The horse in this clip is being offered peppermint essential oil. Notice how he alternates between the upper and underside of the tongue. By using the underside (sublingual) this leads to rapid absorption due to the rich vascular supply. The horse then proceeds to reject the peppermint by showing no further interest, demonstrating how he guides his own dosage.

The behavioural response of an animal to a plant extract that has been offered will indicate whether it has been selected. The horse in this clip is showing an interest in the essential oil that is being offered. Note how the horse is flaring the nostrils and changing the side of nostril that he is choosing to inhale the aroma. He has also responded to the essential oil by the use of the vomeronasal organ which can be seen by the horse raising his upper lip. This is known as the flehmen response.
Herbivores such as the horse have evolved to forage on a wide range of plants. This video demonstrates the behavioural response when a selection of dried plant extracts is offered (left-right: marshmallow root, bladderwrack seaweed, marigold flowers, rosehip shells, wheatgrass powder). This video clip also shows how the horse alternates the selection to suit needs. Note the end bowl containing marshmallow root has not been selected.