Aggression among sows at mixing is a major concern for group-gestation housing systems. The elevated aggression is detrimental to both health and performance of sows because it can cause injuries, increase cortisol levels and reduce pregnancy rate of the sows. However, mixing-induced aggression among sows in group housing systems is inevitable, regardless of space allowance, group size, bedding, pen design or feeding regimens.

Aggression among unfamiliar pigs is necessary to develop a dominant hierarchy within a group, a minimal level of aggression in sows at mixing cannot be eliminated. So our management strategies should focus on protecting vulnerable sows from aggression.

In most group-housing systems, gilts (i.e. female pigs that have not given birth to piglets) are housed separately to prevent aggression from older sows. However, after first farrowing, these young sows (first-parity sows) are usually housed in pens with older sows. Since young sows are smaller than mature sows, with the average body weight approximately 75 per cent of the body weight of mature sows, they are usually subordinate in group-housing systems. They lose most fights at mixing, suffer more injuries and have higher cortisol levels than mature sows.

The initial aggression results in the subordinate young sows becoming fearful of further conflicts while attempting to obtain feed and water, which leads to inadequate feed intake and reproductive failure. Failure of conception and lameness caused by initial aggression can result in young sows being culled prematurely, which reduces lifetime productivity and production efficiency of a sow herd. In terms of body weight and size, first parity sows are more like gilts rather than mature sows. So it may be appropriate to house first-parity sows in gilt-pens rather than in sow-pens to prevent these young sows from aggression so that their performance and welfare would not be compromised in group-housing systems.

West Central Research and Outreach Center has been conducting several research projects on reducing aggression among gestating sows in group-housing systems. One of the research projects was to investigate whether grouping sows by parity can improve performance and well-being of young sows in group-housing systems. This two-year project was financially supported by the Minnesota Pork Board, and the animal trial on its research farm has recently been completed. The preliminary data indicate that by housing young sows (first parity) in gilt-pens, young sows had fewer skin lesions caused by aggression at mixing than young sows in sow-pens. Although young sows in gilt-pens were involved in more fights, they won more fights than young sows in sow-pens. These young sows in gilt-pens also had greater farrowing rate (i.e. the number of sows farrowed as a percentage of the number of sows used for breeding) and gained more weight during the gestation period than young sows in sow-pens.

All these results suggest that grouping sows by parity can improve performance and well-being of young sows in group-gestation housing systems. The improvement of performance in young sows can contribute to improved lifetime productivity of these sows, and ultimately, contribute to improved production efficiency of a sow herd using group housing systems.

August 2012