Dominance relationships between sows dependent on the time interval between separation and reunion

One hundred thirty-six sows were weaned from farrowing crates into groups of eight sows of mixed origin and housed in a straw-bedded stimulation pen. The groups were transferred to individual stalls after 4 days in loose housing and artificially inseminated. After either 7 or 28 days in single housing, the sows were reunited in the stimulation pen. During the first 48 h after introduction and reunion the occurrence and the outcome of all agonistic interactions (AI) were observed using an infrared video technique and data of wins and defeats was transformed into an 8 × 8 (winner-looser) matrix. The sociometric parameters (DC, h, h′, K) and the percentages of unknown, one-way, two-way and tied relationships were registered for eight groups of sows which had a 7 days interval in single housing between separation and reunion and nine groups which spent 28 days in single housing. The number of AI per sow and 48 h was significantly lower after reunion (7 days = 6.2; 28 days = 16.6) compared with the period after the introduction (26.8, 32.6 respectively) and the frequency of AI was significantly lower after 7 days than after 28 days in single housing. The sows established a semi-linear hierarchy after first mixing with high values for h′ (up to 0.698), K (up to 0.615) and DC (up to 0.922). Only in the 7 days group, there was a significant decrease in the values of h, h′ and K, when comparing the introductory period with the period after the reunion but an increase in DC was calculated for both groups (7 and 28 days). The percentage of unknown relationships after introduction had an average maximum of 22.8%, reaching 32.1% in two groups. After reunion these had risen to 56.7 and 30.6% for the 7 and 28 day groups respectively, one group even reached a maximum of 75%. Up to 71.8% of all relationships after introduction were unidirectional and up to 11.1% were bi-directional, these percentages were lower after reunion and also differed between the 7 and 28 days groups.