TEMPORAL CHANGES IN GROWTH ENHANCEMENT BY RACTOPAMINE IN PIGS - PERFORMANCE ASPECTS

Ractopamine is a phenethanolamine with beta-adrenergic agonist activit y which stimulates growth in pigs. In order to test whether the growth -enhancing properties of ractopamine change over time, 40 Large White crossbred gilts (47.0 kg liveweight) were assigned to five treatment g roups: CON, control diet for 6 weeks; RAC, ractopamine(20 mg/kg feed) for 6 weeks; C/R, CON for 3 weeks, then RAC for 3 weeks; R/C, RAC for 3 weeks, then CON for 3 weeks; ALT, CON for weeks 1, 3. and 5, and RAC in weeks 2, 4 and 6. The control diet was a commercial wheat and soy- based grower ration (13 MJ DE/kg, 16% crude protein, 0.9% lysine, 0.6 g available lysine/MJ DE); all pigs were fed 2.25 kg/day. Weekly measu rements included body weight and P2 backfat (by ultrasound). Due to un certainty about the data from the final week, results for weeks 1 to 3 and 3 to 5 are presented. Average daily gain (ADG) was increased by R AC (+16%) during the first 3 weeks, with no further improvement therea fter. C/R produced higher ADG (+21%) during weeks 3 to 5 compared with controls, and ALT tended to do the same (+12%, P < 0.10). Over the 5 week period, total weight gain was increased (P < 0.05) by RAC (+18%) and ALT (+15%), and tended to increase with C/R (+12%, P < 0.10) with no significant effect of R/C (+10%, P > 0.05). There were no differenc es among groups in P2 backfat during the feeding period (P > 0.05). Wi th the exception of dressing percentage, which was increased by RAC, C /R and ALT (P < 0.05), carcass and meat quality characteristics (carca ss weight, P2 backfat, loin eye area, pH, colour, drip loss and protei n solubilities) were unaffected by any of the treatments (P > 0.05). I n conclusion, ractopamine stimulated growth in finishing pigs on restr icted intakes; this effect diminished over time, and was more pronounc ed in older and heavier pigs. In addition, meat quality characteristic s were unaffected.