Aggression can impair productivity and well-being. The association between aggression in finishing pigs and the feed additive ractopamine (RAC), a β-adrenoreceptor agonist, is unknown and warrants further investigation. Our goal was to examine behavioral activity, including aggression, in the home pen and concentrations of peripheral amines in barrows and gilts, taking into account diet (RAC) and social rank. Sixty-four finishing pigs, housed in pens of 4 by sex, were fed either a control (CTL) or RAC-added (5 mg/ kg for 2 wk plus 10 mg/kg for another 2 wk) diet. The top dominant and bottom subordinate pigs in each pen were determined at mixing (2 wk pretrial). The behavior of all pigs was recorded continuously during the pretrial week (baseline) and for the following 4 wk. These behavioral data were used to evaluate home pen aggression, including the number of agonistic interactions (AINX) and constituent aggressive actions, during a 3-h period (0800 to 1100 h) once per week and their change in relation to the baseline. Time-budget behaviors and postures were analyzed over eight 24-h periods (2 d/wk) using 10-min instantaneous scan sampling that focused on only the dominant and subordinate pigs in each pen. These 2 pigs were also subjected to blood collection once per week during the trial to determine concentrations of dopamine, norepinephrine, epinephrine, and serotonin (5-HT) using HPLC. Gilts performed more bites and total actions per AINX than barrows, and RAC-fed gilts increased bites and pursuits, whereas these behaviors decreased compared with baseline values in all other subgroups. Gilts fed RAC increased the total number actions per AINX, whereas the occurrence of AINX decreased for all subgroups. Overall, RAC-fed pigs were more behaviorally active, spending more time alert, bar biting, and sham chewing compared with CTL pigs. The dominant RAC-fed pigs tended to have the greatest norepinephrine concentrations among the tested subgroups. Dominant barrows had greater epinephrine concentrations than subordinate barrows. The RAC-fed gilts tended to have lesser 5-HT concentrations than CTL gilts, whereas concentrations were similar in barrows. Greater activity and the increase in oral-related behaviors observed in RAC-fed pigs may be mediated by the increase in arousal caused by RAC. Intensified aggression in gilts, especially when fed RAC, may be linked to reduced central 5-HT and greater noradrenergic activity, and further research on brain neurotransmitters in gilts is needed.

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