Abstract

Eight crossbred gilts (73 kg liveweight) were surgically prepared with indwelling venous catheters to examine the temporal pattern of circulating metabolites and hormones during dietary ractopamine (RAC, 0 or 20 mg kg-1) treatment for 24 days. Pigs were bled every 2 h for 24 h on days 1, 2, 8, 22 of treatment and again 5 days after withdrawal of RAC. Dietary RAC increased liveweight daily gain and decreased feed consumption : liveweight gain by 37 and 25%, respectively. There were no differences in growth performance between the two groups of pigs during the week after withdrawal of RAC. Dietary RAC had no acute or chronic effects upon plasma concentrations of non-esterified fatty acids (NEFA) or glucose. Likewise, plasma NEFA and glucose were not different during the withdrawal period. Plasma urea nitrogen began to decrease after 24 h of dietary RAC treatment, and remained lower than for the controls for the duration of RAC treatment. Withdrawal of RAC caused a marked increase (21%) in plasma urea nitrogen. Dietary RAC decreased plasma insulin and negated the normal increase with age in plasma IGF-I. These metabolic data are consistent with increased protein deposition with little change in fat deposition during dietary RAC treatment.