Understanding the body’s natural temperature mechanisms can help us get rugging just right, explains Rick Farr MRCVS
Autumn is approaching; the evenings are a little cooler and rain is starting to come in sideways, sending a chill down your spine.
With winter looming, it’s time to reach for that massive, all-in-one heavyweight rug to keep your horse nice and toasty. Or is it?
It is important to consider how nature has equipped different species to cope with changing seasons. The horse has developed some unique tolerances to cold and wet weather – conditions which may make us shudder but rarely bother our equine counterparts.
Horses are found throughout every climate on the planet, from the sub-zero north to the tropics and arid deserts. Their adaptations to remaining within their specific thermal neutral zone (TNZ) are impressive.
The term TNZ describes the ambient temperature range at which the body needs to make no additional physiological effort to warm or cool itself. Humans have a TNZ of roughly 18 to 30°C; in other words, in this zone you could stand naked and still maintain your internal core temperature without too much issue.
Horses, however, have a much larger TNZ. The average adult horse can quite happily live unrugged in temperatures down to 5°C, but how many times have you grabbed that heavyweight rug when it has been a balmy 10°C?
Balancing act
A horse’s internal core temperature is a balance between heat generated and lost. Internal body temperature is maintained by metabolic functions such as digestion, movement and cardiovascular activity. The body also tries to conserve heat through the insulating properties of hair, muscle, fat and skin. Heat can be lost via four mechanisms:
- Evaporation: mainly from water on the skin, through sweating or getting wet. Although horses do not pant like some other domestic species, respiration can also play a large role in heat loss.
- Radiation: the normal process of heat moving away from the body without physical contact, such as heat lost from the skin to the environment. This is closely linked to the ambient air temperature and also to body mass. An Arabian horse has a higher body surface to mass ratio than a Norwegian Fjord, for example, so will cool more readily in hot temperatures.A variation of this is radiant heat loss, when heat is transferred to solid surroundings. Keeping a horse in a walled stable in cold weather can actually increase the amount of radiant heat loss.
- Conduction: heat loss with direct physical contact. Water is a fantastic heat conductor, so wetting of the skin can, once again, increase heat loss. A recent study suggested that horseshoes might also make a difference: hind hooves had significantly lower temperatures when shod.
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Convection: airflow across the body also results in heat loss, as with an unrugged horse standing in an exposed field without shelter when the wind chill factor is high. Monitoring your horse’s temperature is good practice. The reading should be between 37.5 to 38.5°C, but it is important to know what is normal for an individual. Checking the rectal temperature with a thermometer is the best way. The practice of feeling a horse’s ears to check if he is warm is a very poor indicator of temperature regulation. Placing a hand on his shoulder or trunk will offer some indication if he is too warm and sweaty under his rugs.When deciding whether to rug, bear in mind that a horse’s hair coat type, length and density will play a large role in his insulating capacity. Unsurprisingly, studies have shown a direct correlation between hair coat length during seasonal changes and mean air temperature, underlining the fact that there is a natural coat cycle for the seasons. However, domestication and clipping have most definitely changed this.
Remember that a full winter coat is not only insulating, but also provides a degree of water resistance. Having visited Iceland, I can testify that the native Icelandic horses live quite happily without rugs in temperatures down to -15°C, with wind and snow blowing horizontally.
These horses have incredibly thick coats that demonstrate piloerection, the ability of individual hairs to trap warm pockets of air. They are also well acclimatised to their environment, a process which can take up to three weeks in unclipped horses.
The term lower critical temperature (LCT) relates to the point at which additional metabolic activity is required to generate more heat to maintain core body temperature. This can range from 5°C for horses in mild climates to -15°C in unclipped horses adapted to very cold temperatures.
Less is more
So what does this all mean when it comes to selecting winter layers? In essence, think twice before grabbing any rug. A horse in good body condition, who has a full coat, access to forage, free-roaming exercise and some form of shelter from the elements does not necessarily need more protection.
If he is young, old, likely to get excessively wet for long periods or exposed to harsh weather conditions, however, he may require extra layers. I would actively encourage people to refrain from rugging excessively unless temperatures consistently drop below 5°C, although clipped horses or warmblood breeds may also need light rugs if they are consistently wet or outside their TNZ.
Where possible, a horse will avoid conditions which promote discomfort or heat loss. Monitor him closely for any subtle changes in his behaviour and adapt your management accordingly, but allow him to be a horse and to benefit from a natural cycle of weight gain and loss over the seasons.
When to add layers
Further factors can affect a horse’s ability to effectively manage his core temperature:
Age: a young horse may not yet have insulating body fat reserves and could therefore be prone to heat loss. Bear in mind that youngsters and foals have much higher LCTs than adult horses, so may struggle in colder temperatures.
At the other end of the spectrum, studies have found that older horses (26 years plus) have increased susceptibility to overheating, especially during exercise. This is due to age-related changes in the physiological mechanisms important for thermoregulation (the body’s ability to return to an optimum temperature range). In addition, changes to hair coat quality and distribution of adipose tissue will affect insulating properties in colder temperatures.
Body condition: while an undernourished horse will have decreased muscle and fat mass, with reduced insulating capabilities, an overweight animal has the opposite problem. As less of his ingested energy will be used for heat production, the excess will consequently be converted into fat – perpetuating obesity.
Activity level: an immobile horse or an animal kept in a small paddock will be unable to walk significant distances, resulting in reduced metabolic activity and heat production.
Breed: a study revealed that surface heat loss was seen more in warmblood horses than cold-blooded types, although it was not known whether this was due to body composition or as a direct result of hair coat thickness.
Clipping: exposed areas of skin will mean reduced hair insulation and increased heat loss. Furthermore, we tend to clip the neck, shoulders and trunk – areas identified as those from which most surface heat is lost. Hence, while clipping works well for exercise, it can be disadvantageous if the environmental conditions result in further heat loss through radiation or evaporation.
Ref Horse & Hound; 3 September 2020
