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Evidence from studies of group-housed sows in unbedded systems indicates that hunger may be a cause of increased aggression between animals. The present study aimed to investigate the effects of food level on performance, aggression and skin damage in a deep-straw system. Sixteen groups offive gilts were introduced over 8 months to one of two dynamic groups in a deep-straw yard (2·35 m2 per sow) receiving different food levels from an electronic sow-feeding system: high (H: 3·0 kg and 38 MJ/day) and low (L: 1·6 kg and 20 MJ/day). Seventy second-parity sows were returned to the same treatments in the same subgroups. The behaviour of the animals was recorded throughout both pregnancies using a time sampling technique. Severity of skin damage was recorded as a number from 0 to 4 for each of nine areas, the average of which being the ‘damage score’. The total number of animals in each dynamic group was maintained at around 30, through the use of additional sows. As expected, H sows were heavier than L sows but no effect offood treatment was found on litter size or performance. L sows were more active (L v. H; standing: 41 v. 31% of total observation time respectively, F114 = 7·65, P < 0·05) and showed more straw manipulation than H sows (L v. H: 17 v. 10% respectively, F 3 34 = 18·4, P < 0·001). No differences between treatments were found either in number of aggressive interactions (L v. H: 3·6 v. 3·8 interactions per sow per observation day, F, u = 0·82, P > 0·05), or in damage score (L v. H: 0·24 v. 0·28; Flu = 0·27; P > 0·05).However, introduction of new animals did influence aggression, with sows being more involved in aggressive interactions on the day they were introduced, than on other sow's introduction days and on no introduction days (Tukey, P < 0·05). Average skin lesions per experimental sow were also higher in the days immediately following introduction. It can be concluded that in a sequential feeding system with plentiful straw, aggression is not influenced by the level offeeding. In these systems, the major factor giving rise to aggression is the introduction of new sows to the resident group.

The need to continuously re-establish the social structure in dynamic groups of sows gives rise to higher levels of aggression compared to stable groups. Previous familiarity of animals could shorten the time it takes to reorganise the social structure, but relies on the animal's social memory.

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