MODIFICATION OF FEED-INTAKE RESPONSE TO A BETA(2)-AGONIST BY BOVINE SOMATOTROPIN IN LACTATING OR DRY DAIRY-COWS

The aim of this experiment was to show whether growth hormone could in crease the effect of the beta(2)-adrenergic agonist clenbuterol on fee d intake. Two groups of Holstein cows [lactating (n = 4) and dry (n = 4)] were used to rule out the possible effects of increased energy req uirements on feed intake. Treatments were administered according to a crossover experimental design with two 13-d periods and an 8-d readjus tment interval. Treatments consisted of daily injections of 40 mg of b ST or a placebo. Moreover, clenbuterol (3.16 mu g/kg of BW) or saline challenges were infused intravenously for 4 h either on d 8 or on d 11 of each period. The cows were offered a dehydrated total mixed diet f or ad libitum intake for 4 h twice daily. For both groups of cows, cle nbuterol reduced DMI on the experimental day only, and bST intensified the effects of clenbuterol on DMI, causing a sharper decrease in DMI over 2 d. This enhancement of clenbuterol activity modified certain me tabolic parameters including higher lipid mobilization during infusion and higher glycogenolysis. For dry cows, the initial rate of eating w as reduced under bST treatment alone. During early lactation, bST, eit her by itself or through increased beta(2)-adrenergic stimulation may restrict the development of intake by dairy cows over a few weeks.