Abomasal Infusion of Cis or Trans Fatty Acid Isomers and Energy Metabolism of Lactating Dairy Cows1,2,3

Diets for dairy cows that provide or induce formation of trans isomers of polyunsaturated fatty acids result in reduced percentages of milk fat. The effect of abomasal infusion of trans-C18:1 fatty acid isomers on energy utilization by mature cows was determined. Six multiparous Holstein cows in midlactation had ad libitum access to a basal diet containing 50% forage and 50% concentrate. Treatments were 1) no infusion, 2) infusion of 630 g/d of a fat mixture high in cis-C18:1 isomers (64% cis-C18:1 68% high oleic sunflower oil and 32% cocoa butter), and 3) infusion of 623 g/d of a fat mixture high in trans-C18:1 isomers (42% trans-C18:1 90% partially hydrogenate soybean oil and 10% high linoleic safflower oil). The experiment was a replicated 3 x 3 Latin square design with 4-wk periods. Measurements of energy balance were made in open circuit respiration chambers during wk 4 of each period. Fat infusion increased milk production by 2.5 kg/d; apparent digestibility of DM, OM, energy, ADF, and ash by 1 to 4 percentage units; metabolizable energy by 11%; and NEL of the diet by 15%. Milk fat percentage and yield were higher when cows were infused with cis-C18:1 than when they were infused with trans-C18:1 (4.12% and 1.41 kg/d vs. 3.15% and 1.06 kg/d, respectively). Infusion of fat increased milk production, but trans-C18:1 reduced milk fat and energy output.