The objective was to determine the effect of increasing intake of a mixture (75:25) of soybean (SO) and linseed (LO) oils on milk production and composition and milk fatty acid (FA) 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 (T0), 2% (T2%), 4% (T4%) and 6% (T6%) of total DM intake equivalent to 0, 0.36, 0.72 and 1.08 kg cow-1 day-1 of the oil mixture. The oil blend was manually mixed to the concentrate (7.04 kg DM cow-1 day-1) 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 production (22.71 kg cow-1 day-1) was not affected (P = 0.46). Milk fat content decreased linearly (P < 0.05) from 3.20 (T0) to 2.67 g 100 g-1 (T6%) 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 content tended (P < 0.08) to be higher in oil supplemented cows and milk urea nitrogen was not affected (P = 0.14). The basal (T0%) concentration (g 100 g-1 FA) of total hypercholesterolemic FA (C12:0, C14:0 and C16:0) of milk averaged 38.93 and decreased linearly (P < 0.0001) with oil intake to 37.81 (T2%), 31.59 (T4%) and 29.18 (T6%). Levels of elaidic (trans-9 C18:1) and trans-10 C18:1 FA resulted low in the basal (T0%) milk (0.21 and 0.20 g 100 g-1 FA respectively) but increased linearly (P < 0.0001) after oil intake reaching the maximum values at T6% (0.73 and 2.23 g 100 g-1 FA respectively). Milk concentration (g 100 g-1 FA) of vaccenic acid (trans-11 C18:1, VA) averaged 3.63 in T0% and increased linearly (P < 0.0001) with oil intake reaching 4.97, 7.05 and 8.38 in T2%, T4% and T6%, respectively. Basal concentration of rumenic acid (cis-9. trans-11 C18:2, RA) was 2.28 g 100 g-1 FA and increased linearly (P < 0.0001) with increased oil dose resulting in maximal plateau in T4% (3.88) and T6% (3.89). The basal atherogenic index (AI) of milk was 1.87 and linearly decreased (P < 0.01) to 1.64 (T2%), 1.18 (T4%) and 0.95 (T6%) after oil intake. The basal n-6/n-3 ratio (3.57) did not differ (P > 0.05) from T2% (3.37) but was higher (P < 0.05) in T4% (4.41) and T6% (4.63) remaining under the recommended value of 5:1. Taken together the results suggest that feeding a blend (75:25; SO:LO) of SO an LO oils at 4% of total DM intake to grazing dairy cows maximize the milk RA content with a concomitant decrease in the hypercholesterolemic FA of milk maintaining a healthy n− 6/n−3 ratio with low levels of the detrimental trans-9 C18:1 and trans-10 C18:1.
Effects of Grass Silage and Soybean Meal Supplementation on Milk Production and Milk Fatty Acid Profiles of Grazing Dairy Cows