Grasses value chains

The relationship between substrate and product for A-9-desaturase is reflected by the desaturase index, defined as [RA a- (RA + VA)]. Various approaches to calculating desaturase index in milk fat are discussed by Kelsey et al. (2003). In the study by Corl et al. (2001), the desaturase index was 0.23 for the hay and concentrate diet and 0.20 when the diet was supplemented with PH VO. Kay et al. (2004) reported a desaturase index of 0.25 for the pasture diet and 0.22 when the diet was supplemented with sunflower oil. Piperova et al. (2002) observed desaturase indices for highland low-fiber diets of 0.40 and 0.35, respectively. Shingfield et al. (2003) reported desaturase indices of 0.18 and 0.15 with a grass silage diet without or supplemented with fish oil these values are probably lower than others because the analytical methods accounted for minor CLA isomers that typically co-elute with RA or because of inhibition of A-9-desaturase by the long-chain PUFA from the fish oil supplement. The desaturase index, as defined earlier, should approximate the proportion of VA desaturated in the tissues. A summary of endogenous RA synthesis estimates and the proportion of VA desaturated in the tissues is in Table III. 

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