A first look at the relationship between skin lesions and cortisol levels in stable groups of pregnant sows

Group housed sows experience acute and severe stress due to aggression at mixing which is reflected in high levels of skin damage and high concentrations of cortisol. The aim of this study was to determine whether skin lesion counts in a stable group are related to concentrations of cortisol in hair prior to farrowing, and to determine whether different aggressive strategies employed by sows in acute and chronic situations are reflected in hair cortisol concentrations. 263 sows in 11 groups of 24 with full-length free access stalls on a commercial farrow-to-finish farm were used in the study. Skin lesions were counted on the rear, middle and anterior regions 24hr and 3 weeks post-mixing (c. 28 days post service). Hair samples and back fat measurements were collected 1 week prior to farrowing. Following extraction of cortisol from the hair, cortisol concentrations were determined using EIA (Salimetrics). Total lesion count data (24hr and 3 weeks post-mixing) were split into high and low lesion counts using the median as cut off, and sows with 4 combinations of lesion counts were identified: low-low (n=32); low-high (n=29); high-low (n=28) and high-high (n=31) for 24hr and 3 week lesion counts respectively. Lesion counts, cortisol and back fat data were available for 125 sows and were correlated using Spearman’s rank correlation, while a one way ANOVA was used to test for differences in hair cortisol concentrations and back fat depths between the 4 aggression strategy groups in R. No statistically significant correlations were found, however, there was a tendency for sows with high 3 week anterior lesion counts to have the lowest cortisol levels (rs = -0.17; p = 0.06). High anterior lesions at 3 weeks post-mixing represent sows which were initiating aggressive attacks long after the dominance hierarchy was established. It appears that this strategy may be linked to lower levels of chronic stress. There were no differences in mean cortisol levels (F(3,116) = 0.52; p = 0.67), or in mean back fat depths (F(3,116) = 0.91; p = 0.44) for sows in each lesion combination category, indicating that under the conditions of this study, the aggressive strategy employed by sows did not influence levels of chronic stress prior to farrowing. Further work is required to validate skin lesion counts as a proxy for chronic stress in group housed sows.