ARS | Publication request: Effects of ractopamine feeding, gender and social rank on aggressiveness and monoamine concentrations in different brain areas of finishing pigs

Submitted to: International Society of Applied Ethology
Publication Type: Abstract
Publication Acceptance Date: March 15, 2008
Publication Date: August 5, 2008
Citation: Poletto, R., Cheng, H., Meisel, R.L., Richert, B.T., Marchant Forde, J.N. 2008. Effects of ractopamine feeding, gender and social rank on aggressiveness and monoamine concentrations in different brain areas of finishing pigs [abstract]. In: Eds. L. Boyle, N. O'Connell and A . Hanlon. Proceedings of the 42nd International Congress of the International Society of Applied Ethology. August 5-9, 2008, Dublin, Ireland. p. 83.

Technical Abstract: This study evaluated the effects of the feed additive ractopamine (RAC), gender and social rank on aggressiveness and brain monoamines levels of serotonin (5HT), dopamine (DA), their metabolites, norepinephrine (NE) and epinephrine (EP) in finishing pigs. Thirty-two pigs (16 barrows/16 gilts) were assigned as either control (CTL) or RAC feeding at 5 ppm for 2 wks, then 10 ppm for another 2 wks (pen=4/gender). Dominant and subordinate pigs in each pen were determined at assignment by aggressive encounter outcomes over 36 h post-mixing. Aggressiveness was measured by latency to first bite (max. 300s) using resident-intruder (R-I) tests performed for 2 days on the wk prior to start (T1), wks 2 (T2) and 4 (T3) of the trial. Resident outcomes, attack or non-attack, were also recorded. After wk 4, animals were euthanized and frontal cortex (FC), hypothalamus (HYP), amygdala (AMY) and raphe nuclei (RN) tested for monoamine levels using HPLC. A 2×2×2 analysis was computed with repeated mixed models, adjusted by Tukey post-hoc, using individuals as units. Latency to attack tended to be shorter in RAC-fed than CTL pigs (83.6 vs. 115.5±16.6s; p=0.09). Cumulative frequency of R attacks over time were significant for treatment × gender × social rank (p<0.05). By 60s, dominant and subordinate RAC gilts had performed more attacks (54.17% and 66.67%) than other individuals. By 120s, CTL dominant pigs had performed at least 50 % of their attacks. RAC-fed pigs did not differ from CTL at T1, and attacked more at T2 than T3 (treatment × week × time, p<0.05). Gilts, especially dominant ones, had lower 5HT in the FC, AMY and RN (p<0.05). When fed RAC, gilts showed lowest 5HT metabolite and greatest DA turnover in AMY (p<0.05). RAC dominant pigs had higher NE in FC; dominant barrows showed higher EP than dominant gilts in HYP (p<0.05). Dominant and subordinate gilts, mainly when fed RAC, showed more impulsive aggressive behavior towards unfamiliar intruder animals, potentially linked with 5HT deficiency and enhanced DA metabolism in brain areas essential in aggression regulation leading to impaired welfare.