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Description of the forage characteristics responsible for CLA enrichment in milk

Opportunities to improve the lipid composition of ruminant products, whether reducing saturated fatty acids or increasing beneficial PUFA, are governed by microbial biohydrogenation of unsaturated fatty acids. Altering microbial lipid metabolism in the rumen for the production of higher levels of PUFA and other interesting fatty acids such as conjugated linoleic acid (CLA) has been a major goal for animal scientists for over thirty years. Numerous studies have shown that the milk from cows grazing fresh pasture had significantly higher levels of the beneficial PUFA and CLA than cows fed on conserved forage. Previous studies have reported the nature of the volatile components released during the cutting of grass.

This “green odour” consists of the products of fatty acid oxidation carried out by plant lipoxygenases. These enzymes catalyse the oxygenation of PUFA forming a mixture of volatile hydroperoxides, alcohols, aldehydes and ketones. These compounds have been shown to have antimicrobial activities and recently have been shown to have an effect on ruminal lipid metabolism. We have shown the effect of two typical fatty acid oxidation products (hydroperoxides and long chain aldehydes) on the biohydrogenation of grass silage C18:2 (n-6) and C18:3 (n-3) when incubated in vitro with strained rumen fluid.

The increase in the biohydrogenation of C18:2 (n-6) and C18:3 (n-3) and the associated increase in trans-vaccenic acid (TVA, a pre-requisite for the formation of CLA in animal tissues such as the mammary gland) may be attributable to the proliferation of biohydrogenating micro-organisms as a consequence of the toxic nature of the “green odour” compounds to competing micro-organisms reducing inter-specific competition. This “green odour effect” may offer a partial explanation for the changes in milk fat profiles when animals graze fresh pasture as opposed to conserved forage.

Reported by

Institute of Biological, Environmental and Rural Sciences
United Kingdom
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