Pig Aggression

by 5m Editor
22 March 2004, at 12:00am

By The Scottish Agricultural College - Under modern commercial conditions, pigs are mixed with unfamiliar animals several times during their lives, and this leads to aggressive behaviour such as fighting and bullying. The aggression that occurs at mixing results in skin lesions. Losers of fights show signs of social stress, such as trying to escape from their attackers and reduced activity. This article provides the Introduction to work done at the SAC on Pig Aggression and links to the other pages of the report.









Mixing Piglets


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Following the UK ban on individual housing, pregnant sows are now housed in social groups where mixing occurs. As well as the clear welfare problems of the aggression that this can cause, there is also an economic cost to the producer including sow fertility and piglet production problems. Information on reducing aggression between sows at mixing can be found in the 'Management of sows at mixing' booklet.

In growing pigs the production problems can include reduced weight gain, increased disease risks, reduced carcass gradings (if pigs are mixed prior to slaughter), and sometimes death (Stookey & Gonyou 1994). More information on the effect of group size on aggression can be found in the 'Housing growing pigs in large groups' booklet.

Previous approaches to the problem of aggression have involved designing production systems to limit the need for re-grouping (e.g. Stolba & Wood-Gush 1984), but these have not been widely adopted, largely because of cost, and the need for increased labour inputs. In any case, it is difficult to eliminate mixing entirely from a production system.

A variety of other approaches which involve intervening in some way at mixing- for example by using tranquillising drugs or distracting odours have also met with limited success, because they simply delay rather than prevent the aggression occurring. These interventionist approaches to reduce aggression on mixing have had limited success (Petherick & Blackshaw 1987), perhaps because unfamiliar pigs are motivated to assess one anothers' competitive abilities (Rushen 1988).

Experiments at SAC

At SAC, work has focussed on the finding that individual pigs react to the challenge of mixing in different ways (Mendl et al. 1992). Some show a propensity to be highly aggressive, whereas other pigs integrate into a new group with very little fighting. Individual differences in the aggressive ‘personality’ of pigs such as these can be measured using a standard test: the resident-intruder attack latency test (Erhard and Mendl 1997).

Pigs are tested individually with a strange ‘intruder’ pig in a portion of their home pen. Whether or not the pig attacks the intruder, and how quickly they attack (the attack latency) are used to measure aggressiveness. When pigs were then mixed according to their test scores, more aggressive pigs fought more and took longer to integrate into a new group (Erhard et al. 1997). The resident-intruder test appears to be a promising method for measuring and investigating aggressiveness in pigs, and one strand of recent research has focussed on some issues surrounding the use of this test. The test was used successfully, and additional behavioural measures of aggressiveness have been developed.

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Source: Scottish Agricultural College - February 2004