Effect of different types of forages, animal fat or marine oils in cow's diet on milk fat secretion and composition, especially conjugated linoleic acid (CLA) and polyunsaturated fatty acids

This review summarises the known effects of forages, animal fats or marine oils on bovine milk fat secretion and composition. Special attention is giv en to fatty acids that could play a positive role for human health, such as butyric acid, oleic acid, C18 to C22 polyunsaturated fatty acids and conju gated linoleic acid (CLA). The efficiency of the transfer of n-3 polyunsatu rated fatty acids from diet to milk is reviewed. Milk fat from pasture fed cows seems to be higher in linolenic acid than milk fat from cows receiving preserved grass or maize, but the magnitude of this difference is limited. Indirect comparisons show that milk fat from maize silage diets is richer in short-chain FA and linoleic acid when compared to grass silage diets. Co mpared to fresh grass, grass silage favours myristic and palmitic acids at the expense of mono- and polyunsaturated FA, including CLA. Protected tallo w allows for a large increase in milk fat yield, and in the percentage of m ilk stearic and oleic acids, at the expense of medium chain FA. Non-protect ed tallow has a similar effect on medium chain FA without increasing so muc h C18 FA yield, which explains that it does not increase milk fat yield. Do se-response curves of milk CLA are reviewed for marine oil supplements, as well as the relationship between milk CLA and trans-C18:l contents. The pot ential of marine oil supplementation to increase the mean CLA content in co w milk fat is large (more than 300% above basal values). A specific role fo r dietary C20:5 n-3 in the sharp decrease in milk fat secretion after fish oil supplementation is suggested. However, there is a need to evaluate how the different feeding strategies could change the other aspects of milk fat quality, such as taste, oxidative stability or manufacturing value. (C) 20 01 Elsevier Science B.V. All rights reserved.