Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/12/2009
Publication Date: 3/1/2010
Citation: Poletto, R., Meisel, R.L., Richert, B.T., Cheng, H., Marchant Forde, J.N. 2010. Aggression, behavior and peripheral amine concentrations in relation to ractopamine feeding, sex, and social rank of finishing pigs. Journal of Animal Science. 88:1184-1194. Interpretive Summary: Ractopamine is a feed additive commonly used in the swine production for its enhancing effects on growth performance and carcass yield in finishing pigs. Ractopamine also mimics the effects of norepinephrine and epinephrine, the fight or flight hormones, increasing arousal. Thus, this increase in arousal may also contribute to more frequent inter-individual interactions and heighten occurrence of fights. The effects of ractopamine feeding of finishing pigs on aggression, an issue of concern for its potential to impair productivity and well-being, warrants further investigation. The current study showed that ractopamine feeding caused the finishing pigs to become more active, spending more time alert, bar-biting and sham-chewing (an abnormal oral behavior). Engagement in agonistic interactions was also more frequently observed in the ractopamine-fed pigs and subordinate female pigs. Ractopamine feeding of female, but not male pigs, increased retaliatory and aggressive behaviors such as bites and pursuits, while the number of fights decreased for all pigs in relation to pre-trial behavioral observations. Dominant social status of ractopamine-fed pigs was associated with greater norepinephrine concentration. Ractopamine feeding also lowered peripheral serotonin, which is essential for aggression inhibition, in female pigs. Greater activity and increase in oral-nasal behaviors observed in the ractopamine-fed pigs are indicative of the augmentation of arousal caused by the feed additive-mediated effects. Furthermore, the increase in aggressive behaviors in the ractopamine-fed female pigs may be linked to central serotonin deficiency and greater sympathetic activity. Intensified aggression associated with ractopamine feeding in gilts is likely to impact the pigs’ well-being by increasing social stress.
Technical Abstract: The effects of the widely used feed additive ractopamine (RAC) on behavioral and physiological responses in finishing pigs are limited. Aggression is of concern for its potential to impair pig’s productivity and well-being; thus its relation with feeding of RAC, an adrenoreceptor agonist, warrants further investigation. Our goal was to evaluate the effects of RAC feeding, gender and social rank on behavioral activity, on the levels of aggression at the home pen and peripheral concentrations of amines in finishing pigs. Sixty-four pigs housed in pens of 4/gender were fed either control (CTL) or RAC (5 mg/kg for 2 wk plus 10 mg/kg for another 2 wk) diets. Social rank of the pigs in each pen was determined post-mixing and their behavior was recorded for 5 wk to determine behavioral activity and postures using 10-min scan sampling method. Aggressive behavior at the home pen was continuously recorded for a 3-h period once/wk for 5 wk and the number of fights and agonistic actions displayed per fight were taken. Blood was collected from the dominant and subordinate pigs once/wk for 5 wk and concentrations of tryptophan (Trp), serotonin (5-HT), dopamine (DA), norepinephrine (NE) and epinephrine (EPI) were measured by high performance liquid chromatography. The RAC-fed pigs spent less time inactive or more time alert, bar-biting and sham-chewing compared to the CTL pigs (P < 0.05). Engagement in fights was greater in subordinate gilts and RAC-fed pigs (P < 0.05). Gilts showed more bites/interaction compared to barrows, and the RAC-fed gilts increased bites while others decreased when compared to baseline values (P < 0.05). Pursuits increased greatly from the baseline in RAC-fed gilts but decreased for other subgroups (P < 0.01). Overall, gilts performed more total actions per fight and the RAC-fed gilts increased the total number actions performed (P < 0.01). The RAC-fed dominant pigs tended to have highest NE concentrations among the tested groups (P = 0.07). The dominant barrows had higher EPI than subordinate barrows (P < 0.05). The RAC-fed gilts had lower 5-HT concentration than CTL gilts (P < 0.05) while it was similar in barrows (P > 0.10). Greater activity and increase in oral-related behaviors observed in RAC-fed pigs may be mediated by the increase in arousal caused by RAC. Furthermore, gilts, especially when fed RAC, showed more aggressive behaviors with potential linkage to lower 5-HT availability and greater noradrenergic activity, leading to increased impairment of pig well-being.