Factors affecting fatty acids composition in milk of grazing cows

通过4项试验研究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）。试验1于4 ~ 12月对全日放牧奶牛场（WG）、限时放牧奶牛场（TG）和非放牧奶牛场采集的牛奶进行脂肪酸组成测定。5 - 10月在温带牧场放牧。NG试验奶牛全季节饲喂混合日粮。整个放牧季节，WG、TG和NG乳中CLA占总脂肪酸的比例分别为1.4、0.9和0.4 mol%。试验2：选取6头奶牛，分别饲喂放牧和非放牧两种条件，研究采食亚油酸和亚麻酸对牛奶中CLA含量的影响。综上所述，放牧条件下乳中CLA的增加是由于瘤胃中由亚麻酸和亚油酸合成的异丙酸向乳腺的供应增加所致。试验3，饲喂3头牛新鲜牧草，研究其十二指肠食糜中脂肪酸的组成。采食新鲜牧草的牛十二指肠内的牛酸流量约为牛酸流量的10倍，说明放牧牛乳中的牛酸主要由牛酸合成。实验4以放牧奶牛的奶为原料制作奶酪，考察牛奶中的CLA能否残留在奶酪中。随着牛奶中CLA含量的增加，奶酪中CLA占总脂肪酸的比例呈线性增加，表明利用放牧奶牛的牛奶生产高水平CLA的奶酪是有可能的。

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.