Reproduction performances and conditions of group-housed non ...

- Paper III -
Previous work have found that mixing of sows into small groups (12 and 3 sows respec-
tively) lead to the highest level of skin injuries (Olsson & Svendsen, 1997) and the highest
and longest lasting elevated level of cortisol (Tsuma et al., 1996) in low ranked sows. In
contrast, Mendl et al. (1992) reported that if sows where mixed into a large group (37 gilts),
the intermediate sows received the highest level of aggressions and had the highest level of
cortisol. The dominating sows received the lowest level of aggressions and the submissive
sows were in between. These results indicate that when little free space is available, the
submissive sows are the ones receiving most aggressions probably because they receive
aggressions from both the dominating and the intermediate sows, whereas the intermediate
mainly receives aggressions from the dominating sows. In contrast, in large groups the
most submissive sows have more chances to avoid the more dominant sows due to more
available free space. Accordingly, this study found that in herds with no escape possibilities
(small group sizes and no feeding stalls) in the pregnancy unit, first parity (and probably
low ranked) sows had significant more lesions compared to older sows three weeks after
mating and at farrowing, whereas in herds with escape possibilities, the second-third parity
(and probably middle ranked) sows had the highest number of skin injuries.
Although not significant, more skin lesions were observed in herds with escape possibilities
in the pregnancy unit, especially at farrowing. One explanation for this could be that the
group of herds with escape possibilities include the two herds with ESF and large dynamic
groups with constantly introduction of new sows into the group. As seen in Figure 1, these
two herds (herd 7 and 15) had the highest level of lesions at farrowing. This is in agreement
with Leeb et al. (2001) who found significant more lesions in herds with dynamic groups
compared to herds with stable groups in a field trial that involved 55 herds and O'Connell et
al. (2003) who found a higher injury level in dynamic compared to stable groups in an
experimental study.
In accordance with Olsson & Svendsen (1995), the frequency of lying solitary was highest
on the day of weaning. Further, the likelihood of lying solitary at weaning was highest for
sows older than third parity, which were also the sows involved in most aggressions at
weaning. Accordingly, Bonde (2004) found that solitary lying behaviour was positive correlated
to involvement in aggressive interactions. Since there was no effect of parity group
on the probability of lying at weaning (P=0.8, results not shown) the results was not solely
a consequence of old sows lying more often than younger sows.
Fear tests. According to the results from the human approach test performed at weaning,
first parity sows were more likely to be categorised as fearful compared to older sows. This
trend could however not be rediscovered in the two fear tests three weeks after mating in
the home arena of the sows. The test arena for the human approach tests differed inevitable
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