Effect of group size on feeding behaviour, social behaviour, and performance of growing pigs using single-space feeders

Use of computerized feed intake recording (CFIR) equipment allows automatic recording of individual feed intake of animals kept in groups. However, current CFIR systems have only one feeding space per social group which may lead to increased competition for access to the feeder. The present experiment examined the effect of increased competition around a single-space feeder on individual performance and behaviour by manipulating the number of pigs per feeder. 150 male crossbred pigs were penned at 34 ± 0.3 kg (mean ± SE) in groups of 5, 10, 15, or 20 for 29 days with one single-space computerized feeder per pen and 1.06 m2/pig. Pigs kept in groups of 20 made fewer but longer visits to the feeder and ate more and faster than pigs kept in the smaller groups (7.1 vs. 14.3 visits/day, P < 0.01; 6.91 vs. 4.64 min/visit, P < 0.05; 214 vs. 119 g/visit, P < 0.05; 31.6 vs. 25.9 g/min, P < 0.05; means of group size 20 vs. means of group size 5, 10, and 15, respectively). No differences were found between group sizes in daily feed intake, daily live-weight gain and feed conversion ratio (overall means: 1490 g/day, 725 g/ day and 2.09 g/g, respectively). Mean number of aggressive interactions initiated and won were significantly lower for the two largest group sizes on the day of grouping. A rank index based on pairwise aggressive interactions was not correlated to any of the production or feeding behaviour variables. No significant effect of group size was found on mean number of attempts to displace other pigs from the feeder. The change in feeding pattern in the largest groups appears to represent an adaptation to the constraint placed on their feeding behaviour and they succeed in adjusting to the lack of feeding space as there were no significant differences between groups in production variables. A correlation between performance and social behaviour may have been disguised by environmental factors such as space allowance and straw provision or, alternatively, aggression and growth may be independent characteristics.

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