Mohammed R, Stanton C S, Kennelly J J, Kramer J K G, Mee J F, Glimm D R, O'Donovan M, Murphy J J
Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada.
J Dairy Sci. 2009 Aug;92(8):3874-93. doi: 10.3168/jds.2008-1613.
Six rumen-cannulated Holstein cows in early lactation were assigned to 3 treatments: grazing (G), zero-grazing (ZG), and grass silage (GS) harvested from the same perennial rye grass sward in a 3 x 3 Latin square design with three 21-d periods. The objectives of this study were to investigate the underlying mechanisms for the reported elevation in milk rumenic acid (RA) concentration associated with G compared with ZG and GS, and to identify the important variables contributing to the milk RA response. Grazing animals were offered 20 kg of dry matter/cow per day; indoor animals were offered ad libitum grass or silage. A concentrate at a rate of 3 kg/d was also offered to all cows. Rumen, plasma, and milk samples were collected in the third week of each period. Data were analyzed by the MIXED procedure of SAS. Dry matter intakes were less for GS with no difference between G and ZG. Milk yield was greater for G than for ZG or GS. Milk fat and protein contents were less for GS with no difference between G and ZG. The combined intake (g/d) of linoleic and linolenic (18:3n-3) acids was different across the treatments (G: 433; ZG: 327; and GS: 164). Rumen pH was less for G with no difference between ZG and GS. Concentrations of volatile fatty acids and ammonia nitrogen in rumens were not different across the treatments. Wet rumen fill was less for G with no difference between ZG and GS. Vaccenic acid concentrations were different across the treatments in rumen (G: 12.30%, ZG: 9.31%, and GS: 4.21%); plasma (G: 2.18%, ZG: 1.47%, and GS: 0.66%) and milk (G: 4.73%, ZG: 3.49%, and GS: 0.99%). Milk RA concentrations were greater for G (2.07%) than for ZG (1.38%) and GS (0.54%). Milk desaturase index based on the ratio cis-9-14:1/14:0 was not different across the treatments. Milk RA yield per 100 g of linoleic acid and linolenic acid intake (efficiency) was 2.23, 1.50, and 0.62 g in G, ZG, and GS, respectively, suggesting that G cows were more efficient than ZG and GS cows in milk RA production. Stepwise regression analysis of a group of variables revealed that plasma vaccenic acid accounted for 95% of the variation in milk RA production. Milk desaturase index did not enter into the model. Overall findings suggest that substrate intake influenced milk RA production but it was not the only factor involved. There were differences in efficiency of milk RA production, which appears to depend on the factors regulating ruminal vaccenic acid production and its supply to the mammary tissue.
放牧（G）、舍饲（ZG）和从同一多年生黑麦草草地收获的青贮草（GS），采用3×3拉丁方设计，共三个21天周期。本研究的目的是探究与舍饲和青贮草相比，放牧导致乳中瘤胃酸（RA）浓度升高的潜在机制，并确定影响乳中RA反应的重要变量。放牧动物每天每头供应20千克干物质；室内动物自由采食青草或青贮料。所有奶牛还按每天3千克的量供应精饲料。在每个周期的第三周采集瘤胃、血浆和乳样。数据采用SAS的MIXED程序进行分析。青贮草组的干物质采食量较低，放牧组和舍饲组之间无差异。放牧组的产奶量高于舍饲组或青贮草组。青贮草组的乳脂肪和蛋白质含量较低，放牧组和舍饲组之间无差异。不同处理方式下亚油酸和亚麻酸（18:3n - 3）的联合摄入量（克/天）不同（放牧组：433；舍饲组：327；青贮草组：164）。放牧组的瘤胃pH值较低，舍饲组和青贮草组之间无差异。各处理方式下瘤胃中挥发性脂肪酸和氨氮的浓度无差异。放牧组的湿瘤胃充盈度较低，舍饲组和青贮草组之间无差异。瘤胃中反式vaccenic酸浓度在不同处理方式下不同（放牧组：12.30%，舍饲组：9.31%，青贮草组：4.21%）；血浆中（放牧组：2.18%，舍饲组：1.47%，青贮草组：0.66%）和乳中（放牧组：4.73%，舍饲组：3.49%，青贮草组：0.99%）也不同。放牧组乳中RA浓度（2.07%）高于舍饲组（1.38%）和青贮草组（0.54%）。基于顺式 - 9 - 14:1/14:0比值的乳去饱和酶指数在各处理方式下无差异。每摄入100克亚油酸和亚麻酸的乳RA产量（效率）在放牧组、舍饲组和青贮草组中分别为2.23、1.50和0.62克，这表明放牧奶牛在乳RA生产方面比舍饲和青贮草奶牛更高效。对一组变量进行逐步回归分析表明，血浆反式vaccenic酸占乳RA产量变化的95%。乳去饱和酶指数未纳入模型。总体研究结果表明，底物摄入量影响乳RA生产，但它不是唯一涉及的因素。乳RA生产效率存在差异，这似乎取决于调节瘤胃反式vaccenic酸产生及其向乳腺组织供应的因素。
