Comparison of milk fatty acid responses during fish oil- and trans-10 cis-12 18:2-induced milk fat depression in dairy ewes

The direct comparison between milk fat depression (MFD) caused by the addition of trans-10 cis-12 18:2 (a conjugated linoleic acid –CLA– isomer) or marine lipids to the diet of dairy ewes may help to elucidate the origin of this syndrome. Therefore, 12 lactating sheep were divided in 3 lots and offered a diet without supplementation (control) or supplemented with 2% DM of fish oil (FO) or 1.1% DM of a rumen-protected product rich in trans-10 cis-12 18:2 (CLA) for 27 days to compare the responses in terms of animal performance and milk fatty acid (FA) profile. Both supplemented diets (FO and CLA) decreased the milk fat content in a similar manner (–18% compared with the control). On the other hand, responses in milk FA profiles differed significantly and support that marine lipid-induced MFD is not mediated by the effects of trans-10 cis-12 18:2. However, a comparison of changes in the molar production of milk FA show that de novo FA synthesis was affected similarly in FO and CLA treatments and more strongly than FA uptake, which implies that both types of MFD might share common mechanisms. The results point to the involvement of less well-known potentially antilipogenic metabolites (such as intermediates of 18:3n-3 biohydrogenation or ruminal hydration and oxidation) in the low-milk fat syndrome in ewes fed FO and seem to downplay the relevance of changes in the milk fat melting point as a major mechanism responsible for FO-induced MFD.