Dietary and ruminally derived trans-18:1 fatty acids alter bovine milk lipids.

The objectives of this study were to determine whether dietary fish oil enhances production of trans-18:1 isomers in the bovine rumen and whether flow of ruminally derived and/or dietary trans-18:1 to the small intestine is correlated with milk fat production. Four lactating Holstein cows with cannulas in the rumen and proximal duodenum were fed diets with 0% supplemental fat (control diet), 3% hydrogenated tallow fatty acids (HTFA diet), 1.5% menhaden oil plus 1.5% stearic acid (MO+SA diet), or 1.5% soybean oil plus 1.5% partially hydrogenated soybean oil (SBO+HSBO diet) in a 4 x 4 Latin square design. Intakes of trans-18:1 were 0, 12, 0 and 69 g/d when cows were fed the control, HTFA, MO+SA and SBO+HSBO diets, respectively. However, the extent of 18:1 + 18:2(n-6) + 18: 3(n-3) biohydrogenation in the rumen was reduced by consumption of the MO+SA and SBO+HSBO diets. As a result, flow of trans-18:1 to the duodenum was 163 g/d for MO+SA-fed cows and 152 g/d for SBO+HSBO-fed cows compared with 38 g/d for cows fed the HTFA and control diets. Incomplete biohydrogenation accounted for all of the trans-18:1 flow when the MO+SA diet was fed. Compared with results when the HTFA and control diets were fed, milk fat percentage was lower and concentration of trans-18:1 in milk fat was higher when the MO+SA and SBO+HSBO diets were fed. Across all treatments, milk fat percentage decreased linearly with the amount of trans-18:1 flowing to the duodenum and the concentration of trans-18:1 in milk fat. Results indicate that bovine milk fat percentage is depressed by trans-18:1, whether derived from the diet or from incomplete biohydrogenation of unsaturated fatty acids.