Interrelationships between dietary protein and ractopamine on protein and 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 protein content (DPC; 8.5, 11.2, 14.0, 16.7, 19.5, and 22.2%) on performance 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 approximately 7.0 Mcal of DE/d). The ADG increased with increasing DPC for both control and RAC-treated gilts. The RAC further increased ADG at DPC > 14%; improvements were most apparent during the first 3 to 4 wk. There was a significant linear interaction between DPC and RAC for average daily carcass gain. Although carcass gain was not affected by RAC at the three lowest DPC, it was significantly increased for the three highest 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 deposition increased with DPC at the same rate for both the control and RAC-treated gilts over at least the two lowest levels of DPC (< 11.2%). At higher DPC maximal rates of protein deposition were at least 21% greater in RAC-treated gilts; these rates were achieved at 3% higher DPC for RAC-treated gilts. Fat deposition decreased with increasing DPC but was unchanged by dietary RAC. Because the biological efficiency of protein use was unchanged, RAC-treated gilts have greater dietary protein requirements than controls do.