Explorative behaviour in horses when presented with unfamiliar poisonous and non-poisonous plants

Poisonous plants are an integral part of many pastures destined for horses around the world. However, fundamental knowledge of the role of horses’ senses in discriminating these plants is still lacking. It is, therefore, of interest to study whether, besides sight, the olfaction used by animals before ingesting may prevent them eating a poisonous plant. The aim of the study was to determine whether stabled horses are capable of distinguishing poisonous plants solely by odour among the unfamiliar plants presented. Twenty adult warmblood mares and geldings were involved in the study. The novel plants presented for exploration by the horses were three poisonous species: Taxus baccata, Buxus sempervirens and Thuja occidentalis, as well as three non-poisonous species: Petroselinum crispum, Anethum graveolens and Eruca sativa. Each plant was presented in a small box for one minute on three days, two plants daily. The plants were unavailable to see or touch by the horses and only smell was perceptible. The horses were habituated and positively conditioned to approach the box. The intensity of exploration was measured by the number of exploration events, total exploration time and occurrence of additional behaviours, such as chewing. The method used made it possible to exclude other senses than smell and a previous experience with the plants tested from the analysis. The time of exploration in subsequent trials was found to be shortened (9.0, 5.5, 3.0 s for poisonous plants and 14.5, 7.0, 5.5 s for non-poisonous plants). The differences in the time spent exploring boxes with different plants show that horses discriminate new odours individually and undertake increased olfactory behaviours when encountering a novel odour. The horses spent significantly more time exploring non-poisonous than poisonous plants (7.0 and 5.0 s, respectively). They were also chewing and licking the crib notably more often when non-poisonous plants were presented compared to poisonous plants (0.24 and 0.13 versus 0.15 and 0.08, respectively). The shortened and weaker exploration in the case of poisonous plants may indicate that these plants caused a reluctance in the horses. Hence, our findings suggest that the horses’ sense of smell has some potential for differentiating poisonous from non-poisonous plants independently of the sense of taste, touch and sight.