Fuentes M C, Calsamiglia S, Cardozo P W, Vlaeminck B
Universitat Autònoma de Barcelona, Bellaterra, Spain.
J Dairy Sci. 2009 Sep;92(9):4456-66. doi: 10.3168/jds.2008-1722.
Milk fat depression in cows fed high-grain diets has been related to an increase in the concentration of trans-10 C(18:1) and trans-10,cis-12 conjugated linoleic acid (CLA) in milk. These fatty acids (FA) are produced as a result of the alteration in rumen biohydrogenation of dietary unsaturated FA. Because a reduction in ruminal pH is usually observed when high-concentrate diets are fed, the main cause that determines the alteration in the biohydrogenation pathways is not clear. The effect of pH (6.4 vs. 5.6) and dietary forage to concentrate ratios (F:C; 70:30 F:C vs. 30:70 F:C) on rumen microbial fermentation, effluent FA profile, and DNA concentration of bacteria involved in lipolysis and biohydrogenation processes were investigated in a continuous culture trial. The dual-flow continuous culture consisted of 2 periods of 8 d (5 d for adaptation and 3 d for sampling), with a 2 x 2 factorial arrangement of treatments. Samples from solid and liquid mixed effluents were taken for determination of total N, ammonia-N, and volatile fatty acid concentrations, and the remainder of the sample was lyophilized. Dry samples were analyzed for dry matter, ash, neutral and acid detergent fiber, FA, and purine contents. The pH 5.6 reduced organic matter and fiber digestibility, ammonia-N concentration and flow, and crude protein degradation, and increased nonammonia and dietary N flows. The pH 5.6 decreased the flow of C(18:0), trans-11 C(18:1) and cis-9, trans-11 CLA, and increased the flow of trans-10 C(18:1), C(18:2n-6), C(18:3n-3), trans-11,cis-15 C(18:2) and trans-10,cis-12 CLA in the 1 h after feeding effluent. The pH 5.6 reduced Anaerovibrio lipolytica (32.7 vs. 72.1 pg/10 ng of total DNA) and Butyrivibrio fibrisolvens vaccenic acid subgroup (588 vs. 1,394 pg/10 ng of total DNA) DNA concentrations. The high-concentrate diet increased organic matter and fiber digestibility, nonammonia and bacterial N flows, and reduced ammonia-N concentration and flow. The high-concentrate diet reduced trans-11 C(18:1) and trans-10 C(18:1), and increased C(18:2n-6), C(18:3n-3) and trans-10,cis-12 CLA proportions in the 1 h after feeding effluent. The increase observed in trans-10,cis-12 CLA proportion in the 1 h after feeding effluent due to the high-concentrate diet was smaller that that observed at pH 5.6. Results indicate that the pH is the main cause of the accumulation of trans-10 C(18:1) and trans-10, cis-12 CLA in the effluent, but the trans-10,cis-12 CLA proportion can be also affected by high levels of concentrate in the diet.
给奶牛饲喂高谷物日粮时出现的乳脂降低现象，与牛奶中反式-10 C(18:1)和反式-10,顺式-12共轭亚油酸（CLA）浓度升高有关。这些脂肪酸（FA）是日粮不饱和FA瘤胃生物氢化作用改变的结果。由于饲喂高浓度日粮时通常会观察到瘤胃液pH值降低，因此决定生物氢化途径改变的主要原因尚不清楚。在一项连续培养试验中，研究了pH值（6.4对5.6）和日粮粗饲料与精饲料比例（F:C；70:30 F:C对30:70 F:C）对瘤胃微生物发酵、流出物FA谱以及参与脂肪分解和生物氢化过程的细菌DNA浓度的影响。双流连续培养包括2个为期8天的阶段（5天适应期和3天采样期），采用2×2析因设计处理。采集固体和液体混合流出物样本，测定总氮、氨氮和挥发性脂肪酸浓度，其余样本进行冻干。对干燥样本分析干物质、灰分、中性和酸性洗涤纤维、FA以及嘌呤含量。pH 5.6降低了有机物和纤维消化率、氨氮浓度和流量以及粗蛋白降解，并增加了非氨和日粮氮流量。pH 5.6降低了C(18:0)、反式-11 C(18:1)和顺式-9,反式-11 CLA的流量，并增加了采食后1小时流出物中反式-10 C(18:1)、C(18:2n-6)、C(18:3n-3)、反式-11,顺式-15 C(18:2)和反式-10,顺式-12 CLA的流量。pH 5.6降低了解脂厌氧弧菌（32.7对72.1 pg/10 ng总DNA）和溶纤维丁酸弧菌vaccenic酸亚群（588对1394 pg/10 ng总DNA）的DNA浓度。高浓度日粮提高了有机物和纤维消化率、非氨和细菌氮流量，并降低了氨氮浓度和流量。高浓度日粮降低了反式-11 C(18:1)和反式-10 C(18:1)，并增加了采食后1小时流出物中C(18:2n-6)、C(18:3n-3)和反式-10,顺式-12 CLA的比例。由于高浓度日粮导致采食后1小时流出物中反式-10,顺式-12 CLA比例的增加幅度小于pH 5.6时观察到的增幅。结果表明，pH值是流出物中反式-10 C(18:1)和反式-10,顺式-12 CLA积累的主要原因，但反式-10,顺式-12 CLA比例也会受到日粮中高浓度精饲料的影响。
