Effects of supplemental 18:0 on milk fat content in dairy ewes fed a diet rich in fish oil

Abstract

Diet supplementation with fish oil (FO) inhibits the saturation of trans-18:1 to 18:0 in the rumen, increasing the accumulation of t11-18:1 and, consequently, the concentration of the potentially health-promoting c9t11-conjugated linoleic acid (CLA) in milk. However, this feeding strategy also induces milk fat depression (MFD), which has been associated with a shortage of 18:0 for mammary c9-18:1 synthesis and its possible impact on the maintenance of milk fat fluidity. Thus, with the aim of studying whether FO-induced MFD can be alleviated by increased availability of 18:0 for mammary Δ9-desaturation (i.e., for its conversion to c9-18:1), an experiment was performed in dairy ewes. The trial followed a 3×3 Latin square design (4 ewes/group) with 3 periods of 4 weeks each and 3 experimental diets: non-supplemented, supplemented with 2% FO and supplemented with 2% FO plus 2% 18:0. Milk production and composition were analyzed on the last 3 days of each period. At the end of the experiment, the digestibility of supplemental 18:0 was measured using 6 lactating sheep. Supplemented diets had no significant effect on milk yield but, compared with the control, both of them reduced milk fat content in a similar proportion (-20%), which suggests that the addition of 18:0 to the diet does not alleviate FO-induced MFD. Since this result cannot be fully explained by the relatively low digestibility coefficient of the 18:0, further research would be required to elucidate if the lack of response to this fatty acid was attributable to a low mammary uptake or to other factors.