This study examined the effects of supplementation of fish oil and canola oil in the diet on milk yield, milk components and fatty acid composition of Holstein dairy cows in early lactation. Eight multiparous early lactation Holstein cows (4212 DIM, 406 kg daily milk yield) were fed a total mixed ration supplemented with either 0% oil (Control), 2% fish oil (FO), 1% canola oil +1% fish oil (FOCO), or 2% canola oil (CO) according to a double 44 Latin square design. Each period lasted 3 wk; experimental analyses were restricted to the last week of each period. Supplemental oils were added to a basal diet which was formulated according to NRC (2001) and consisted of 20% alfalfa, 20% corn silage and 60% concentrate. Milk yield was similar between diets (p>0.05), but dry matter intake (DMI) was lower (p<0.05) in cows fed FO diet compared to other diets. Milk fat percentage and daily yield decreased (p<0.01) with the supplementation of fish and canola oil. The daily yield and percentage of milk protein, lactose and solids-not-fat (SNF) were not affected by diets (p>0.05). The proportion (g/100 g fatty acids) of short chain fatty acids (SCFA) decreased and polyunsaturated fatty acids (PUFA) increased (p<0.05) in milk of all cows fed diets supplemented with oil. The proportions of 6:0, 8:0, 10:0 12:0 and 14:0 fatty acids in milk fat decreased (p<0.01) for all diets supplemented with oil, but the proportions of 14:1, 16:0 and 16:1 fatty acids were not affected by diets (p>0.05). The proportion of trans(t)-18:1 increased (p<0.01) in milk fat of cows fed FO and FOCO diets, but CO diet had the highest proportion of cis(c)-11 18:1 (p<0.01). The concentration of t-10, c-12 18:2, c-9 t-11 18:2, 18:3, eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6) increased (p<0.05) in FO and FOCO diets in comparison with the other two diets. These data indicate that including fish oil in combination with canola oil significantly modifies the fatty acid composition of milk. (Key Words : Fish Oil, Canola Oil, Milk Fatty Acid Composition, Holstein Dairy Cows)

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