A study of the relative aversiveness of a new daily injection procedure for pigs

Abstract

This experiment examined the aversiveness of a new injection procedure (“Injection treatment”), developed for the daily administration of procine somatotropin to pigs, which was imposed on pigs over a 3 week period late in the growing phase. The injection was via a 13mm, 16 gauge needle and utilized a low penetration gas injection gun. Observations were conducted on the escape-avoidance responses of treated pigs to this injection procedure and to humans. In addition, the cortisol response to treatment and the cortisol response of pigs to an ACTH challenge were determined and the adrenal glands were weighed. To gauge a relative measure of the magnitude of these behavioural and physiological responses, a positive control (“Positive treatment”, a treatment with rewarding components such as a human patting and stroking approaching pigs), a negative control (“Negative treatment”, a treatment that is clearly aversive involving regular electric shocks) and a neutral control (“Control treatment”, a treatment with minimal human contact, similar to the amount of contact which occurs during routine husbandry) were included in the evaluation. Based on the relative behavioural and cortisol responses to treatment, the Injection treatment was judged to be moderately aversive. This relative assessment is based on three findings. Firstly, the escape-avoidance responses of pigs to the Injection treatment were intermediate between those of pigs to the Negative treatment and to the Control and Positive treatments. Secondly, the cortisol response of pigs to the Injection treatment was similar to that of pigs to the Control and Positive treatments but was lower ( P ≤ 0.05) than that of pigs to the Negative treatment. Thirdly, in response to humans in a standardized test, pigs in the Injection treatment behaved similarly to those in the Control treatment for three of the four variables measured and significantly ( P ≤ 0.05) different than those in the Negative treatment for three of the four variables. Although there was evidence that the pigs in the Negative treatment may have experienced a chronic physiological stress response with a significant ( P ≤ 0.05) depression in growth rate, there was neither physiological nor production evidence to indicate that the Injection treatment adversely affected the long-term stress physiology of the pigs. In conclusion, the daily imposition of this new injection procedure over a 3-week period is moderately aversive to pigs and the welfare of these pigs is similar to that of pigs receiving minimal human contact as occurs in the routine husbandry of growing pigs.