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

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hour after mixing (Kongsted et al., 2004). However, sows involved in many aggressions at
weaning had the fewest skin lesions three weeks after mating (Kongsted et al., 2004). As a
consequence, any stress experienced by these old, and probably high ranked sows (Arey &
Edwards, 1998) might have been too short lasting to influence their reproduction. The
number of skin lesions has previously shown to correlate with incidences of aggressive acts
(Barnett et al., 1992). However, not only physical contact but also treats and visual contact
may cause stress in the submissive sows (Ruis et al., 2002). Skin lesions per se may there-
fore be inadequate to determine levels of stress. Finally, the lack of relation between repro-
duction and fear could partly be due to the timing of the fear tests. Fearfulness towards hu-
mans around oestrus might decrease the chance of a correct timing of ovulation and in-
semination (Kongsted, 2004a). However, the fear tests were not performed on the observation
day around mating because the oestrus behaviour of the sows made the test results unreliable
(Kongsted et al., 2004). Therefore, the fear tests were only carried out at weaning
and three weeks after mating, and it cannot be excluded that the fearfulness of the sows at
that time were different compared to around mating.
In conclusion, with the heterogeneity of the herds involved and the non-standardisation of
possible influential factors related to layout and management in mente, the found relations
are noteworthy. Taken together, the results indicate that the individual variation in feed
intake in sows group housed in commercial herds may be large enough to impair pregnancy
rate and perhaps also litter size. However, it cannot be excluded that the found associations
between back fat and reproduction are linked to social stress. The relations between indicators
of feed intake and reproduction performance may not only be a consequence of feed
intake per se but a combination of low feed intake and a high level of social stress in the
low ranked sows. Additional studies are needed to illuminate the role of social stress in
commercial sow herds further. Special attention should be aimed at developing additional
indicators of social stress suitable for use in practical pig production.
Acknowledgements
The author wishes to thank the participating farmers and the technicians Henrik K. Andersen,
Kristine R. Hansen, Michael Hansen, Orla Nielsen, Niels H. Thomsen, and Helge Yde
for their much appreciated assistance in carrying out the data collection. The author also
wishes to express her gratitude to John E. Hermansen and Troels Kristensen for valuable
comments on this paper and to Erik Jørgensen for valuable contributions to the statistical
analyses.
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