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. 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. 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.

中文翻译：

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使用 RUSITEC 技术研究商业缓释尿素产品对体外瘤胃发酵和瘤胃微生物群落的影响

本研究的目的是使用瘤胃模拟技术 (RUSITEC) 确定商业缓释尿素 (SRU) 对体外发酵特性、养分消化率、产气、微生物蛋白质合成和细菌群落的影响。该实验是一个完全随机的设计，有四个处理和每个处理四个重复。处理是：对照饮食（不添加 SRU）、对照饮食加 0.28% SRU (U28) 或加 0.56% SRU (U56)，以及用相当于 0.35% SRU 的部分豆粕进行改良的对照饮食 (MU35;干物质 [DM] 基础）。实验包括 8 天的适应和 7 天的数据和样本收集。瘤胃接种物是从三头瘤胃造瘘的安格斯奶牛获得的，这些奶牛用相同的饮食喂养孵化的底物。DM的消化率，有机质（OM）、粗蛋白（CP）、纤维和淀粉不受影响，但随着SRU补充量的增加，日产量（P < 0.07）和甲烷（P < 0.05）呈二次方增加。SRU添加量的增加不影响发酵pH和总挥发性脂肪酸（VFA）的产生，而线性（P < 0.01）降低丙酸盐的比例，线性（P < 0.01）增加乙酸与丙酸盐的比率和氨氮（N）专注。随着 SRU 补充量的增加，微生物 N 效率也呈线性（P < 0.03）提高。与对照日粮相比，SRU 替代部分豆粕提高了（P < 0.05） DM、OM 和 CP 的消化率，降低了（P < 0.02）总产气量。对照和MU35的总VFA产量和乙酸与丙酸的比例没有差异，而MU35的丁酸盐比例较低（P <0.05），支链VFA的比例高于对照饮食（P <0.05）。MU35 的总和与液体相关的微生物 N 产量以及氨氮浓度高于对照饮食（P < 0.03）。观察到的操作分类单位 (OTU)、香农多样性指数和微生物群落的 β 多样性在处理之间没有差异。分类学分析显示，添加 SRU 对门水平细菌的相对丰度没有影响，而在属水平，SRU 对饲料颗粒相关细菌中 Rikenellaceae 和 Prevotellaceae 的相对丰度的有益影响，MU35在液体伴生菌中Roseburia的丰度更高（P < 0.05）。在奶牛日粮中添加 SRU 显示出增加氨氮浓度和微生物蛋白质产量的潜力，并将发酵模式改变为更多的醋酸盐产量。在奶牛日粮中添加 SRU 对提高 OM 和纤维的消化率也显示出有益效果。结果表明，SRU 可以部分替代奶牛日粮中的豆粕，在不影响瘤胃发酵的情况下增加微生物蛋白产量。在奶牛日粮中添加 SRU 对提高 OM 和纤维的消化率也显示出有益效果。结果表明，SRU 可以部分替代奶牛日粮中的豆粕，在不影响瘤胃发酵的情况下增加微生物蛋白产量。在奶牛日粮中添加 SRU 对提高 OM 和纤维的消化率也显示出有益效果。结果表明，SRU 可以部分替代奶牛日粮中的豆粕，在不影响瘤胃发酵的情况下增加微生物蛋白产量。