Conjugated linoleic acid and other beneficial fatty acids in milk fat fromcows fed soybean meal, fish meal, or both

Twelve multiparous Holstein cows at 48 +/- 8 DIM were used in a 4 x 4 Latin square with 21-d periods to determine the effect of replacing soybean meal with fish meal on feed intake, milk yield, and milk composition. Fish meal substituted for soybean meal on an isonitrogenous basis at 0, 25, 50, and 100% of supplemental protein. Total mixed diets were (DM basis) 25% corn si lage, 25% alfalfa hay, and 50% concentrate mix. Intake of DM (27.9, 27.8, 2 6.1, and 25.8 kg/d for diets 1 to 4, respectively) was similar for all diet s. Milk yield (37.5, 37.8, 37.2, and 37.7 kg/d) was not affected by diets. Milk protein percentages (3.23, 3.24, 3.31, and 3.35) increased with 100% f ish meal supplementation and tended to be higher with 50% fish meal supplem entation compared with 100% soybean meal diet. Milk fat percentages (3.18, 2.99, 3.04, and 2.87) and yield were lower with the 100% fish meal than wit h the 100% soybean meal diet. Concentration of n-3 fatty acids in milk fat (0.54, 0.56, 0.63, and 0.72 g/100 g fatty acids) increased as the proportio n of fish meal in the diet increased. Concentrations of c9,t11 conjugated l inoleic acid (CLA; 0.39, 0.44, 0.46, and 0.72 g/100 g fatty acids) and tran svaccenic acid (TVA; 1.09, 1.19, 1.28, and 1.54 g/100 g of fatty acids) wer e higher with the 100% fish meal diet than with the 100% soybean meal diet. A total replacement of soybean meal with fish meal in the diet of lactatin g cows increased milk protein percentages and the beneficial fatty acids (C LA, TVA, and n-3 FA) in milk fat.