In group-housed sows, when similar groups of sows are maintained throughout the gestation period, aggression between groups is reduced, and the culling rate is reduced due to non-productive parameters, increasing the sows' lifetime productivity.

Researchers at the Prairie Swine Centre in Saskatchewan, Canada studied aggression and injuries resulting from grouping sows after breeding.

Searching for methods to reduce aggression, sows were regrouped within a few days of breeding, using five experimental social groups.

Groups contained 16 bred sows. All animals had previous experience in group housing with an electronic sow feeder. Sows were regrouped at 11 days after breeding and placed in a mixing pen on 80 sq. ft. of partially slotted floor.

Each group consisted of approximately equal numbers of Parity 1-2 sows and Parity 3-6 sows. Less than 50% of all penmates within the respective groups were familiar, having shared a pen during the previous gestation.

The five treatment groups included:

  • Control: Group of sows formed as above;

  • Familiar: Sows from the same previous gestation group;

  • Dominant: Standard group of sows and three socially dominant animals that were large (Parity 5 or higher), housed together for at least six weeks, and well acquainted with the mixing pen;

  • Protected: Standard group of sows, but provided with seven free-access half-stalls to offer protection to head and shoulders; and

  • Exposed: Standard group of sows, except these sows were weaned directly into the mixing pen and held there for 48 hours before being moved to the breeding stall.

After regrouping, saliva samples and data on aggression and injuries were collected.

Scores for aggression and injuries among the five social management treatments are detailed in Table 1. The “familiar” treatment, where group integrity is maintained, appears to provide the best opportunity for reducing aggression. Researchers believe that the relatively short fights that occurred among familiar sows probably represented reinforcement of social position rather than the establishment of new hierarchy.

In the “dominant” group, the presence of three older animals from a well-established social order produced fewer aggressive events, particularly on the first day of group formation. The concept behind this treatment is that sows would avoid aggression when in the presence of a clearly dominant individual.

The “exposed” treatment, where sows had spent 48 hours together after weaning, but before being placed in stalls for breeding, did not reduce the incidence of aggression compared to the “control” group, except on the first day. But the level of injuries was reduced.

Researchers noted the short period of pre-exposure used in this study may have only provided a weak social order that required additional establishment after the subsequent regrouping.

This study confirmed other reports showing the ineffectiveness of protective stalling on the aggression among regrouped sows, as in the “protected” group.

No differences were observed in overall salivary cortisol concentrations — a measure of stress levels — among the five treatment groups (Table 1).

However, there were differences in cortisol levels on different days, with the lowest concentration prior to regrouping, and the highest levels on all the days following regrouping (Table 2).

In conclusion, maintaining sows in similar groups from gestation period to gestation period increases the odds of reduced aggression, compared to the other regrouping strategies tested.

But this method would not always be practical. Having dominant sows within the group tended to reduce aggression and injuries on the first day following regrouping. A similar trend was found when sows were exposed to each other before breeding. Protection during regrouping didn't prevent aggression or injuries.

Researchers: H.W. Gonyou and S.M. Hayne, both of the Prairie Swine Centre (PSC). Contact Ken Engele of the PSC by phone (306) 373-9922, fax (306) 955-2510 or e-mail ken.engele@usask.ca.