Traditionally pigs have been housed in group sizes of approximately 25 pigs per pen. However, the swine industry is beginning to shift towards housing grow-finish pigs in groups as large as 100 to 1000. With increasing group size has come concerns that pigs in these groups will suffer a higher degree of injuries, such as lameness, and reduced health status.
We conducted a series of studies on pigs in groups of 18 and 108 examining a number of factors in relation to the pigs’ health and welfare. Injury scores were carried out on a biweekly basis, at the same time as weighing. The pigs were scored for the presence of flank bites, tail bites, lesions on the legs and lameness. Scores increased as severity increased. Twice daily walk-through health assessments were also conducted, and any illnesses were recorded in detail and treated as necessary.
Overall flank bite and tail bite scores were not affected by group size (Table 1). Group size did, however, have an effect on lameness scores (Table 1, Figure 1a). Overall, pigs housed in the large groups experienced more lameness. This was particularly evident during the second and final scoring periods, when the pigs weighed approximately 50 kg and 95 kg, respectively. One possible explanation may be that pigs in the large groups spent more time inactive than pigs in small groups, which may have increased the occurrence of limb stiffness resulting in lameness. Another possibility is that large group housing allows more space for running. If the pigs’ feet were to get caught in the slats while running, injury to the limb would be more likely. Casual observations of pigs running through a large group indicate that they also run into walls and other pigs more often, likely because they are traveling too fast to stop in time.
Overall leg lesion scores were higher among large group pigs (Table 1, Figure 1b). The difference in lesion scores was most evident during the second scoring period. Large group pigs may have experienced a higher score for these injuries because they spent more time lying down than small group pigs, which would have allowed their legs to rub on the concrete more frequently than the legs of pigs in the small groups.
Although there were significant differences in leg lesion and lameness scores among small and large groups, the severity did not justify antibiotic treatment or a pig’s removal from the trial (Tables 2 and 3). It is possible that the higher overall scores were an artefact of a large number of low lameness or lesion scores that would not justify antibiotic treatment or animal removal, rather than a minimal number of high lameness or lesion scores that would justify treatment or removal. Mortality rates ranged from 0.7 to 0.9 % and did not differ between the group sizes.
Large group housing for grow-finish pigs is not as detrimental to pig vitality as once presumed. When provided with adequate space, large group pigs experience a marginal increase in lameness and leg lesion prevalence. However, the occurrence does not appear severe enough to justify treatment. Overall, large group housed pigs do not seem to suffer reduced welfare as long as regular and thorough health checks are performed.