INTERRELATIONSHIPS BETWEEN DIETARY-PROTEIN AND RACTOPAMINE ON PROTEINAND LIPID DEPOSITION IN FINISHING GILTS

Fifty-two gilts were used to investigate the effects of two levels of dietary ractopamine (RAC; 0 and 20 mg/kg) and six levels of dietary pr otein content (DPC; 8.5, 11.2, 14.0, 16.7, 19.5, and 22.2%) on perform ance over the live weight range from 60 to 90 kg. Pigs were housed in individual pens and restrictively fed their treatment diets according to a sliding scale related to live weight (average intake approximatel y 7.0 Mcal of DE/d). The ADG increased with increasing DPC for both co ntrol and RAC-treated gilts. The RAC further increased ADG at DPC > 14 %; improvements were most apparent during the first 3 to 4 wk. There w as a significant linear interaction between DPC and RAC for average da ily carcass gain. Although carcass gain was not affected by RAC at the three lowest DPC, it was significantly increased for the three highes t DPC. Feed to gain decreased with increasing DPC in a similar manner for both control and RAC-treated gilts. The relationship between empty body protein deposition and DPC could be described by a quadratic and rectilinear model. Regardless of the model employed, protein depositi on increased with DPC at the same rate for both the control and RAC-tr eated gilts over at least the two lowest levels of DPC (< 11.2%). At h igher DPC maximal rates of protein deposition were at least 21% greate r in RAC-treated gilts; these rates were achieved at 3% higher DPC for RAC-treated gilts. Fat deposition decreased with increasing DPC but w as unchanged by dietary RAC. Because the biological efficiency of prot ein use was unchanged, RAC-treated gilts have greater dietary protein requirements than controls do.