Milk Fatty-Acid Profile after Feeding Increasing Doses of a Mixture of Soybean and Linseed Oils to Pasture Dairy Cows

The goal was to determine the effect of growing intake of a mixture (75:25) of soybean (SoOi) and linseed (LiOi) oils on milk production and composition and milk fatty-acid (MF-A) profile in grazing dairy cows. Twenty-four Holstein cows were assigned to 4 treatments in a completely randomized design with three weeks of adaptation to oil doses and one week of experimental measurements. On a dry matter (DM) basis, cows were fed pasture (63%), energy concentrate (37%) and the SO-LO oil mixture at zero (Tr0%), 2% (Tr2%), 4% (Tr4%) and 6% (Tr6%) of total DM intake equivalent to 0, 0.36, 0.72 and 1.08 kg/cow/day of the oil mixture. The oil mixture was manually mixed-up to the concentrate (7.04 kg DM cow/day) and supplied by halves during each milking time without refusals. Pasture (P = 0.49) and total DM intakes (P = 0.31) were similar between treatments averaging 11.27 and 18.85 kg DM cow-1 day-1 respectively. Milk output (22.71 kg/cow/day) was not affected (P = 0.46). Milk fat content reduced linearly (P < 0.05) from 3.20 (Tr0%) to 2.67 g 100 g-1 (Tr6%) without effects (P = 0.73) on fat or fat corrected milk (4%FCM) yields. Milk protein concentration (P < 0.56) or yields (P < 0.11 were not affected. Lactose contents tended (P < 0.08) to be higher in oil supplemented cows and milk urea nitrogen was not affected (P = 0.14). The basal (Tr0%) concentration (g /100 g MF-A) of totaly hypercholesterolemic MF-A (C12:0, C14:0 and C16:0) of milk averaged 38.93 and decreased linearly (P < 0.0001) with oil intake to 37.81 (Tr2%), 31.59 (Tr4%) and 29.18 (Tr6%). Levels of elaidic (trans-9 C18:1) and trans-10 C18:1 MF-A resulted low-slung in the basal (Tr0%) milk (0.21 and 0.20 g/100g MF-A, respectively) but increased linearly (P < 0.0001) after oil intake reaching the maximum values at Tr6% (0.73 and 2.23 g/100g MF-A, respectively). Milk concentration (g/100g MF-A) of vaccenic acid (trans-11 C18:1, VA) averaged 3.63 in Tr0% and increased linearly (P < 0.0001) with oil intake reaching 4.97, 7.05 and 8.38 in Tr2%, Tr4% and Tr6%, respectively. Basal concentration of rumenic acid (cis-9. trans-11 C18:2, RA) was 2.28 g/100g MF-A and increased linearly (P < 0.0001) with increased oil dose resulting in maximal plateau in Tr4% (3.88) and Tr6% (3.89). The basal  atherogenic index (AI) of milk was 1.87 and linearly decreased (P < 0.01) to 1.64 (Tr2%), 1.18 (Tr4%) and 0.95 (T6%) after oil intake. The basal Ω6/Ω3 ratio (3.57) was no different (P > 0.05) from T2% (3.37) but was upper (P < 0.05) in Tr4% (4.41) and Tr6% (4.63) remaining under the recommended value of 5:1. Taken together the results suggest that feeding a blend (75:25; SoOi:LiOi) of SiOi an LiOi oils at 4% of total DM intake to pasture dairy cows maximize the milk RA content with a concomitant decrease in the hypercholesterolemic MF-A of milk maintaining a beneficial for health Ω6/Ω3 ratio with low levels of the detrimental trans-9 C18:1 and trans-10 C18:1.