Most studies on feeding and social behaviours of pigs have been conducted on groups of fewer than 40 pigs/group. However, these group sizes are much smaller than some that are now used in some commercial operations (100-1000 pigs/group) in North America and elsewhere. The social dynamics of feeding and other behavioural activities of pigs in large social groups are not well understood and the competition for and the utilization of important resources such as feeders by pigs in larger social groups is therefore unclear. The present study was conducted to gain a better understanding of feeding and other behavioural activities and the impacts of larger social groups on social stress in grower-finisher pigs.

To address this question, two blocks, which consisted two group-size treatments, 18 (Small Group) and 108 (Large Group) grower-finisher pigs per pen, were carried out. Each block, which lasted 10 weeks in duration, consisted of two pens of Large Group and four pens of Small Group size. A total of five hundred seventy six barrows and gilts (Pig Improvement Canada) were used in the experiment at the PSC Elstow facility. The animals were weaned at approximately 18-day of age, were then held in nurseries for eight weeks, before being used in the experiment. The ratio of barrows to gilts was kept constant (1:1) between the two group sizes and the average starting weight of pigs was 34.6 kg ± 4.1 kg (S.D). Pigs were housed on fully-slatted floors with floor space allowance per pig of 0.76m2.

Wet/dry feeders supplied feed and water to the animals, with a pig to feeder space ratio of 9 to 1. Feeders were spread equidistantly along the central line in large groups with four feeder holes per feeder location. This maintained an equal distribution of feeders within the large group, giving an equal opportunity for all the pigs to access the feeders without any difficulty.

The individual pig feeding behaviour and group feeding patterns were studied during weeks 1, 5, 7 and 10 of the grower-finisher cycle. In addition, other behavioural activities such as percentage of time spent on eating/drinking, resting (lying) and standing/walking and diurnal patterns of these activities of pigs in both large and small groups were studied during weeks 2, 5 and 10 following re-grouping.

To evaluate the group size effect on social stress, salivary cortisol levels were measured periodically throughout the grower-finisher cycle i.e. during weeks 1, 2, 5 and 10. In addition, morphological parameters of the adrenal glands were measured at the end of grower finisher cycle to understand any effects of chronic stress on pigs that were formed into larger groups.

The pigs in large groups had more bouts of feeding (35 vs. 25, P<0.05) and the feeding bouts were shorter in duration (232 vs. 301 sec, P<0.05) during day 3 following re-grouping. However, no differences in number of feeding bouts and bout lengths were found during weeks 5, 7 and 10. More importantly, we found that the percentage of pigs queuing at the feeders to be high in larger groups than in smaller groups during day 3 (0.90 vs. 0.59, %, P <0.05). This trend of higher percentage of queuing at feeders were also apparent during day 6 following re-grouping (0.79 vs. 0.60, %, for large and small groups, P=0.08) but not thereafter. There were similar 24 hr group feeding patterns in pigs of both SG and LG during weeks 1, 5, 7 and 10 (Figure 1). Furthermore, the average percentage of feeder spaces occupied (mean day 3 and 6 and week 5, 7 and 10) was also similar between the two group sizes (55.7 vs. 56.2, %, for large and small groups). The average times spent on eating/drinking (5.2 vs. 5.2 %, for small and large groups), standing/walking (5.1 vs. 5.4 %, for small and large groups) and resting (89.6 vs. 89.3 %, for small and large groups) did not differ between the two group sizes. Furthermore, the diurnal patterns of these activities were also not affected by the large groups. One main concern of large group sizes for pigs is the potential for increased social stressors. Interestingly however in our study, during the entire 10 wk experimental period, pigs in larger groups did not demonstrate any short-term (acute) or long-term (chronic) responses of social stress (Table 1). Therefore, it was apparent that the pigs had not gone through any adverse social stressors by living in larger groups.