Influence of different dietary forages on the fatty acid composition of rumen digesta as well as ruminant meat and milk

We review literature on effects of dietary forages on milk and tissue fatty acid composition of cattle and sheep, with particular emphasis on changes induced by leguminous and biodiverse forages versus intensive ryegrass. Differences are discussed in relation to changes in rumen or duodenal digesta to explain the origin of the differences as, in most cases, increased omega-3 PUFA (i.e., linolenic acid and/or long chain omega-3 PUFA) in milk and intramuscular fat due to feeding of red or white clover and botanically diverse forages could not be attributed to increased dietary supply of linolenic acid (C18:3 n-3). Hence, increased forestomach outflow of C18:3 n-3 has been suggested to originate from reduced rumen lipolysis, with literature providing some evidence for the role of polyphenoloxidase, which is particular active in red clover, to inhibit rumen lipolysis. Increased proportions of CLA c9t11 in milk and intramuscular fat of ruminants fed botanically diverse forages have been associated with increased forestomach outflow of vaccenic acid (C18:1 t11), which is the main precursor of endogenous CLA c9t11 production. Despite the lack of direct evidence, some plant secondary metabolites, present in herbs of botanically diverse forages, are suggested to be potential modifiers of rumen biohydrogenation based on their effects on rumen methanogenesis.