Individual variation of the extent of milk fat depression in dairy ewes fed fish oil: Milk fatty acid profile and mRNA abundance of candidate genes involved in mammary lipogenesis

Abstract

Dairy ewes are less prone than cows to milk fat depression (MFD) but suffer from this syndrome when marine lipids are added to their diet to modulate milk fatty acid (FA) profile. However, there are large individual differences in MFD extent, and the reasons behind this variability are uncertain. On this basis, a study was conducted in lactating sheep to test the hypotheses that individual susceptibility to the low-fat milk condition may be explained by differences in (1) the milk concentration of some FA, particularly antilipogenic FA, or (2) the transcriptional regulation of mammary lipogenesis. For 5 wk, 15 ewes received a total mixed ration supplemented with 0 (control; n = 5) or 20 g of fish oil/kg of dry matter [10 animals selected out of 22 and divided into those showing marked (RESPON+; n = 5) or mild (RESPON-; n = 5) MFD]. Milk production and composition, including a comprehensive FA profile, were examined on 3 consecutive days before and after treatments. Candidate gene expression was also analyzed before the start of the trial and at its end using RNA isolated from milk somatic cells. According to the experimental design, the fish-oil-induced decrease in milk fat concentration was much stronger in RESPON+ (-25.4%) than in RESPON- (-7.7%). Milk from all ewes fed the supplemented diet showed rather uniform changes in the proportion of potentially healthy FA (such as cis-9,trans-11 18:2, trans-11 18:1, or 20:5n-3) and of those with confirmed or putative antilipogenic effects (e.g., cis-9 16:1, trans-10 and cis-11 18:1, trans-9,cis-11 18:2, and 10-oxo-18:0), without significant variation between RESPON+ and RESPON-. It was not possible to relate the very few exceptions to this general trend (e.g., in cis-7 16:1 and 22:6n-3) to responsiveness. Major mechanisms involved in mammary lipogenesis, specifically the uptake and de novo synthesis of FA, appeared to be unequally inhibited in ewes displaying different degrees of MFD, with molar yields of >16C FA being unaffected in RESPON-. However, this was not reflected in candidate gene expression. Supplementation with fish oil showed a tendency to lower the mRNA abundance of lipogenic genes such as ACSS2, FASN, LPIN1, FADS2, and INSIG1, but only SCD and GPAT4 tended to differ between RESPON- and RESPON+. Overall, these results offer no convincing support for the initial hypotheses, so further research must be pursued to explain the individual variation in MFD severity.