The horse thermoregulation ?
When autumn comes, it is normal practise to start to rug until the mild spring returns. However, it is difficult to create a universal rule defining when a horse should wear a rug and what thickness is required for the horse to be warmer. This topic is often debated as it depends on many parameters, such as the breed, age, physical condition, location (meadow, exposed grazing, stable) or geographical location. In order to appreciate the complexity of this problem, it is first necessary to understand how the horse adapts to face the cold.
A limited number of scientific publications have been interested in the natural regulation of body temperature in winter, known as thermoregulation. Below, a brief bibliographic review summarizes the salient points from the results of such research. Conducted and written by Dr Christine Briant, who is a Development Engineer at the IFCE (L’Institut français du cheval et de l’équitation/French Institute of Horse and Equestrianism) who works in partnership with ARIONEO as part of an ongoing collaboration.
How do horses adapt to face temperature variations?
Horse’s adapt in one of two ways, either by thermogenesis (heat production) or by thermolysis (loss of heat). Physiological mechanisms will modulate this adaptation or trigger responses which occur due to the heat transfer from the external environment to the horse’s body or vice versa, which is dependent on the outside temperature:
– radiation is a mechanical transfer of the heat from the body to the environment, if the body temperature is higher than the environmental one,
– conduction is a loss of heat by contact, which can be very important with water contact,
– convection is a loss of heat due to the wind replacing warm air by colder air
In a horse, skin, muscles, fat and hair all allow regulation. Regulation occurs dependent on the changes between the horse’s body and its environment.
How does a horse produce heat?
When winter is approaching, especially if the horse is outdoors, hair grows longer and thicker to protect the horse from the temperature decrease. This photoperiodic phenomenon is the first progressive visible manifestation of cold adaptation.
Other mechanisms are involved in production of heat as an immediate response to cold exposure:
– Shivers are a reflex mechanism of muscle contraction that produce heat from the degradation of sugars and fatty acids (multiplication of the metabolism by 4 or 5), this moderate mechanism can be maintained for a long time. Unlike intense (anaerobic) muscular exercise which also allows a rapid rise in body temperature, but produces lactic acid responsible for muscle fatigue and is highly energy consuming (multiplication of the metabolism by 25).
– Piloerection is characterized by bristling hairs and increases the insulating effect by increasing hair thickness up to 30%. Thus, it will contain more air. However, in cases of abundant rainfall, where the horse’s coat is very wet, it will lose this ability, so the water will reach the skin and cool the body.
– Vasoconstriction is a constriction of the blood vessels of the limbs extremities which limit the loss of heat. The blood flows back to the central organs (this is the phenomenon that induces white fingertips in humans in cold temperature) and reduces the temperature of the extremities by up to 1.7 ° C, which avoids tissue damage when horses have their feet in the snow.
– Decreasing of the respiratory rate, that can reduce the caloric loss by evaporation.
– Mobilization of the neuro-endocrine system: the release by the adrenal glands of adrenaline and norepinephrine allows the mobilization of glycogen which will serve as “fuel” to the muscles.
How does the horse remove heat?
Evaporation is carried out by breathing (increases in the breathing rate) and sweating, which cools the temperature of the skin by evaporation. Conversely, exposure to cold will produce a decrease in respiratory rate, to decrease the heat loss.
What is the most comfortable temperature range for horses?
Scientists call it the Thermal Neutral Zone or Thermal Comfort Zone or Minimum Thermoregulation Zone. This is the temperature range within the body that does not consume extra energy to maintain the internal body temperature.
If the body has to set up mechanisms to warm up (shivering or slowing down of the respiratory rhythm) or to cool (sweating, increased breathing rhythm), it means that the horse is out of its comfort zone. The lower limit of this zone is the lower critical temperature and the upper limit is the upper critical temperature.
In humans, the thermal comfort zone is very restrictive and is located at around 25°C.
In temperate regions, for unclipped horses, the thermal neutrality zone range is approximately between +5°C and +25°C.
However, it varies according to breed, morphology, age, health status and food intake. Thus, if the horse is outside during the natural decrease of the temperature in autumn, they will gradually adapt, by changing their coat, distributing their fat tissue differently, modifying their behaviour as well as their food intake. Horses need around 10 to 20 days to adapt to a temperature drop of around 15°C.
For newborn foals, the zone of thermal comfort is located between +16°C to +22°C.
Finally, attention should be paid to older horses whose thermoregulation functions might be impaired.
For clipped horses living in a temperate climate, a study showed that under +6°C, shivering might appear in some horses. In this way, the authors recommend to use a rug or increase food intake under +6°C.
Moreover, some breeds or morphological types are more suited for hot or cold climes:
– Horses most adapted to the cold are heavier and more compact with shorter limbs, longer hair especially the fetlocks, thick skin that does not allow the blood vessels to be seen and a fat subcutaneous tissue. These breeds have the capacity to moderate their effort intensity over a long time period by mobilizing the fatty reserves without the production of lactic acid (for example Draft or Native ponies),
– Horses adapted to warm temperature are lighter with long limbs, less insulating with silkier hair and clearly visible vessels under the skin. They are more difficult to fatten and adapt to rapid tempweature changes a by using the glycogen in the muscles (Warmbloods).
During transit in a trailer or horsebox, the temperature can be 10°C higher than the outside temperature. If a rug is necessary when boarding, during transit the horse’s comfort zone may change leading to a requirement to change the rug or remove the rug completely during transport. An inability to react to this change in need, may even create thermal stress.
Christine Briant link to the blog: https://comportementbienetreifce.wordpress.com
How to look after the welfare of your horse?
We have read above that the thermal neutral zone has been studied and identified in scientific studies.
However, on a daily basis, each individual and many external parameters need to be taken into account, so it is pretty difficult to predict which rug will be the most appropriate and suitable for the approaching night for any given horse. Thus, two horses of different breeds living in the same place or two horses of the same breed living in different geographical areas may have very different thermoregulation processes and needs.
Orscana analyses these parameters and can help you to choose the most suitable rug using algorithms based on parameters of breed, age, sex, coat combined with the up-to-date weather forecast. Moreover, over time, Orscana gets to know your horse and can thus adapts to the physiology of each individual.
Scientific Director at Arioneo