INFLUENCE OF LEVEL OF DIETARY-PROTEIN OR ENERGY ON EFFECTS OF RACTOPAMINE IN FINISHING SWINE

The effect of ractopamine, a beta-adrenergic agonist, on growth, nutrient utilization, and carcass composition was studied in pigs fed either 18% CP, 12% CP, or 18% CP restricted (RES = 67% of ad libitum) diets. The 18 and 12% CP diets provided 3.52 and 3.68 Mcal of DE/kg, respectively. All pigs were fed a low-protein (12% CP) diet during pretreatment growth from 15 to 60 kg. Ractopamine at 20 or 30 ppm (30 ppm for RES pigs) in the diet was fed from 60 kg live BW until slaughter at 105 kg (9 pigs/treatment). No ractopamine treatment effect (P > .05) was observed for either daily gain or gain/feed, although gain/feed was improved by 8% in both of the ad libitum groups. Ractopamine treatment resulted (P < .01) in an overall reduction of carcass lipid by 8%, an increase of carcass protein by 5%, and a 21% improvement in the efficiency of protein utilization; the greatest changes occurred in the pigs fed the 12% CP diet (-17%, +11%, and +32%, respectively). The ad libitum daily feed intake was 15% less for pigs fed the 12% CP diet than for those fed the 18% CP diet (P < .01), and there was a 10% reduction in intake of both diets with the addition of ractopamine (P < .05). Both carcass lipid and protein deposition seemed to be closely related to energy intake (P < .01). Throughout the range of protein and energy intake, the repartitioning effect of ractopamine resulted in a reduction in carcass lipid accompanied by an equivalent increase in energy retention in the form of carcass protein.