Lipid metabolism and intake behavior of dairy cows: effects of intravenous lipid and beta-adrenergic supplementation.

The goal of this experiment was to determine whether modulation of beta 2-adrenoceptors influenced DMI of lactating dairy cows. Because stimulation of these receptors induces mobilization of body fat stores, the effect of such stimulation on feed intake was compared with the effect of an intravenous administration of lipid nutrients. Four treatments were infused intravenously over a 4-h period: 1) a beta 2-agonist (clenbuterol), 2) a beta-blocker (propranolol), 3) triglyceride emulsion (lipids), or 4) saline. Two trials were carried out for the same four cows to compare the effects of two expected lipolytic situations. Trial 1 used cows at 60 DIM treated with infusions from 0300 to 0700 h, and trial 2 used cows at 92 DIM treated with infusions from 0800 to 1200 h. Each trial was composed of three consecutive replications of a 4 x 4 balanced Latin square design with 2-d periods. A complete diet (70% maize silage and 30% concentrate) was offered for ad libitum intake. In both trials, clenbuterol reduced DMI on the day of infusion (-7.9 and -11.2% for trials 1 and 2, respectively) and on the following day (-5.1 and -6.3% in trials 1 and 2, respectively). This decrease occurred some time after the end of infusion. Intravenous lipid supplementation reduced DMI during infusion in both trials and during the entire day in trial 2. Both clenbuterol and lipids increased plasma NEFA only during infusion. Propranolol had no effect on DMI or basal lipolysis. The data demonstrated that stimulating beta 2-adrenoceptors reduced DMI but in a manner different from that of lipid energy supplementation.