The objective of this study was to supplement dairy cow diets with a mixture of rapeseed oil and fish oil at a low dose to modulate rumen and milk fatty acids without affecting other rumen fermentation and productive parameters. Our study was carried out on 14 lactating Polish Holstein-Friesian cows during their productive lives. One cow within each group was fitted with rumen cannulas. Animals were subjected to one of the treatments: 1) CON; total mixed ration (TMR) without oil supplementation,2) FRM; consisting of TMR + 360 g/day/animal of rapeseed oil and fish oil in a 1:1 mixture. Milk production recorded throughout the experiment was not affected by oil mix supplementation; additionally, the whole tract digestibility did not differ when compared with the control group. No negative effects were also observed on the total rumen protozoan population, volatile fatty acids and methanogens; however, total bacterial counts were slightly affected (P>0.05) after 3h of feeding as an immediate effect of oil mix addition, which later disappeared at 6 h post-feeding. Feeding dairy cows with rich sources of long chain polyunsaturated fatty acids at a low dose (360 g/day/animal) resulted in favourable changes in milk fatty acid content without affecting milk fat concentration. Milk from dairy cows fed a diet with oil supplement had more (P>0.05) C18:2 c9t11 and C18:2 t10c12, by 30% and 38%, respectively, when compared to the control group. In addition, the level of supplement used in our study increased the n-3 fatty acids proportion significantly when compared to the control diet.Hence, we can conclude that using 360 g/day/animal (about 1.8% of dry matter intake) positively influenced the milk fatty acid content without any adverse effect on cows’ productivity.
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