Salivary Cortisol Concentrations of Group-Housed Gestating Sows: Individual Vs. Group Samples

The objectives of this study were to determine salivary cortisol concentrations and evaluate differences between individual and group samples collected from gestating sows. Mixed parity sows (n = 928, Parity 1 to 9) from 20 breeding groups were used. Sows of each breeding group were moved from stalls and housed in pens (42 to 51 sows/pen) with an electronic sow feeder after pregnancy confirmation at 5 weeks postmating. Sows remained in pens until d 109 of gestation when they were moved to farrowing rooms. Within each breeding group, 8 focal sows balanced for parity were identified for saliva sampling. Saliva samples were collected from focal sows using absorbent cotton swabs in stalls before mixing, 2 d after mixing and about 109 d of gestation in pens. Once sows were moved to pens, group samples of saliva were collected on the same day that individual samples were collected using a cotton rope hung on the front partition of the pen for 30 min until the rope was fully saturated with saliva. Saliva was removed by centrifugation, and frozen for subsequent analysis of cortisol concentration by ELISA assay. Sensitivity of the assay was 0.04 ng/mL. The intra-assay CV was less than 10%. For individual focal sows, salivary cortisol concentrations were lower before mixing when sows were housed individually compared with 2 d after mixing and d 109 of gestation when sows were housed in groups (0.65 vs. 3.03 and 3.44 ng/mL, SE = 0.258; P < 0.001), suggesting that stress was induced by mixing and by physiological changes with the progress of pregnancy. Group cortisol concentrations were similar (3.04-group vs. 3.44-individual ng/mL, SE = 0.536; P = 0.88) to individual concentrations on d 109 of gestation, but tended to be lower (1.78 vs. 3.03 ng/mL, SE = 0.529; P < 0.10) 2 d after mixing than individual concentrations. No interactions of group and gestation period were detected (P = 0.29). Pearson correlation was moderate for d 109 (r = 0.732, P < 0.001) but not significant at d 2 after mixing between group and individual cortisol concentration. These results suggest that salivary cortisol concentrations in group samples tend to be lower than in samples collected from individual sows which may be due to lower stress to animals during sample collection for groups of sows. Cortisol concentrations in group saliva samples may be a good indicator of animal welfare at the group level.