54 The effect of weaning age on physiological, behavioral, and performance indicators of welfare in weaned piglets.

Abstract For the past 30 years, research has been conducted to determine the effect of weaning age on piglets’ performance. Nonetheless, the literature is full of contradictory results, and thus there remains no clear consensus on the optimal weaning age. The objective of this study was to evaluate the effect of weaning piglets at 3 and 4 weeks of age (approximately 21 and 28 days) on a set of physiological, behavioral, and performance indicators of welfare, to gain a holistic understanding of the impact of weaning age on piglets’ overall welfare. Sixteen litters were randomly assigned to be weaned at 3 (21-25 days; n=80) or 4 (28-34 days; n=80) weeks of age. At weaning, piglets were blocked by weight and randomly housed in mixed-sex pens of 5 piglets. Blood samples were collected from one focal male and female, per pen, prior to weaning and at 24 hours and 7 days post-weaning to evaluate a treatment effect on blood leukocytes. A subset of pens was video recorded for the first 48 hours post-weaning to evaluate their behavior. Weight gain and feed intake were monitored at 7-, 14-, 21-, and 28-days post-weaning. A series of mixed models were used for statistical analysis. The models included the fixed effects of weaning age, time, and their interaction, and the random effect of pig or pen. Piglets weaned at 3 weeks expressed a higher neutrophil-lymphocyte ratio (3 weeks = 1.93 ± 0.20 SE, 4 weeks = 0.98 ± 0.20 SE; P < 0.001) at 24 hours post-weaning, and less feeding behavior (3 weeks = 0.35 ± 0.08% SE, 4 weeks = 1.34 ± 0.29% SE; P < 0.05) during the first 24 hours post-weaning, compared to piglets weaned at 4 weeks. Overall, piglets weaned at 3 weeks also had lower average daily feed intake (3 weeks = 0.47 (95% CI 0.43, 0.50), 4 weeks = 0.69 (95% CI 0.64, 0.73) kg/day; P < 0.0001) and lower average daily gain (3 weeks = 0.32 (95% CI 0.28, 0.37), 4 weeks = 0.48 (95% CI 0.44, 0.52) kg/day; P < 0.0001) during the 28-day nursery phase. Our results indicate that weaning piglets at 4 weeks of age, as opposed to 3 weeks, has a positive impact not only on their performance, but also on behavioral and physiological indicators of welfare, highlighting the welfare benefits of delaying weaning.