Effects of diet and handling on behavioral and neurophysiological indicators of aggression in growing pigs

Rosangela Poletto, Purdue University

Abstract

Aggression among pigs is associated with formation of the social hierarchy, especially during mixing events and competition for resources. It can be a major problem for the pigs’ health and welfare due to injuries and social stress, and may reduce profitability of the swine industry. Therefore, any factor that may impact the regulation and amount of aggression, for instance, feeding of the β-adrenoreceptor agonist ractopamine (RAC), is worthy of investigation. Ractopamine is fed to finishing pigs prior to slaughter and improves growth performance and carcass leanness. By mimicking the action of catecholamines, RAC increases arousal and thus the “fight-or-flight” response, which may also 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. The effects of gender and social rank of the finishing pigs were also taken into account for the analysis of all these variables. The results of this study further support the role that RAC plays in enhancing growth performance previously reported, especially in barrows. Pigs fed RAC peaked Enterobacteriaceae shedding during the first week of feeding, which then decreased until slaughter. Pigs fed RAC had twice the front and rear hoof lesions at slaughter compared to control-fed pigs. Similarly, barrows and dominant individuals had increased hoof lesions. Ractopamine feeding increased behavioral activity (i.e. alertness and display of oral-nasal behaviors) and led pigs to engage in agonistic interactions more frequently; subordinate gilts engaged more often in non-aggressive interactions. Ractopamine feeding of gilts, but not barrows, increased the display of bites and pursuits during home pen aggression assessment, while all subgroups decreased the total number of agonistic interactions in relation to baseline. During the resident-intruder test, gilts had a higher frequency of attacks toward unfamiliar intruder pigs; this aggressiveness was intensified in gilts fed RAC, regardless of their social rank. Physiological measures showed that dominant RAC-fed pigs and dominant barrows had greater plasma norepinephrine and epinephrine concentrations, respectively. Meanwhile, RAC feeding lowered blood serotonin (5-HT) and the 5-HT metabolite (5-HIAA) concentrations while raising the dopamine turnover rate in the amygdala. The 5-HT concentration was also lower in the raphe nuclei and frontal cortex of gilts. A suppressed expression of serotonergic receptors essential for aggression inhibition (e.g. 5-HT1B), and the enzyme monoamine oxidase-A was observed in the brain of gilts. The dopamine D1 receptor expression reduced in the raphe and increased in frontal cortex, varying only as a function of RAC feeding. Thus, when combining enhanced aggressive behavior of gilts with neurophysiological profiling, it is relevant to speculate that a deficient serotonergic system, and potentially an enhanced dopaminergic activity, in brain areas vital for aggression inhibition may be linked to the behavioral pattern observed in gilts. Ractopamine may be intensifying aggression through activation of central dopaminergic and noradrenergic systems; but no alterations in the catecholamine concentrations were detected in the brain of the tested pigs due to RAC feeding. These findings led to the last experiment which targeted using tryptophan (TRP) feeding to minimize aggressive behavior in growing gilts (3 and 6 month old) through indirect augmentation of the serotonergic system. Additionally, long-term social handling was carried out as an attempt to interfere with aggression and reduce management stress. Feeding TRP at 250% of the control diet doubled blood TRP concentration and reduced behavioral activity. Additionally, aggressiveness was minimized by the high-TRP diet in 3 month old gilts. These results are likely mediated by activation of the cerebral serotonergic system; however, the reason why TRP feeding did not affect similarly the behavior of gilts at both ages 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.

Degree

Ph.D.

Advisors

Marchant-Forde, Purdue University.

Subject Area

Neurosciences|Physiology|Animal sciences

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