Seasonal variation in milk conjugated linoleic acid and Δ9-desaturase activity in dairy cows

The objective of this work was to study changes in the fatty acid profile of cows' milk throughout the year with particular emphasis on cis-9, trans-11 conjugated linoleic acid (CLA), a proven anticarcinogen, found predominantly in milk and meat from ruminants. During the winter months, a total mixed ration of grass and maize silages, brewers grains, cereals, soya and dairy concentrates was fed. Through the summer months, fresh grass was fed, with increasing levels of buffer feeding given as the summer progressed. The CLA content of milk in May, June and July was significantly higher (P < 0.05) than all other months, averaging 1.50 g CLA/100 g FAME compared with a mean of 0.77 g/100 g for the other months. Delta(9)-Desaturase activity was also greater in the summer. Milk fat produced during the summer contained significantly (P < 0.05) greater amounts of short-chain fatty acids at the expense of medium-chain fatty acids indicating that fresh grass may alter the pattern of fatty acids produced de novo in the mammary gland. Results suggest that fresh grass promotes the synthesis of CLA.in the dairy cow through an increase in Delta(9)-desaturase activity in the mammary gland and possibly other unknown, factors. (C) 2002 Elsevier Science B.V. All rights reserved.