Effects of food and time of day on aggression when grouping unfamiliar adult pigs

The effects of manipulating bioschedules on the number of aggressive interactions and skin lesions following grouping of four unfamiliar, adult ovariectomized pigs were examined in an experiment involving 24 pigs in five replicates. There were six treatments: food present at the time of grouping in the morning or the afternoon (Food-am and Food-pm), food present ad libitum for either 24 or 48 h (Food-24h and Food-48h), pigs grouped 30 min after sunset (Dark) and a Control. The latter treatment pigs were introduced into the pen in the morning after feeding. All treatments were in rectangular pens providing a space allowance of 1.4 m 2 per pig. Aggressive interactions were observed for 90 min immediately following grouping and for 40 min periods around feeding on the day following grouping and 9 days later. Skin lesions were assessed 3 days after grouping. Social behaviour and spatial use of the pens were also determined during a 60 min period between 14:00 and 16:00 h on the day following grouping and 9 days later. Fewer ( P < 0.05) aggressive interactions occurred during the period 30–90 min after grouping in the Dark than the Food-am and Food-48h treatments. The Dark and Control treatments were quicker (0.001 < P < 0.05) to first lie down and to concurrently lie down compared with the other treatments where food was provided. Provision of ad libitum food for either 24 or 48 h reduced (0.01 < P < 0.05) aggressive interactions around feeding only while food was present; on Day 10 of grouping there were no treatment effects ( P > 0.05). There were no effects of treatment on the number or length of lesions, or the spatial use of the pen during a rest period ( P > 0.05). However, the number of aggressive interactions during the initial 15 and 90 min of grouping were significantly regressed with the length of lesions measured 3 days after grouping (0.01 < P < 0.05).