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please follow this link:

http://www.sciencedirect.com/science/article/pii/B978008100596500915X

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http://www.sciencedirect.com/science?_ob=ArticleListURL&_method=list&_ArticleListID=-912503131&_sort=r&_st=13&view=c&md5=a53658b21f05c9d2a1b127674f13c3e9&searchtype=a

The effect of age on milk fat content is less important compared with that on Solids Not Fat (SNF) content, which is more marked. Generally, age of a cow and its milk fat content are inversely related. Accordingly, as cows get older or as parity advances their milk fat content is reported to decrease by about 0.02 to 0.2% per lactation (based on various sources). This is reportedly caused by increasing lactation milk yield as parity advances and related increased incidence of udder infections.

For farther information get connected to the following links:

https://www.uaex.edu/publications/pdf/FSA-4014.pdf

http://www.aaabg.org/livestocklibrary/1997/AB97058.pdf

Look for quantity and quality, not only age but also seasons affects the fat quanity and quality ? 

Dear Scholars, thank you very much for your interesting responses and suggestions!!!

For any individual animal, fat content decreases slightly during successive lactations, by ~0.2 % over a typical productive lifetime (~5 lactations). In practice, this

factor usually has no overall effect on the fat content of a bulk milk supply because

herds normally include cows of various ages. (Fox et al 2015)