Factors affecting fatty acids composition in milk of grazing cows

为探讨放牧奶牛乳中脂肪酸组成的影响因素，特别是共轭亚油酸(CLA)的影响因素，对放牧奶牛乳汁中的共轭亚油酸(CLA)含量进行了研究。试验一测定了4~12月全天放牧牛场(WG)、限时放牧牛场(TG)和非放牧牛场(NG)牛奶中的脂肪酸组成。于5~10月在温带草地放牧，试验组和试验组的奶牛均为放牧奶牛。在整个季节，给NG组的奶牛提供全混合日粮。在整个放牧季节，WG、TG和NG牛奶中CLA占总脂肪酸的比例分别为1.4%、0.9%和0.4%。在试验2中，6头奶牛在放牧和非放牧条件下饲养，研究了亚油酸和亚麻酸摄入量对牛奶中CLA含量的影响。这些结果表明，放牧条件下牛奶中共轭亚油酸含量的增加是由于瘤胃内亚麻酸和亚油酸合成的牛乳酸量增加所致。饲喂新鲜牧草的奶牛十二指肠中的疫苗酸流量是共轭亚油酸流量的10倍左右，这表明放牧奶牛乳中的共轭亚油酸主要是由疫苗合成的。在实验4中，以放牧奶牛的乳汁为原料制成干酪，以测试牛奶中的共轭亚油酸是否能留在干酪中。干酪中共轭亚油酸占总脂肪酸的比例随着牛奶中共轭亚油酸比例的增加而呈线性增加，这表明利用放牧奶牛的乳制备共轭亚油酸含量较高的干酪是可能的。

Four trials were conducted to investigate the factors affecting fatty acids composition, especially focused on CLA (conjugated linoleic acid), in milk of grazing cows.In experiment 1,fatty acids composition in milk collected from whole day grazing dairy farms (WG), time restricted grazing farms (TG) and non grazing farms (NG) were measured from April to December. The cows in WG and TG were grazed on temperate pasture from May to October. The cows in NG were offered total mixed ration throughout the season. Across the grazing season, proportions of CLA to total fatty acids in milk of WG,TG and NG were 1.4,0.9 and 0.4 mol%, respectively.In experiment 2,six milking cows were kept under grazing and non-grazing condition to investigate the effect of linoleic and linolenic acids intake on the CLA content in the milk. These results suggested that the increase of CLA in milk under grazing condition was due to the increase of vaccenic acid supply, which was synthesized form linolenic and linoleic acids in the rumen, to mammary glandIn experiment 3,three steers were offered fresh herbage to investigate the fatty acid composition in duodenum digesta. The amount of vaccenic acid flow to the duodenum of steers fed fresh herbage was about ten times more than that of CLA flow, suggesting that the CLA in the milk of grazing cows are mainly synthesized from vaccenic.In experiment 4,cheeses were made from the milk of the grazing cows to test whether the CLA in milk could remain in the cheese. The proportion of CLA to total fatty acids in cheese increased linearly with increase of the proportion of CLA in milk, suggesting that it could be possible to make a cheese containing high level of CLA using the milk produced from grazing cows.