ABOMASAL INFUSION OF CIS OR TRANS FATTY-ACID ISOMERS AND ENERGY-METABOLISM OF LACTATING DAIRY-COWS

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-C-18:1 fatty acid isomer s on energy utilization by mature cows was determined. Six multiparous Holstein cows in midlactation had ad libitum access to a basal diet c ontaining 50% forage and 50% concentrate. Treatments were 1) no infusi on, 2) infusion of 630 g/d of a fat mixture high in cis-C-18:1 isomers (64% cis-C-18:1; 68% high oleic sun-flower oil and 32% cocoa butter), and 3) infusion of 623 g/d of a fat mixture high in trans-C-18:1 isom ers (42% trans-C-18:1; 90% partially hydrogenated soy-bean oil and 10% high linoleic safflower oil). The experiment was a replicated 3 x 3 L atin square design with 4-wk periods. Measurements of energy balance w ere made in open circuit respiration chambers during wk 4 of each peri od. Fat infusion increased milk production by 2.5 kg/d; apparent diges tibility of DM, OM, energy, ADF, and ash by 1 to 4 percentage units; m etabolizable energy by 11%; and NE(L) of the diet by 15%. Milk fat per centage and yield were higher when cows were infused with cis-C-18:1 t han when they were infused with trans-C-18:1 (4.12% and 1.41 kg/d vs. 3.15% and 1.06 kg/d, respectively). Infusion of fat increased milk pro duction, but trans-C-18:1 reduced milk fat and energy output.