There is considerable interest in altering the fatty acid composition of milk with the overall aim of improving the long-term health of consumers. Important targets include reducing the amounts of medium-chain saturated fatty acids, enhancing cis-9 18:1 to reduce cardiovascular risk, as well as increasing concentrations of trans-11 18:1 and cis-9, trans-11 18:2 which have been shown to exert anti-carcinogenic properties in a range of human cell lines and animal models. Most studies have examined use of plant or marine oils, vegetable oilseeds or rumen protected or inert lipids in the diet to modify milk fatty acid composition, with much less attention paid to the fatty acid composition of the basal forage. We review recent progress in this area and identify potential for increasing the levels of mono- and poly-unsaturated fatty acids (MUFA and PUFA) in milk produced by dairy cows in high-forage systems. We also review the range of levels of important MUFA and PUFA in milk achieved by feeding the major classes of forage, as well as considering effects of less common forages to reveal potential new approaches to manipulate rumen fatty acid metabolism. Even though forages contain relatively low levels of lipid, they are often the major source of fatty acids in ruminant diets. We describe ways in which herbage species, cultivar, conservation method and level of forage in the diet of dairy cows affect rates and extents of ruminal biohydrogenation of dietary fatty acids and milk fatty acid composition. Discussion of the potential to increase recovery of forage PUFA in milk first considers genetic approaches to increase PUFA in forage, including in a range of animal management systems. Losses of fatty acids during forage conservation and storage are described along with strategies to reduce such losses. We describe plant traits, such as polyphenol oxidase, tannins and outflow rates from the rumen, which are associated with reduced rumen biohydrogenation. Similarly, we describe plant factors associated with increased levels of biohydrogenation intermediates in the rumen. The second aspect of this review focuses on effects of forage composition on metabolism of supplementary PUFA showing that both the level and type of forage in the diet are important determinants of ruminal lipid metabolism and milk fatty acid responses to plant and marine oils. Whilst increased use of forage in the diet does not produce the large 18:3 n − 3, conjugated linoleic acid or cis-9 18:1 enrichment, or saturated fatty acids depletion in milk, that can be achieved with dietary oil supplementation, beneficial changes can be made without substantial increases in milk trans fatty acids. Trans fatty acids are an important issue for future research, both because of their generally negative effect on human health, as well as because trans-11 18:1 appears to be an exception to this generalisation.
- fatty acid