Dietary control of milk fat nutritional quality in the dairy cow : trans, and polyunsaturated fatty acids, and conjugated linoleic acid

Abstract

After a survey of metabolic pathways and nutrient fluxes involved in mammary lipogenesis, this review summarises the known effects of diet on ruminant milk fatty acid (FA) composition, and particularly polyunsaturated FA, trans monounsaturated FA and conjugated linoleic acid (CLA). The main dietary factors which were studied are the nature of forages (including pasture), and the supplementation of dairy rations with protected or unprotected tallow, and protected or unprotected vegetable or marine oils (fish or algae). Tallow supplementation increases the secretion of oleic acid. Milk fat percentage of linoleic acid varies from 2 to 3% in basal diets. It is increased by only 1.5 point when the diet is supplemented with oils or seeds rich in C18:2, because of an extensive ruminal biohydrogenation. Milk fat from grass diets (rich in C18:3) is higher (0.5 to 1 point) in linolenic acid than milk fat from maize silage diets. Linseed increases the milk fat percentage of linolenic acid by 0.3 point. Milk fat percentage of C20:5 (EPA) and C22:6 (DHA) is enhanced up to 0.5 % of total FA by the supply in the diet of marine oils. Milk fat content can be greatly decreased by low fibre, high grain diets and/or by feeding unprotected marine oils or vegetable oils rich in unsaturated FA. A role of trans-C18:1 isomers, including trans-11 and trans-10 C18:1, but also of trans 10, cis12 CLA, in the inhibition of mammary gland lipogenesis is suggested. Diets which increase milk trans C18:1 or CLA percentage in milk fat are those which provide lipid substrates for the formation of CLA or trans C18:1 in the rumen, or those which change the microbial activity associated with ruminal biohydrogenation of polyunsaturated FA, or those which change the mammary desaturase activity. The influence of dietary factors on the different C18:1 and CLA isomers in milk fat is not well known.