Use of high doses of 18:0 to try to mitigate the syndrome of milk fat depression in dairy ewes fed marine lipids

Abstract

Despite the benefits of adding marine lipids rich in n-3 polyunsaturated fatty acids (PUFA) to ruminant diets to improve milk fatty acid (FA) composition, this strategy induces milk fat depression (MFD), precluding its application under practical conditions. The MFD elicited by marine lipids has tentatively been explained by a shortage of available ruminal 18:0 for mammary uptake and Δ9-desaturation to cis-9 18:1, which might increase milk fat melting point and impair fat secretion. This hypothesis was challenged in a recent experiment in dairy ewes, as diet supplementation with 2% DM of 18:0 did not prove useful to alleviate fish oil-induced MFD. However, further research with higher levels of 18:0 inclusion seemed advisable. Therefore, in this study, 16 lactating ewes were allocated to 4 treatments lasting 27days: a total mixed ration containing no additional lipid (control) or 2% DM of fish oil alone (FO) or in combination with 3% (FOSA3) or 4% (FOSA4) of stearic acid. Fish oil supplementation induced MFD, but addition of 18:0, regardless of the dose, was not able to mitigate it: decreases in milk fat content reached 19% in FO, 20% in FOSA3 and 27% in FOSA4. The reduction in milk 18:0 concentration due to FO (−81%) was completely reverted neither by FOSA3 (−41%) nor by FOSA4 (−25%), and something similar occurred with cis-9 18:1 concentrations (12.53, 5.91, 9.50 and 11.28g/100 FA, in control, FO, FOSA3 and FOSA4, respectively). Increases in some candidate milk fat inhibitors in FO and FOSA diets (e.g., cis-9 16:1, cis-11 18:1, trans-10 18:1, 10-oxo-18:0, or trans-10 cis-15 18:2) might account for the absence of a positive effect of dietary 18:0. The estimated milk fat melting point was lower in the three supplemented diets (on average, −2.6°C compared with the control). In conclusion, addition of high doses of stearic acid to the diet (at 3 and 4% DM) was not able to alleviate the MFD caused by the concomitant supplementation with fish oil. This lack of response would further challenge the hypothesis suggesting that fish oil-induced MFD is mainly explained by decreased ruminal production of 18:0 and subsequent problems of milk fat fluidity, but further research would be still needed.