Soybean oil supplement induces increased approaching behavior to humans and alters serotonin concentrations in horses

Enhancement of human-horse interaction is crucial for safety in equine management, as poor relationships between humans and horses can lead to accidents. Serotonin is a neurotransmitter that is highly related to social affinity in animals and several studies have been documented that supplementation of tryptophan, which is a precursor of serotonin, can increase calmness of horses. This study aimed to assess the effect of tryptophan through soybean oil supplementation on serotonin concentrations and the behavior of horses. Six horses participated and the horses were treated with or without soybean oil diets in a cross-over design. Daily feed intake was 2 % of the body weight of horses, with a timothy hay to grain ratio of 8:2. Behavioral observations and blood collections were conducted and serotonin and cortisol concentrations were quantified using enzyme-linked immunosorbent assays. Data was analyzed using two-way repeated measures analysis of variance to explore the effects of diet, time, and their interaction. The difference was considered statistically significant when p-value was less than 0.05. The results demonstrated that soybean oil supplementation significantly increased approaching behaviors towards humans (5.57 ± 1.12) compared to baseline scores (1.2 ± 0.57). Moreover, serotonin concentrations were elevated following the soybean oil diet (150.09 ± 21.32 ng/mL) compared to concentrations on day 1 (115.89 ± 22.3 ng/mL). Cortisol concentrations remained stable throughout the experimental period when horses were treated with the soybean oil diet (405.29 ± 35.29 ng/mL vs. 427.64 ± 87.34 ng/mL). In contrast, cortisol concentrations increased significantly in horses on the non-oil diet (405.29 ± 35.29 ng/mL vs. 427.64 ± 87.34 ng/mL). These findings suggest that soybean oil supplementation positively influences horse behavior towards humans, indicating that dietary modification may enhance human-horse interactions.