Agriculture and Agri-Food Canada, Dairy and Swine Research and Development Centre, Sherbrooke, Québec, Canada J1M 1Z3.
J Dairy Sci. 2010 Feb;93(2):693-700. doi: 10.3168/jds.2009-2399.
Insufficient readily fermentable energy combined with extensive degradation of proteins in alfalfa (Medicago sativa L.) may result in poor forage N utilization by ruminants. Using the inherent genetic variability and differences between harvests, our objective was to compare the effect of contrasting concentrations of nonstructural carbohydrates (NSC) in alfalfa on rumen fermentation and microbial protein synthesis. Individual genotypes of the alfalfa cultivar AC Caribou grown near Québec City, Québec, Canada, were harvested at the vegetative and early flowering stages, dried at 55 degrees C, ground, and analyzed for soluble carbohydrates (fructose + sucrose + glucose + pinitol) and starch. Approximately 20 genotypes having, respectively, the highest and lowest NSC concentrations were pooled to constitute 2 contrasted 1-kg forage samples. Samples of high- (17.9% DM) and low- (7.4% DM) NSC alfalfa were respectively allocated to separate dual-flow fermenters in a completely randomized design with 3 replications. Rumen inoculum was obtained from 4 ruminally fistulated cows in early lactation that were fed a TMR with a 50:50 forage to concentrate ratio. A 10-d incubation period was used, with the first 6 d serving as an adaptation period followed by 4 d of sampling with solid and liquid dilution rates in the fermenters set at approximately 2.0 and 4.3%/h, respectively. High versus low NSC concentration in alfalfa significantly enhanced the apparent digestibility of OM (59.1% for high-NSC alfalfa vs. 54.4% for low-NSC alfalfa) and DM (60.0 vs. 54.3%) and the true digestibility of DM (74.1 vs. 64.7%). Increasing NSC concentration in alfalfa (high vs. low) significantly decreased ruminal pH (6.85 vs. 7.08) and NH(3)-N concentration (26.0 vs. 33.6 mg/dL) and increased total VFA concentration (94.9 vs. 83.0mM). Molar proportions of acetate, isobutyrate, and isovalerate significantly decreased, whereas molar proportions of propionate and butyrate significantly increased with high-NSC alfalfa, resulting in a more glucogenic fermentation. More importantly, microbial N flow (263 vs. 230 mg/d) and bacterial N efficiency (41.1 vs. 29.6% of available N), measured using (15)N as a microbial marker, both significantly increased with the high-NSC alfalfa. These results indicate that increasing the concentration of NSC in alfalfa promotes a glucogenic fermentation and enhances microbial N synthesis in the rumen.
饲用苜蓿（Medicago sativa L.）中可发酵能量不足，且蛋白质大量降解，这可能会导致反刍动物对饲草氮的利用率降低。本研究利用苜蓿品种间固有的遗传变异性和不同收获期的差异，旨在比较饲用苜蓿中不同非结构性碳水化合物（NSC）浓度对瘤胃发酵和微生物蛋白合成的影响。在加拿大魁北克市附近种植的紫花苜蓿（AC Caribou）的个体基因型，分别在营养生长期和初花期收获，在 55°C 下干燥、粉碎并分析可溶性碳水化合物（果糖+蔗糖+葡萄糖+肌醇）和淀粉。具有最高和最低 NSC 浓度的约 20 个基因型分别混合组成 2 个具有对比浓度 NSC 的 1kg 饲草样本。高 NSC（17.9% DM）和低 NSC（7.4% DM）苜蓿样本分别分配到完全随机设计的 2 个独立的双流通发酵器中，每个处理重复 3 次。瘤胃液接种物取自 4 头处于泌乳早期、采用 50:50 粗饲料：精饲料比的 TMR 饲养的奶牛。使用 10d 的孵育期，前 6d 为适应期，然后进行 4d 的采样，发酵器中固体和液体稀释率分别设定为约 2.0%/h 和 4.3%/h。苜蓿中 NSC 浓度的高低显著提高了有机物（高 NSC 苜蓿为 59.1%，低 NSC 苜蓿为 54.4%）和干物质（高 NSC 苜蓿为 60.0%，低 NSC 苜蓿为 54.3%）的表观消化率，以及干物质的真消化率（高 NSC 苜蓿为 74.1%，低 NSC 苜蓿为 64.7%）。苜蓿中 NSC 浓度的增加（高 vs. 低）显著降低了瘤胃 pH 值（6.85 vs. 7.08）和 NH3-N 浓度（26.0 vs. 33.6mg/dL），增加了总挥发性脂肪酸浓度（94.9 vs. 83.0mM）。乙酸、异丁酸和异戊酸的摩尔比例显著降低，而丙酸和丁酸的摩尔比例显著增加，这表明具有高 NSC 的苜蓿发酵更具生糖性。更重要的是，利用 15N 作为微生物标记物测量的微生物氮流量（263 vs. 230mg/d）和细菌氮效率（可用氮的 41.1% vs. 29.6%）均随高 NSC 苜蓿而显著增加。这些结果表明，苜蓿中 NSC 浓度的增加可促进生糖发酵，并增强瘤胃中微生物氮的合成。
