Effect of birth order on performance and affective state of pigs
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In pigs, birth order is associated with higher pre-weaning mortality. However, knowledge on the effect of birth order on welfare of surviving piglets is limited. The aim of this study was to investigate the possible link between birth order and both piglet performance and affective state. Firstly, the following data was collected from 393 piglets: stillbirth rate, intactness of the umbilical cord, cyanosis, drinking time, birth weight, teat order, weaning and end weight. Secondly, an active-choice judgement bias test was performed with low birth order (n=10) and high birth order (n=10) piglets. During the preceding discrimination training, the pigs learned to associate two tone cues with either a high or low reward, provided at different locations. After training, ambiguous intermediate tones were introduced and the pig’s choice of location was observed. Results showed that last-born piglets had a higher birth weight than middle-born piglets. They also drank from more caudal teats than first-born piglets. In the judgement bias test, low and high birth order piglets showed a similar learning rate, latency and optimistic bias. Before the first individual trial, salivary cortisol levels were highest in low birth order pigs. However, after the trial, levels had decreased to the high birth order pigs’ level. Additional behavioral tests and further study of stress response are proposed for validation of these results.