ALPHA-LINOLENIC ACID TRANSFER INTO MILK-FAT AND ITS ELONGATION BY COWS

It was the purpose of this study to explore to what extent alpha-linolenic acid is transferred into bovine milk fat if either fed as intact linseed or if infused as linseed oil (alpha-linolenic acid content almost-equal-to 55 wt%) into the abomasum. Two cows consumed 2000 g/d linseed for 9 wk. Then two doses of linseed oil (500 or 250 g/d) were infused for 14 or 4 wk into the abomasum of the cows, respectively. There was no effect of linseed oil on milk production, milk fat and protein content, whereas linseed caused a slight decline of milk protein content simultaneously with a pronounced rise of trans fatty acids in milk fat. While during the linseed period only 1% of consumed alpha-linolenic acid (C18:3n-3) appeared in milk fat, on average 49 or 56% of infused C18:3 was transferred into milk fat depending on the infusion rate of linseed oil. There was only modest rise of about 0.25 wt% of desaturation/elongation metabolites (C20:5 plus C22:5) of alpha-linolenic acid in milk fat indicating a low DELTA-6-desaturase activity of the cow. Docosahexaenoic acid (C22:6) was not found in milk fat. It is concluded that bovine lipid metabolism allows surprisingly high transfer rates of n-3 fatty acids of linseed oil into milk fat and that there is the possibility to enrich milk fat with n-3 fatty acids without affecting milk yield provided ruminal biohydrogenation is prevented.