Fatty acid profiles of milk and rumen digesta from cows fed fish oil, extruded soybeans or their blend

Four fistulated primiparous cows (two Holstein and two Brown Swiss) averaging 102 DIM were used in a 4 x 4 Latin square with 3-wk periods to determine the effect of feeding fish oil, extruded soybeans, or their combination on fatty acid profiles of milk and rumen digesta. Experimental diets consisted of: 1) control diet; 2) a diet with 2% (DM basis) added fat from menhaden fish oil; 3) a diet with 2% added fat from extruded soybeans; and 4) a diet with 1% added fat from fish oil and 1% fat from extruded soybeans. All diets consisted of 25% corn silage, 25% alfalfa hay, and 50% concentrate. Milk yields (28.6, 29.7, 29.2, and 28.1 kg/d for control, fish oil, extruded soybeans, and combination diets, respectively) were similar for all fat supplements and control. Milk fat and protein percentages (3.49, 3.08; 3.25, 2.96; 3.47, 3.01; 3.48, 2.99 for diets 1, 2, 3, and 4, respectively) were not affected by fat supplements compared with control. Dry matter intake (23.0, 21.6, 22.7, and 21.6 kg/d) was reduced when diets containing fish oil were fed. Concentrations of conjugated linoleic acid [CLA; cis-9, trans-11 CLA, 0.40, 0.88, 0.87, and 0.80 g/100 g fatty acids (FA)] and transvaccenic acid (TVA, 1.02, 2.34, 2.41, and 2.06 g/100 g of FA) were increased in milk fat by all fat supplements, with no differences in milk CLA and TVA observed among fat supplements. As with milk fat, proportions of ruminal CLA (0.09, 0.26, 0.18, and 0.21 g/100 g of FA) and TVA (2.61, 4.56, 4.61, and 4.39 g/100 g of FA) increased with fat supplements. The effects of fat supplements on ruminal TVA and CLA concentrations were also reflected in rumen FA-salts, free fatty acids, and neutral lipids. The higher TVA to CLA ratio in the rumen compared with milk indicated that fat supplements increased milk CLA concentration mainly by increasing ruminal production of TVA, which also implied the significant role that mammary delta-9 desaturase plays in milk CLA concentrations.