Seasonal Dynamics Of Milk Fatty Acids From Tianzhu White Yaks

Ten white yaks were selected to explore the effect of seasonal factors to yak milk fatty acids profile. Also the relationship between forage quality and yak milk fatty acid content in different seasons was studied. The selected yaks were grazing freely in natural grassland in the region of wushaoling mountain area belongs to Tianzhu county. The milk samples were collected from10lactating yaks in different seasons and during each period each yak was sampled10days continuously at morning milking. At the same time natural forage samples from pasture were also collected during each sampling period by mimicking yak grazing. The forage samples were stored at-20℃until freeze-drying for further analysis. The results were showed at the following:(1) Yak milk production shows significantly different with the change of season and the results tells that the milking in warm season (summer and autumn) were significantly higher than in winter and spring (p<0.05).It was concluded that the seasonal change of milk yield has a direct relationship with food restrictions and physiological states of the yak.(2) Both yak milk fat and milk protein content changing with seasons show relatively consistent. The contents of milk protein and milk fat in the warm season (summer and autumn) are markedly lower than it in cold season (winter and autumn)(p<0.05), and contents in hay period is apparently higher. Yak milk fat in spring reaches up to9.88%, while half of the content in summer was tested out which less than the half of it in spring. The protein content was6.53%and6.74%in content respectively in spring and winter, significantly higher than5.85%and6.17%of the summer and autumn (p<0.05). These results may be related to the physical characteristics of milk in winter and spring and also types of supplementary feeding in some way.(3) Saturated fatty acids (SFA) contribute to the majority of FA composition in yak milk fatty acid nutrition. And the content of unsaturated fatty acids (UFA) also shows significantly seasonal characteristics. In summer and autumn, the content of unsaturated fatty acids in yak milk reaches up to40.94g/100gFA and41.07g/100gFA respectively, significantly higher than that in winter and spring (p<0.05), which were35.68g/100gFA in winter, and37.84g/100gFA in spring.(4) Polyunsaturated fatty acids (PUFA) in yak milk shows that the content in summer and autumn significantly trend (p<0.05) higher than that in winter and spring. And in summer it has the highest content, which was significantly higher than other seasons (p<0.05).The content of the PUFA in milk is mainly affected by the condition of the pasture. Polyunsaturated fatty acids in forage have the same seasonal dynamics as it is in milk.(5) Functional fatty acids content in yak milk also shows significant seasonal differences. The contents of CLA in the summer and autumn are significantly higher than that in winter and spring (p<0.05), In the summer milk samples the content of CLA is1.95%and2.24%in autumn, in winter0.84%and only0.61%in spring. The contents of CLA in summer and autumn are markedly higher than that in winter and spring, which was considered to be related with the forage quality and pasture condition; a-linolenic acid (LNA) in the spring milk samples has the minimum content while the y-linolenic acid (GLA) maximum inversely, in which the content is significantly higher than other seasons (p<0.05). In addition, arachidonic acid (C20:4) and the EPA were examined out in the spring and winter milk samples while it haven’t be ruled out in summer and autumn’s. These results not only shows the relationship between yaks and forage with seasonal change but also a reflection of yak adaptation to the special environment in Qinghai-Tibet plateau.