Trans and conjugated fatty acids in milk from cows and goats consuming pasture or receiving vegetable oils or seeds

This review summarises the effects of nutritional factors on fat content, different isomers of trans 18:1 and conjugated linoleic acids (CLA) in milk fat from dairy cows and goats. Main dietary factors taken into account are the nature of forages and pasture, and supplementation with oil seeds, vegetable or marine oils. The effects of interactions between the nature of forages and lipid supplements in cows or goats are reviewed with particular attention. Lipid supplements could highly increase mean trans-vaccenic acid (TVA) and cis-9, trans-11 CLA content in ruminant milk fat, and interact with the quantity of concentrate in the diet or the type of preserved forage. Vegetable or fish oil supplementation increased milk cis-9, trans-11 CLA up to 4% (in total milk fatty acids (FA)). The content of copper or vitamin E in the diet could modify the milk percentage of TVA and cis-9, trans-11 CLA. Other isomers of trans-18:1 (e.g. trans-10) and CLA (e. g. cis-11, trans-13 or trans-11, trans-13 or trans-7, cis-9 or cis-9, cis-11 CLA) may be manipulated by diet composition. However, further studies are needed to establish their responses to different feeding strategies. The trans-10, cis-12 CLA in percentage in milk fat was always lower than 0.15% of total FA. The proportions of different trans-18:1 isomers in milk fat are similar in caprine and bovine species. Recent studies showed that the CLA content of goat milk fat was similar to values (0.4 - 0.9% and up to 4% for winter and lipid-supplemented diets, respectively) observed in the cow.