GLUCOSE AND NOREPINEPHRINE CHALLENGES DURING ABOMASAL INFUSION OF CISOR TRANS OCTADECENOATES IN HOLSTEIN COWS

This experiment determined the effects of infusion of mixtures of fat containing predominantly cis-C-18:1 or trans-C-18:1 fatty acids into t he abomasum on responses of cows to glucose and norepinephrine challen ges administered i.v. Six lactating Holstein cows, each with a rumen c annula, were arranged in two Latin squares with 21-d periods. The comm on basal diet contained 40% forage and 60% concentrate. Treatments wer e the uninfused control, 750 g/d of a cis fat mixture (65% high oleic sunflower oil and 35% cocoa butter), and 750 g/d of a trans fat mixtur e (93% shortening and 7% corn oil) infused into the abomasum via a tub e that passed through the rumen cannula. Glucose challenges (0.4 mg/kg of BW, administered i.v.) were conducted on d 18, and norepinephrine challenges (0.7 mu g/kg of BW, administered i.v.) were conducted on d 19 of each experimental period. Despite a lower percentage of fat in m ilk for trans than for cis treatment, disappearance rates of glucose, secretion of insulin after glucose challenge, and appearance rates of NEFA and triglycerides after norepinephrine challenge mere similar bet ween treatments. Thus, these data support the hypothesis that trans-C- 18:1 fatty acids affect the synthesis of milk fat in the mammary gland of lactating cows.