In the domestic environment, horses are often kept in sub-optimal conditions that limit the expression of species-typical behaviors and compromise their welfare. Environmental enrichment strategies have been identified as an option to make the domestic environment more interactive and complex, resulting in an improvement in the animal’s quality of life. Olfactory stimulation, a form of sensory environmental enrichment that aims to trigger the sense of smell through the introduction of odors, has shown promise for horses. (source) The following highlights are from of a review of the scientific research studying the impact of essential oils odors on the well-being of domesticated horses.
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Essential oils improve welfare of horses
According to the results of recent studies, essential oils and other plant-derived odors may be able to improve the welfare of certain horse species. Some of these odors seem to encourage relaxation and relieve stress for domesticated horses, while others seem to have a more stimulating effect on the animals.
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Research using lavender to reduce stressful events in horses
Most of the research carried out with horses focused on the use of lavender essential oil. This essential oil, known for its anxiolytic-like effect in animal models, has been shown to have the potential to reduce the intensity of stress reactions in horses according to recent studies. The use of this type of olfactory stimulus has the potential to help animals deal with challenges posed by the domestic environment and reduce unwanted behaviors.
According to some researchers, horses treated with lavender essential oil (Lavandula augustifolia) after an acute stress response induced by an auditory stimulus (air horn) showed a significant reduction in heart rate, while other researchers have reported that in horses treated with lavender essential oil (Lavandula augustifolia) during transport experienced cortisol levels suppressed.
In one experiment, horses were subjected to a series of stress tests in which they were exposed to common stressors in the domestic environment, such as social isolation, novel area, novel object, and the sudden appearance of an object. In this study, stress indicators such as heart rate, alert postures, and defecation were lower in horses treated with lavender* (Lavandula augustifolia) before stress tests. The lavender essential oil also modified salivary cortisol. (source)
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*A differential of this work was the dilution of the essential oil in vegetable oil and the application with the aid of a roll-on around the nostrils of the treated animals. From the point of view of environmental enrichment, this type of application is not ideal, as it does not constitute a change in the environment itself. Furthermore, the use of roll-on forces the animal to smell the odor, not allowing individuals to have autonomy while interacting with the olfactory stimulus.
In horses that were not subjected to an external stressor, some researchers observed that the use of lavender essential oil (Lavandula augustifolia) significantly increased the parasympathetic component of heart rate variability (RMSSD). This effect was not maintained after lavender removal, which highlights that its use should be considered only as a short-term solution. (source)
Some researchers have concluded that spikenard (Nardostachys jatamansi) and Roman chamomile (Anthemis nobilis) appeared to be better at calming horses than lavender, which has been the best-documented essential oil to date. While spikenard was better at inducing a relaxed facial expression, Roman chamomile was better in terms of muscle relaxation.
Unlike other studies, whose focus was on testing the ability of essential oils to encourage relaxation and relieve stress, the study conducted by some researchers aimed to test the olfaction of horses that were presented with the odors of the following oils: orange (Citrus sinensis), peppermint (Mentha piperita), cedarwood (Cedrus), and lavender (Lavandula angustifolia). Their results demonstrated that the horses were able to detect and distinguish between all four odors. Young horses sniffed cedar longer than old horses and pregnant mares sniffed less lavender than non-pregnant mares. Furthermore, animals showed greater interest in peppermint essential oil, which calls for further investigation, especially since this oil is also known for its stimulating effect in horses. (source)
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Conclusion
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The researchers conducting the review concluded that (1) olfactory stimuli have the potential to increase behavioral diversity and the ability of animals to deal with challenges, as well as to stimulate species-typical behaviors; and (2) when using olfactory stimulation as environmental enrichment, professionals working with horses should seek to use olfactory stimuli that have already been tested in scientific settings and are considered safe and effective for this species.
Journal reference: Ana Caroline Bini de Lima, Vanessa Cristini Sebastião da Fé, Maria Simara Palermo Hernandes and Viviane Maria Oliveira dos Santos. Olfactory Stimulation as Environmental Enrichment for Domestic Horses—A Review Animals 2023, 13(20), 3180; DOI: 10.3390/ani13203180
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