Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: July 11, 2010
Publication Date: March 15, 2010
Citation: Poletto, R., Richert, B.T., Cheng, H., Meisel, R.L., Marchant Forde, J.N. 2010. Effects of Diet and Handling on Behavioral and Neurophysiological Indicators of Aggression in Growing [abstract]. Journal of Animal Science, American Society of animal Science, Midwest Meeting, March 15-17, 2010, Des Miones, Iowa. 88:48-49 (E-Suppl. 3). Technical Abstract: Aggression among pigs is associated with formation of the social hierarchy, especially during mixing events, and competition for resources, and can interfere with pigs’ health and welfare due to injuries and social stress, thus reducing profitability of the swine industry. Therefore, any factor that may impact the regulation and amount of aggression, for instance, feeding of the ß-adrenoreceptor agonist ractopamine (RAC), is worthy of investigation. Ractopamine is fed to finishing pigs prior to slaughter and is very efficient in improving growth performance and carcass leanness. But by mimicking the action of catecholamines, RAC increases arousal and thus the “fight-or-flight” response, which may lead to increased aggression. In order to test this hypothesis, experiments investigating the production performance measures, health indicators (Enterobacteriaceae shedding and hoof lesions), and behavioral and neurophysiological measures related to aggression were carried out, also taking into account the effects of gender and social rank of the finishing pigs. The results of this study further support the role that RAC plays in enhancing production performance, especially in finishing barrows (P < 0.05). Pigs fed RAC peaked Enterobacteriaceae shedding during the first week of feeding, which then decreased until slaughter; meanwhile RAC-fed pigs had twice the front and rear hoof lesions at slaughter compared to control-fed pigs (P < 0.05). Similarly, barrows and dominant individuals had increased hoof lesions (P < 0.05). Overall, RAC increased behavioral activity (i.e. alertness and display of oral-nasal behaviors) and RAC-fed gilts, but not barrows, increased the display of bites and pursuits during home pen aggression assessment, while decreasing the number of agonistic interactions, suggesting augmentation in severity of fights among gilts (P < 0.05). During the resident-intruder test, which measures aggressiveness, gilts also attacked more unfamiliar intruder pigs; this aggressiveness was intensified in gilts fed RAC, regardless of their social rank (P < 0.05). Both dominant RAC-fed pigs and dominant barrows had greater plasma catecholamine concentrations and greater dopamine turnover rate in the amygdala (P < 0.05), the brain center for emotion control. Meanwhile, RAC feeding lowered concentrations of serotonin (5-HT), a key inhibitor of aggression, in the blood of gilts. Concentrations of 5-HT and its metabolite 5-HIAA were also lower in the brain of gilts. A suppressed expression of serotonergic receptor genes essential for aggression regulation (e.g. 5-HT1B), and the enzyme monoamine oxidase-A was observed in the brain of gilts. Thus, when combining enhanced aggressive behavior with neurophysiological profiling, it is relevant to speculate that an enhanced catecholaminergic system but more importantly a deficient serotonergic system may be linked to the behavioral pattern observed in gilts. Ractopamine may be indirectly intensifying aggression through activation of central regulatory mechanisms of aggression. On the other hand, feeding tryptophan (TRP) at 250% of the control diet to 3 and 6 month old maternal line gilts doubled blood TRP concentration and reduced behavioral activity (P < 0.05). Additionally, aggressiveness was minimized by the high-TRP diet but only in 3 month old gilts (P < 0.05). These results are likely mediated by activation of the cerebral serotonergic system; however, the reason why TRP feeding did not affect the behavior of gilts at both ages similarly remains unclear. Long-term social handling of gilts improved growth performance and reduced time-budget behavioral activities, especially when associated with TRP feeding. Linking behavioral with neurophysiological profiling of growing pigs is a valuable approach for elucidating potential mechanisms related to aggression. The understanding of these underlying mechanisms, and their relation with production practices including dietary manipulation (i.e. feed additives such as RAC and TRP) and social handling, may be an essential component to improve productivity and welfare of growing and finishing pigs.