Effect of feeding ractopamine hydrochloride on growth performance and responses to handling and transport in heavy-weight pigs

The impact of feeding ractopamine hydrochloride (RAC) on growth performance and responses to handling and transport in heavy BW pigs was evaluated in a study performed as a split-plot design with a 3 × 3 factorial arrangement of treatments: 1) RAC level (0 vs. 5 vs. 7.5 mg/kg of feed) and 2) handling intensity (HI; gentle vs. moderate vs. aggressive); RAC level was the main plot and HI was the subplot. A total of 288 pigs housed in groups of 8 were used to evaluate growth performance over a 28-d RAC feeding period (98.5 ± 4.58 to 131.5 ± 7.45 kg BW). On d 29 of the study, the HI treatment was applied to 216 pigs (6/pen; 2/pen on each HI). This was followed by transportation for 1 h on a livestock trailer at the end of which pigs were subjected to a final handling procedure. Blood samples (to measure acid-base, cortisol, and catecholamine levels) were collected and rectal temperature was measured 2 h before the HI treatment (baseline) and after the final handling procedure (final). Feeding RAC (5 and 7.5 mg/kg) improved (P < 0.01) ADG (9.9 and 9.0% for 5 and 7.5 mg/kg RAC, respectively) and G:F (8.8 and 11.8%, respectively) compared to controls, with no differences (P > 0.05) between the 2 RAC levels. Increasing the intensity of handling decreased (P < 0.001) final blood pH, bicarbonate, and base excess and increased (P < 0.001) final blood lactate and plasma cortisol and norepinephrine levels. Aggressive compared to gentle handling increased (P < 0.05) the incidence of pigs exhibiting open-mouth breathing and skin discoloration after the final handling procedure but had no effect (P > 0.05) on the incidence on nonambulatory, noninjured pigs. There was no effect (P > 0.05) of feeding RAC on final rectal temperature or blood acid-base measurements. Feeding 7.5, but not 5, compared to 0 mg/kg RAC increased (P < 0.05) final plasma epinephrine levels and the incidence of nonambulatory, noninjured pigs. This study confirms the improved growth performance of pigs fed RAC and the negative effects of aggressive handling on physical, metabolic, and physiological responses of pigs. It also suggests that pigs fed 5 compared to 0 mg/kg RAC showed similar responses to transport and handling. However, pigs fed 7.5 mg/ kg of RAC had a greater incidence of nonambulatory, noninjured pigs when subjected to the handling/transport model and this warrants further investigation.