THE SELECTION OF PROTEIN-INTAKE BY PIGS TREATED WITH PORCINE SOMATOTROPIN

The hypothesis that pigs can select a protein intake to meet their lean growth requirement when given a choice of two diets, differing in protein content, was tested in this study. Seventy-two gilts (PIG Camborough x Canabrid), of an average initial body weight of 20 kg, were allotted to one of four pens with half-slatted concrete floors and two electronic feeders. Two pens of pigs were assigned to a choice-feeding system (choice) and two pens to a single-feeding system (single). Half of the pigs in each pen received a daily injection of 100 mu g/kg BW of porcine somatotropin (ST) and the other half received a daily injection of saline. Feed intake of individual pigs was recorded on a daily basis with the Feed Intake Recording Equipment (FIRE) system. Pigs assigned to the choice-feeding system were given two feeds of different protein content, 10 or 24%, but with similar digestible energy content (13.7 MJ of DE/kg). The single diet contained 16% CP during the grower period and 14% throughout the finisher period. Overall weight gain of pigs was not affected by feeding system or ST injection; however, feed intake was decreased by injection of ST (P = .006) and by choice feeding(P = .005). Choice-fed pigs showed decreased dietary protein intake compared with single-fed pigs. The increased deposition rate of lean and protein with injection of ST did not stimulate an increase in protein consumption. Choice feeding and ST injection lowered the amount of feed or protein required per unit gain, or per unit of lean or protein deposited. Injection of ST significantly decreased the amount of fat and increased the lean and bone content of the wholesale prime cuts. Choice feeding and ST injection resulted in a decrease (P = .001 to .09) in backfat thickness. There was a significant increase in protein and unsaturated fatty acid (UFA) and a decrease in DM, crude fat, and saturated fatty acid (SFA) content in lean tissue of ST-treated pigs. Choice feeding resulted in (P = .004) a decrease in UFA and an increase of SFA. Injection of ST increased plasma urea nitrogen, GH, IGF-I, and insulin concentration and decreased plasma glycerol concentration. The deposition rates of lean, fat, and bone were not affected by choice feeding. Injection of ST increased (P = .003 to .001) the deposition rates of lean, bone, and protein and decreased (P = .001) the deposition rate of fat tissue.