Diet, rumen biohydrogenation and nutritional quality of cow and goat milk fat

The potential to modify the milk fatty acid (FA) composition by changing the cow or goat diets is reviewed. Ruminal biohydrogenation (RBH), combined with mammary lipogenic and D-9 desaturation pathways, considerably modifies the profile of dietary FA and thus milk composition. The pasture has major effects by decreasing saturated FA and increasing FA considered as favorable for human health (c9-18:1, 18:3n-3 and c9t11-CLA), compared to winter diets, especially those based on maize silage and concentrates. Plant lipid supplements have effects similar to pasture, especially linseed, but they increase to a larger extent, simultaneously several trans isomers of 18:1 and, conjugated or non-conjugated 18:2, especially when added to maize silage or concentrate-rich diets. The goat responds better for milk 18:3n-3 and c9t11-CLA, and sometimes less for c9-18:1, and is less prone to the RBH trans-11 to trans-10 shift, which has been shown to be time dependent in the cow. The respective physiological roles of most milk trans FA have not been studied to date, and more studies in rodents and humans fed dairy products modified by changing ruminant diet are required before recommending a larger use of lipid sources and how to combine themwith the different feeding systems used by dairy farmers.