EFFECTS OF RACTOPAMINE ON THE GROWTH AND CARCASS QUALITY OF ENTIRE MALE AND FEMALE PIGS FED AD-LIBITUM OR AT A RESTRICTED LEVEL

The efficacy of ractopamine (Rac) at 0, 5, 12.5, or 20 mg/kg of the di et for entire male and female pigs of a lean genotype was assessed in two experiments. In Experiment 1 (n = 56) pigs were fed a diet contain ing 16.5% crude protein ad libitum for 40 days from an initial weight of c. 50 kg, whereas in Experiment 2 (n = 80) the same diet was fed at a restricted level (c. 87% ad libitum) according to liveweight from 5 0 kg to a final weight of 87 kg. For Experiment 1 (ad libitum) Rac had no effect on growth rate of either sex and no effect on carcass compo sition for males, but for females the carcass lean content increased a nd carcass fatness decreased. For Experiment 2 with pigs fed at a rest ricted level, Rac increased the rate and efficiency of growth, the kil ling-out percentage, and the carcass lean percentage for both sexes, b ut levels of fatness were not affected. Rac had no effect on Warner-Br atzler shear force values for either experiment, but for Experiment 1, the longissimus muscles from females receiving Rac had higher ultimat e pH values, lower cooking losses, and darker colour. Entire males in contrast showed no effect of Rac on pH, but drip loss values were high er and the meat was a lighter colour for pigs fed Rac. It is concluded that the response of pigs to Rac under pig production systems in use in New Zealand may depend on the sex of the pig and on whether or not they are fed at a restricted level.