Effect of linseed, sunflower, or fish oil added to hay-, or corn silage-based diets on milk fat yield and trans-C18:1 and conjugated linoleic fatty acid content in bovine milk fat

It has been well established that milk fat yield and composition are affected by different types of forage and lipid supplementation of cow diets, but there are very few direct comparisons, including different forages and lipid supplements. In this experiment, we analysed directly the interaction among 2 forage types and 3 types of lipid supplements on the responses of milk fat and fatty acid yields. Twelve Holstein cows were divided into 2 groups and received diets based on either mountain grassland hay (H, 60% of DM intake) or corn silage (C, 69%) and concentrate mixture. They were used in two replicated 3 x 3 Latin square designs (2 cows per block) to evaluate responses to feeding two basal diets (one for each Latin square) supplemented with either 2.5% fish oil (FO), 5.0% sunflower oil (SO), or 5.0% linseed oil (LO). Responses of milk fat and FA yields were different according to the interaction between different types of forage and lipid supplementation. In fact, milk fat yield was decreased by FO (when compared to other oils) with H diets, whereas it was decreased by LO with C diets. We observed a shift of transl1- to trans10-18:1 yields, due to changes in ruminal biohydrogenation (RBH) pathway, after 2 weeks with CSO and CLO diets. The milk 18:2n-6 yield was increased to a larger extent with HSO than with CSO diets. Linseed oil supplementation increased more the trans13/14- and cis15-18:1, and 18:3n-3 yields with H than with C diets. Linseed oil decreased more even saturated fatty acids (FA) than SO or FO with C diets, whereas no change was observed with H diets. Effects on milk FA yields were more pronounced with C than H diets, notably with the shift of transl1- to trans10-18:1 with C diets. For a regular milk production with a stable composition, the adaptation period for feeding a diet supplemented with vegetable lipids should be not lower than 3 weeks. We can conclude that the dose of vegetable oils that does not cause milk fat depression is under 5%. Finally, the milk nutritional quality could be improved for HLO diet (versus other diets) with lower saturated FA (SFA) and higher milk 18:3n-3 levels.