Aggression can be a major problem for swine production as it negatively impacts the pigs’ health and welfare. Increasing tryptophan (TRP) intake to raise brain serotonin (5-HT)—key for aggression control, and long-term positive social handling can reduce stress in pigs. Objective was to feed a short-term high-TRP diet to grower (3 months) and finisher (6 months) maternal gilts that were either socially handled or not and measure their behavioural activity and aggressiveness. Eight pens of six unrelated gilts were split into two blocks balanced for litter, social handling (non- vs. handled) and dietary treatment (control vs. high-TRP). Social-handling was applied three times per week, from day 45 until 6 months of age. At 3 months, two handled and two non-handled pens were assigned to control while the other four pens were assigned to the high-TRP diet fed ad libitum for 7 days. At 6 months of age, pen assignment to dietary treatments was swapped. Body weights and blood were taken at the start and at the end. Blood samples were analyzed for TRP and 5-HT concentrations using high pressure liquid chromatography. Behaviour was recorded from days 1 to 5 and scan-sampling used to determine time-budget behaviours and postures in a 12-h period each day. Aggression evaluation in the home pen focused on counts of agonistic interactions, bites and head-knocks per interaction during three, 30-min intervals (08:00, 12:00, and 16:00 h) from days 1 to 5. Resident–intruder test was carried out for a maximum of 300s at days 6 and 7 to measure aggressiveness, predicted by the latency to the first attack and attack outcomes. A 2×2 factorial arrangement of dietary treatment and social handling within age was analyzed by repeated measures of mixed models and Tukey adjustments. The TRP-added diet raised blood TRP concentration of 3- and 6-month-old gilts by 180.7% and 85.2% respectively, reduced behavioural activity and time spent standing, while increasing lying behaviour, mostly in grower gilts. High-TRP diet reduced the number of agonistic interactions, and aggressiveness in 3-month old gilts, which took longer to attack the intruder pig, and displayed fewer attacks on the first day of testing. Long-term positive social handling improved growth performance and had a slight effect on behaviour (P< 0.05). Provision of enhanced TRP diet reduced behavioural activity and aggressiveness of grower gilts, and these results are likelymediated by activation of brain serotonergic system. Short-term high-TRP dietary supplementation may be used to reduce aggression at mixing in young pigs.

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