Effects of Dietary Melissa officinalis Supplementation on Hematological, Neuroendocrine, and Behavioral Parameters in Weaned Piglets and in the Beginning of the Fattening Period

This study aimed to evaluate the effects of dietary supplementation with Melissa officinalis on the haematological, biochemical, neuroendocrine, and behavioural parameters of weaned pigs, as well as on behaviours associated with the establishment of social hierarchy following transfer to the fattening stage under conditions of social competition. The study included 48 pigs allocated to control and experimental treatments. Over the 55-day experimental period, pigs in the experimental treatment received dried Melissa officinalis leaf material incorporated into the diet, with the daily amount gradually increasing from 3.20 to 9.60 g/animal/day. The mean daily amount administered throughout the experimental period was 6.99 g/animal/day, approximately 7.0 g/animal/day. During the growing period, behavioural responses, serum serotonin and cortisol concentrations, and haematological and biochemical parameters were assessed. Following transfer to the fattening stage, agonistic behaviour associated with the establishment of social hierarchy was monitored. The data were analysed using one-way analysis of variance and correlation analysis. Dietary supplementation with Melissa officinalis significantly increased white blood cell (WBC; p = 0.048), lymphocyte (LYM; p = 0.008), and monocyte counts (MON; p = 0.004), as well as serum serotonin concentrations (p < 0.001). Serum cortisol concentrations were 37.73% lower in the experimental treatment than in the control treatment, although the difference was not statistically significant (p = 0.304). Pigs in the experimental treatment exhibited lower locomotor activity (p = 0.001), a tendency towards less climbing behaviour (p = 0.059), and a longer duration of play fighting (p = 0.001). Following transfer to the fattening phase, pigs in the experimental treatment exhibited a lower frequency of agonistic interactions during the establishment of social hierarchy (p = 0.008). In the control treatment, cortisol was negatively correlated with active behaviour (r = −0.845; p < 0.01) and positively correlated with inactive behaviour (r = 0.845; p < 0.01), whereas serotonin was positively correlated with play fighting (r = 0.462; p < 0.05). These findings indicate that the inclusion of dried Melissa officinalis leaf material in the diet improves immune status, beneficially modulates neuroendocrine regulation, and facilitates the social adaptation of pigs by reducing agonistic behaviour. Therefore, Melissa officinalis may represent a promising phytogenic feed additive for mitigating social stress and improving pig welfare during critical production stages.