation pH and total volatile fatty acid (VFA) production, whereas linearly (P < 0.01) decreased proportion of propionate, and linearly (P < 0.01) increased acetate to propionate ratio and ammonia nitrogen (N) concentration. The microbial N efficiency was also linearly (P < 0.03) improved with increasing supplementation of SRU. In comparison with control diet, the dietary substitution of SRU for part of soybean meal increased (P < 0.05) the digestibility of DM, OM and CP and decreased (P < 0.02) the total gas production. The total VFA production and acetate to propionate ratio did not differ between control and MU35, whereas the proportion of butyrate was lower (P < 0.05) and that of branched-chain VFA was greater (P < 0.05) with MU35 than control diet. Total and liquid-associated microbial N production as well as ammonia N concentration were greater (P < 0.03) with MU35 than control diet. Observed operational taxonomic units (OTUs), Shannon diversity index, and beta diversity of the microbial community did not differ among treatments. Taxonomic analysis revealed no effect of adding SRU on the relative abundance of bacteria at the phylum level, while at the genus level, the beneficial impact of SRU on relative abundance of Rikenellaceae and Prevotellaceae in feed particle-associated bacteria, and the abundance of Roseburia in liquid associate bacteria was greater (P < 0.05) with MU35. CONCLUSIONS: Supplementation of a dairy cow diet with SRU showed potential of increase in ammonia N concentration and microbial protein production, and change fermentation pattern to more acetate production. Adding SRU in dairy cow diet also showed beneficial effect on improving digestibility of OM and fibre. The results suggest that SRU can partially substitute soybean meal in dairy cow diet to increase microbial protein production without impairing rumen fermentation.