The importance of familiarity when grouping gilts, and the effect of frequent grouping during gestation

The importance of previous acquaintenceship when grouping (Study I) and the effect of frequent grouping during gestation (Study II) on gilts were studied to further elucidate the causes and effects of aggressive interactions when gilts are mixed. In Study I, 2 gilt groups (gilt age about 7 months) were used, A (n=6) and B (n=8), where A and B gilts were unacquainted with each other as they did not have any physical, visual or olfactory contact during the growth period. Different pairs of B gilts were mixed first with Group A for 4 days (first grouping), returned to B for 7 days, and then regrouped with Group A (second grouping) for 1 day. Aggressive interactions on the day of grouping (day 0) were studied at both the first and second grouping. At the first grouping with A more aggressive interactions were noted than at the second one (difference of aggressive interactions different from zero, p=0.02). Few aggressive interactions occurred on day 0 when the B pair was returned to B. When mixed with A, the B gilts generally kept away from the A gilts, and spent more time standing than they did in the B group. In Study II for each of the 6 trials (cortisol 5 trials), 2 groups of 6 gilts (gilt age about 8 months) were established before service where one group was mixed every third week during gestation (4 regroupings) with 3 unfamiliar sows for a week; new sows were used at each regrouping. The 'control' group was not regrouped. Significantly more injuries were noted on the regrouped gilts (p=0.01-0.05) one week after grouping; they also had a greater increase in blood cortisol levels 3 hours after grouping, in comparison to those of the controls. However, the cortisol increase was only significant (p=0.05) for the first regrouping. No significant differences between the two groups were noted with respect to litter size at birth, sow morbidity before, during or after farrowing, or to piglet mortality and morbidity. However, the regrouped gilts tended to have more animals that did not test pregnant at 28 days after service, more abortions, more pigs dying ante- and intrapartum, and more cases of MMA.