Thermal comfort zones of equines – A brief summary of the literature
The Australian climate can often include extreme seasonal temperature variations with hot, sunny days commonplace during the summer. Climate change is expected to increase the number and temperatures of such periods and will bring about a range of stressors to horses kept in outdoor conditions or confined to stalls (Hristove et al 2017).
In Australia, horses, ponies and donkeys are usually kept in outdoor paddocks without stabling or provision of stalls. Many are seen kept in paddocks without trees or other provisions of shade. Whilst equines are able to tolerate a wide range of temperatures, they need to be protected from extreme environmental conditions. This applies in particular to foals and old, frail or sick horses. There is only limited information in the literature on the optimal temperatures and environmental conditions that equines are able to tolerate. We can, however, provide guidelines to protect these animals from unnecessary suffering based on studies that have investigated horses’ physical and physiological responses to very hot and sunny conditions.
Shade, whether man-made or natural, will block the sun’s rays. Horses seek shade in response to overexposure to heat/sun. This minimises the absorption of solar-generated heat via the skin and assists in maintaining normal core temperatures which is vital for normal physiological processes. However, shade-seeking behaviour and the associated physiological effects amongst horses can be difficult to evaluate. Studies evaluating the behavioural responses of horses during hot and sunny conditions have explored variables such as preference, duration and frequency of shade seeing behaviour, as well as insect avoidance behaviour and impact on herd dynamics. Individually housed horses differ in their shade seeking behaviour during extremely hot and/or humid conditions (Holcomb et al 2014). The studies have demonstrated that horses will seek shade when it is available, although not when humans expect them to do so (Holcomb et al 2014, Hartmann et al 2015 a & b; Holcomb and Stull 2016). Physiological responses to heat and sun can be measured using a range of physiological methods including rectal temperature, sweating, respiration rate, skin temperature and hormone levels.
Most studies in the literature have been conducted in adult, healthy horses. These studies have demonstrated that healthy, adult horses kept in paddocks are adapted to tolerate temperatures between -15 – 25 degrees Celsius (NRC 2007). Above 25 degrees C, horses appear to utilise behavioural and metabolic adaptions depending on the degree of heat stress experienced (Hristoc et al 2017). Shelter/shade seeking is part of such behaviour (Holcomb et al 2015). Studies performed on stationary horses under controlled conditions (clearly not representative of horses kept in paddocks typically observed in Australian management practices) have shown that the thermoneutral zone for these horses ranged between 5 degrees C and 30 degrees C (Morgan 1995, Morgan 1998; Autio et al 2007). During these ‘thermal comfort zones’, horses are able to maintain a constant metabolic rate without the need to adapt their physical or physiological responses. In contrast to other animals, horses have different regulatory mechanism to reduce heat stress. This is due to the relatively high muscle mass and high mass-specific rate of heat prediction during metabolism coupled with a relatively low mass-specific surface area which facilitates heat dissipation, for example via sweating (Castanheira et al 2010). There is only limited information on young, very old or frail horses during conditions of extreme heat or humidity. One study has demonstrated that foals, yearlings and horses over 15 years of age are more likely to use shade during hot and humid conditions compared to healthy, adult horses (Heleski and Murtazashvili 2010). More research is needed to define the thermal comfort zones of these subgroups of horses.
A multitude of variables will influence the shade-seeking behaviour of horses but the evidence to date has clearly highlighted that horses will seek out shade to assist them in maintaining their thermal comfort zone. As such, the provision of shade appears to be a basic requirement for the appropriate management of all equines, especially under the changing climatic conditions that are experienced around the world and the forecast of increasing summer temperatures and extreme weather forecasts.
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Castanheira M. paiva S, Louvandini H, Landim A, Fiorvanti M, Paludo G, dallago B, McManusw C. Multivariate analysis for characteristics of heat tolerance in horses in Brazil. Tropical Animal health production 2010; 42: 185-191.
Hartmann E, Hoplins RJ, Blomgren E, Ventorp M, von Bromssen C, Dahlborn K. Daytime shelter use of individually kept horses during Swedish summer. Journal of Animal Science 2015a; 93: 802-810.
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Holcomb KE. Is shade for horses a comfort resource or a minimum requirement? American journal of Animal Science 2017 (95): 4206-4212
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