Consistency of Convenience Sampling Order and its Association With Response to Handling and Weaning in Domestic Pigs (Sus scrofa)

Convenience sampling in animal experiments may affect findings due to individual difference in temperament. The aim in this study was to assess the consistency of convenience sampling order (ORDER), and to study the association between potential factors (i.e., behaviors related to handling, and coping ability to weaning) and ORDER in pigs (n = 325). ORDER was recorded by catching the closest pig in the pen on Day (D) 1, 14, 23, 27, 31, 38, 69 and 79, while weighing pigs. Response to handling was assessed on all weighing days except D69. For D1 to 38, vocalization and attempt of escaping were recorded on a Yes/No basis; for D79, a scoring system from ‘very difficult’ to ‘easy to handle’ was used for 3 handling phases: ‘Moving into the crate,’ ‘In the crate,’ and ‘Leaving the crate’. ORDER within each pen was categorized to 4 quartiles. To study the association between coping ability to weaning and ORDER, salivary cortisol and chromogranin A (CgA) were determined on pre- and post-weaning. The response variables for statistical analysis were ORDER: either 1) the log-transformed percentage of ORDER; or 2) first quartile (1QT, i.e., first-caught) vs. the rest; or 3) fourth quartile (4QT, i.e., last-caught) vs. the rest. An individual effect on ORDER was found (P < 0.001), suggesting that a pig that had been caught either first, middle, or last was very likely to be caught in a similar order in the following handling sessions. On the other hand, neither responses to handling nor coping ability to weaning were associated with ORDER (P > 0.05). To conclude, although ORDER is not associated with responses to human handling and coping ability to weaning, convenience sampling in pigs should be cautious as ORDER is highly consistent and long-lasting.