The modulation of milk fat nutritional quality through fish oil supplementation seems to be largely explained by the action of n-3 very long chain polyunsaturated fatty acids (PUFA) on ruminal biohydrogenation (BH) of C18 fatty acids (FA). However, relationships among this action, disappearance of those PUFA in the rumen, and potential detrimental consequences on ruminal fermentation remain uncertain. This study compared the effect of 20:5n-3 (eicosapentaenoic acid; EPA), 22:5n-3 (docosapentaenoic acid; DPA), and 22:6n-3 (docosahexaenoic acid; DHA) on rumen fermentation and BH of C18 FA and was conducted simultaneously in cows and sheep to provide novel insights into interspecies differences. The trial was performed in vitro using batch cultures of rumen microorganisms with inocula collected from cannulated cows and ewes. The PUFA were added at a dose of 2% incubated dry matter, and treatment effects on ruminal C18 FA concentrations, PUFA disappearances, and fermentation parameters (gas production, ammonia and volatile FA concentrations, and dry matter and neutral detergent fiber disappearances) were examined after 24 h of incubation. A principal component analysis suggested that responses to PUFA treatments explained most of the variability; those of ruminant species were of lower relevance. Overall, EPA and DHA were equally effective for inhibiting the saturation of trans-11 18:1 to 18:0 and had a similar influence on ruminal fermentation in cows and sheep (e.g., reductions in gas production and acetate:propionate ratio). Nevertheless, DHA further promoted alternative BH pathways that lead to trans-10 18:1 accumulation, and EPA seemed to have specific effects on 18:3n-3 metabolism. Only minor variations attributable to DPA were observed in the studied parameters, suggesting a low contribution of this FA to the action of marine lipids. Although most changes due to the added PUFA were comparable in bovine and ovine, there were also relevant specificities, such as a stronger inhibition of 18:0 formation in cows and a greater increase in 18:3n-3 metabolites in sheep. No direct relationship between in vitro disappearance of the incubated PUFA and effect on BH (in particular, inhibition of the last step) was found in either cows or ewes, calling into question a putative link between extent of disappearance and toxicity for microbiota. Conversely, an association between the influence of these PUFA on ruminal lipid metabolism and fermentation may exist in both species. In vivo verification of these findings would be advisable.

The ability of tannins to interfere with ruminal biohydrogenation (BH) and modulate the fatty acid (FA) profile of ruminant-derived products is highly controversial, which is probably related to the type of tannin and the dosage rate. Therefore, this in vitro study was conducted to analyse the effect of 4 commercial extracts of tannins (from chestnut, oak, quebracho and grape) at 4 doses (20, 40, 60 and 80 g/kg diet DM) with the aim of selecting an effective treatment to modulate the BH of unsaturated FA.Two in vitro assays with batch cultures of rumen microorganisms, using cannulated ewes as donors of rumen inocula, were performed. The incubated substrate (a total mixed ration similar to that fed to the animals) was supplemented with 20 g of sunflower oil/kg DM. The first experiment followed a 4 × 4 + 1 design (i.e., 4 types of tannins × 4 doses of each one, and a control), and treatment effects on the FA composition of the ruminal digesta were examined by gas chromatography. On the basis of these results, the second experiment was conducted to make sure that the selected dose and type of tannin would not impair rumen fermentation. To this end, gas production kinetic parameters, extent of degradation, in vitro true substrate digestibility, pH, and ammonia and volatile FA concentrations, as well as the bacterial community (by terminal restriction fragment length polymorphism, T-RFLP) were examined. All tannin extracts were able to modulate the in vitro BH of unsaturated FA. However, the high dose required in many cases suggests that their efficacy would be rather limited in terms of animal feeding. On the other hand, the oak tannin extract, at a dose of 20 g/kg diet DM, increased total polyunsaturated FA, and trans-11 18:1, and decreased trans-10 18:1 and 18:0 rumen concentrations without eliciting any negative response in ruminal fermentation. Although this treatment had no discernible effects on the bacterial community structure and 3 diversity, a few fragments compatible with uncultured Lachnospiraceae species were affected.

A number of studies in dairy cows have shown a relationship between milk fat depression (MFD) and alterations caused in lipogenic gene expression by dietary nutrients.However, information in small ruminants is not only scarce but also inconsistent. Therefore, this experiment was conducted in dairy ewes to study the effect of a diet known to induce MFD on milk fatty acid (FA) composition and mRNA abundance of key candidate genes involved in mammary lipogenesis. Twelve lactating Assaf ewes (63 days in milk; SD = 7.8) were randomly assigned to 2 treatments consisting of a total mixed ration based on alfalfa hay and concentrates (50:50), supplemented with 0 (control) or 17 g of fish oil/kg of diet DM (FO). Profiles of milk FA and mRNA abundance of candidate genes in biopsied mammary tissue were examined before starting the treatments and after 1 and 4.5 weeks on the diets. As expected, FO induced MFD and modified milk FA composition. Compared with the control, 3 reductions in milk fat concentration and yield were not detected on day 7, but reached up to 25 and 22%, respectively, on day 30. However, increases in confirmed or putative antilipogenic FA (trans-10 cis-12 and trans-9 cis-11 18:2, cis-9 16:1, cis-11 18:1 and oxo-FA) were already established on the early stage of the treatment and lasted until the end of the feeding period. These changes were accompanied by decreases in the mRNA abundance of genes encoding lipogenic enzymes. The coordinated nature of the downregulation, which tended to affect most studied metabolic pathways, including FA activation (ACSS2), de novo synthesis (ACACA and FASN), uptake and transport (LPL and FABP3), desaturation (SCD1) and esterification (AGAPT6), supports the involvement of a central regulator of milk fat synthesis. In this regard, without ruling out the potential contribution of PPARG, our results suggest that SREBF1 would have a relevant role in the MFD syndrome in sheep fed FO.Among the other studied transcription factors, the tendency to a downregulation of INSIG1 was associated with that of SREBF1, whereas no variation was detected for SCAP or THRSP.Fish oil had no significant effects on the transcript abundance of CD36, GPAM, DGAT1, LPIN1 and XDH. Overall, changes in potential antilipogenic FA and mRNA abundance of candidate lipogenic genes support a relationship between them and suggest that FO-induced MFD in dairy ewes would be mediated by transcriptional mechanisms.

Supplementation of dairy ewe diet with marine lipids may be an effective strategy for modulating milk fatty acid composition but induces milk fat depression (MFD). This syndrome has been associated with a shortage of 18:0 for uptake and Δ(9)-desaturation that may impair the capacity of the mammary gland to achieve an adequate fluidity for milk fat secretion. On this basis, it was suggested that supplemental 18:0 may contribute to alleviate marine lipid-induced MFD in sheep. To test this hypothesis, 12 lactating ewes were allocated to 1 of 3 lots and used in a 3×3 Latin square design with 3 periods of 28 d each and 3 experimental treatments: a total mixed ration without lipid supplementation (control) or supplemented with 20 g/kg of DM of fish oil alone (FO) or in combination with 20 g/kg of DM of 18:0 (FOSA). Diets were offered ad libitum, and animal performance and rumen and milk fatty acid composition were studied at the end of each period. After completing the Latin square trial and following a change-over design, the in vivo digestibility of supplemental 18:0 was estimated using 6 lactating sheep. As expected, diet supplementation with fish oil increased the milk content of some potentially health-promoting fatty acids (e.g., cis-9,trans-11 18:2, trans-11 18:1, 20:5n-3, 22:5n-3, and 22:6n-3), but reduced milk fat concentration and yield (-20% in both FO and FOSA treatments). Thus, although reductions in milk 18:0 and cis-9 18:1 output caused by FO (-81 and -51%, respectively) were partially reversed with FOSA diet (-49 and -27%, respectively), the addition of 18:0 to the diet did not prove useful to alleviate MFD. This response, which could not be fully accounted for by the low digestibility coefficient of supplemental 18:0, may challenge the theory of a shortage of this fatty acid as a mechanism to explain fish oil-induced MFD in sheep. Effects of FO and FOSA on rumen and milk fatty acid composition would support that increases in the concentration of some candidate milk fat inhibitors (e.g., cis-9 16:1 or 10-oxo-18:0) might play a relevant role in this type of MFD.