Thermoregulation, critical temperatures, winter feeding and the correct use of rugs
With the arrival of winter, one of the questions owners ask most often concerns the need to protect the horse from low temperatures. When is it appropriate to use a rug? Can a horse living outdoors go without protection even below zero? And above all, does the horse suffer more from cold or from heat?
Another aspect, often underestimated, concerns feeding: how much does cold affect the horse's energy requirement? Is it necessary to increase hay? And if so, by how much?
To answer these questions correctly we need to understand some aspects of equine physiology and how the body regulates its own temperature in relation to environmental conditions.
The first consideration is that the horse has a remarkable capacity to adapt to low temperatures, generally greater than that of humans. This, however, does not mean that all horses tolerate cold in the same way, or that the use of rugs is always unnecessary.
1. Is the horse more sensitive to heat or to cold?
From a physiological point of view, a healthy adult horse with a good winter coat and properly acclimatised is particularly efficient at conserving body heat.
Its relatively large body mass, its surface-area-to-volume ratio and the characteristics of its coat all help to limit heat loss.
During the cold months several mechanisms come into play:
- Piloerection: the hairs stand up, increasing the thickness of the insulating layer of air next to the skin.
- Peripheral vasoconstriction: reduced blood flow to the skin limits heat loss.
- Metabolic adaptation: in intense cold, heat production increases, with a consequent rise in the energy requirement.
- Fibre fermentation: microbial activity in the large intestine contributes to metabolic heat production.
Conversely, during physical exercise the horse produces considerable amounts of heat. Only part of the metabolic energy of the muscles is converted into mechanical work, while a substantial proportion is released as heat.
Heat is dissipated through cutaneous circulation, sweating and respiratory exchange.
At high temperatures, especially when combined with high humidity, the evaporation of sweat becomes less efficient. This can lead to a progressive build-up of heat and increase the risk of hyperthermia.
We can therefore say that, given appropriate acclimatisation, the horse is generally better adapted to tolerate cold than situations of intense heat, particularly during physical activity.
This is not an absolute rule, however: thermal risk always depends on the environment, on the activity being performed and on individual characteristics.
2. The lower critical temperature: when does cold become a problem?
One of the fundamental concepts in the physiology of thermoregulation is the thermoneutral zone.
This is the range of ambient temperature within which the body can keep its temperature stable without having to increase metabolic heat production.
The lower limit of this range is known as the lower critical temperature, or Lower Critical Temperature (LCT).
Below this value, the horse must increase heat production to compensate for losses to the environment.
The scientific literature reports different values depending on husbandry and acclimatisation:
- In adult horses well acclimatised to cold, with a well-developed winter coat, the LCT can reach values of around -15 °C, or even lower in particular conditions.
- In horses that are not adequately acclimatised and are usually kept in more sheltered environments, the LCT can be appreciably higher, even in the order of +5 °C.
These figures show how greatly environmental adaptation can alter the horse's physiological response.
It is important to stress that the lower critical temperature is not the temperature at which hypothermia appears.
A horse can be below its own LCT and still maintain a normal body temperature, provided it has enough energy to compensate for the greater heat losses.
Conversely, a debilitated or malnourished animal, or one exposed to particularly unfavourable weather, may develop thermoregulatory difficulties even at relatively mild temperatures.
3. An unclipped horse living outdoors: down to what temperature can it cope?
A healthy adult horse in good body condition, with a full winter coat and an adequate period of acclimatisation, can tolerate ambient temperatures well below zero.
However, air temperature alone is not enough to establish the level of risk.
The following must be considered at the same time:
- The intensity and duration of exposure to cold.
- The presence of wind and precipitation.
- Access to dry shelter.
- Continuous availability of water and an adequate quantity of forage.
- Body condition, age and health status.
- Coat characteristics and degree of acclimatisation.
Indicative guidance for an unclipped, acclimatised adult horse in good condition
| Ambient temperature | Management considerations |
|---|---|
| +10 to 0 °C | Generally well tolerated in dry conditions with shelter available. |
| 0 to -10 °C | Check the availability of forage, unfrozen water and protection from the weather. |
| -10 to -15 °C | Pay closer attention to energy balance, behaviour and body condition. |
| Below -15 °C | Assess individually the need for greater energy intake, protection and supervision. |
These ranges are purely indicative and do not represent universal clinical thresholds.
An acclimatised horse might tolerate temperatures lower than those given, while a thin, elderly horse, or one with a wet coat, might struggle at much higher temperatures.
Good winter management should therefore not be based on a fixed number of degrees, but on an overall assessment of the horse and of the environment in which it lives.
4. Why can rain and wind be more of a problem than frost?
One of the most interesting findings to emerge from research concerns the role of weather conditions.
The insulating capacity of the coat depends largely on the air trapped between the hairs.
When the coat becomes deeply wet, this capacity can be considerably reduced. Wind, moreover, increases heat loss by convection.
As a result, a day with relatively mild temperatures but persistent rain and strong wind can present a greater thermal challenge than a very cold but dry day.
A study published in the Equine Veterinary Journal examined horses' preferences for shelter and for supplementary sources of heat during the Nordic winter.
The observations highlighted the importance of precipitation and wind in driving the search for protection, showing that ambient temperature is not the only relevant factor.
Further studies have confirmed that horses spend more time in shelters during unfavourable weather, even when wearing a rug.
These results demonstrate that thermal discomfort does not depend on temperature alone, but on the interaction between cold, wind, humidity and the insulating capacity of the coat.
For horses that live outdoors permanently, the availability of effective shelter from precipitation and wind is therefore an essential element of welfare.
5. Does clipping alter the capacity for thermoregulation?
Clipping is frequently carried out in sport horses during the winter months to promote heat dissipation during work and to reduce the time the coat takes to dry after exercise.
From a physiological point of view, however, removing the hair reduces natural thermal insulation.
A study published in 2012 in Acta Veterinaria Scandinavica examined the effects of clipping and of rug use on thermoregulation during exercise and recovery.
The study, carried out on three equids subjected to different clipping and rugging conditions, found greater heat dissipation during activity in the clipped animals, while in those with an intact winter coat more marked increases in respiratory rate and rectal temperature were observed during recovery.
Although the sample was small, the results illustrate how clipping can significantly alter heat exchange.
Clipping can therefore be advantageous for some sport horses, but it requires more careful management of thermal protection during rest periods.
It is also necessary to distinguish between:
- Full clip: produces a marked reduction in natural insulation and generally requires greater attention to rugging during the cold months.
- Partial clip: retains part of the winter coat, particularly over the back and hindquarters, allowing greater protection than a full clip.
- Unclipped horse: retains in full the insulating mechanisms provided by the coat, provided this is dry, healthy and adequately developed.
6. Rugs: when are they really necessary?
The use of rugs should not be regarded as compulsory for every horse during the winter.
Their function is to reduce heat loss and, in waterproof models, to protect the coat from water and wind.
The choice must depend on individual and environmental conditions.
In general, a rug can be particularly useful in horses that are:
- Fully clipped.
- Elderly, thin or with reduced energy reserves.
- Debilitated, or affected by conditions that compromise thermoregulation.
- Not acclimatised to low temperatures.
- Exposed to persistent rain, wind and cold, especially where adequate shelter is lacking.
In healthy, unclipped, well-acclimatised horses kept under appropriate management, routine use of a rug may on the other hand be unnecessary.
Which weight should you choose?
Rugs are commonly classified according to the amount of filling, expressed in grams per square metre (g/m²).
| Type | Indicative fill weight | Use |
|---|---|---|
| Waterproof rug with no filling | 0 g | Protection from rain and wind, with no significant additional insulation |
| Lightweight rug | 50–100 g | Limited thermal protection, useful in relatively mild conditions |
| Medium-weight rug | 150–250 g | Greater insulation for clipped horses or those particularly sensitive to cold |
| Heavyweight rug | 300–400 g or more | High protection for particularly cold conditions and for horses that need it |
This classification describes the commercial characteristics of rugs, but it is not a veterinary prescription based solely on outside temperature.
There is in fact no scientifically validated correspondence allowing one to state that a given fill weight is always correct at a specific temperature.
Actual protection also depends on the outer material, breathability, fit, the presence of a neck cover, exposure to wind and the degree of clipping.
Stable rug or turnout rug?
Stable rugs are designed for sheltered environments and are generally not waterproof.
Turnout rugs, by contrast, must be waterproof, hard-wearing and breathable enough for outdoor exposure.
Using a stable rug in the rain can lead to water absorption, with consequent loss of insulation and a possible worsening of thermal comfort.
It is also important to avoid rugs that restrict movement, cause rubbing or stay damp against the body.
An excessively heavy rug can impede normal heat dissipation and encourage sweating.
For this reason it is important to check the skin and coat under the rug regularly, especially when there are large swings in temperature between day and night.
7. What do studies tell us about horses' preferences?
An interesting study published in 2019 in Applied Animal Behaviour Science assessed the preferences of 23 horses regarding the use of rugs.
The animals had previously been trained to communicate, using symbols, whether they wanted to put a rug on, keep it on or have it removed.
The results showed that:
- At temperatures below -10 °C, preference for a rug was high.
- At temperatures between +5 and +10 °C, the presence of rain and wind appreciably increased the probability that horses would choose to be rugged.
- At temperatures above +20 °C, the horses tested preferred to remain unrugged.
This study is particularly interesting because it considers the animals' behaviour and preferences, not only assessments made by owners.
The results should not be interpreted as absolute thermal thresholds, but they do show how greatly wind, precipitation and individual characteristics influence a horse's comfort.
A further study published in 2019 found that even horses wearing a rug continued to seek shelter in unfavourable weather.
A rug, therefore, does not necessarily replace the availability of a sheltered environment.
8. How can you recognise a horse that is suffering from the cold?
Assessment of thermal comfort must be based on observing the animal, not solely on ambient temperature.
Signs that may suggest difficulty with thermoregulation include:
- Persistent muscle shivering.
- A hunched posture and attempts to shelter from the wind.
- Insistent seeking of shelter.
- Reduced spontaneous activity or changes in behaviour.
- Progressive weight loss despite an apparently adequate ration.
- In more severe cases, weakness, depression and changes in body temperature.
It should be remembered, however, that these signs are not specific and may also be associated with other disease conditions.
Shivering in particular is a physiological mechanism of heat production and does not necessarily imply hypothermia.
In horses with significant clinical signs, veterinary assessment is indicated and, where possible, measurement of the rectal temperature.
The opposite risk must also be considered.
An over-rugged horse may sweat under the rug, with increased skin temperature and thermal discomfort. Sweat on a horse at rest should prompt an immediate reassessment of its rugging and of its clinical condition.
9. The role of hay in winter: fermentation, heat production and increased energy requirement
Hay is one of the most important elements in the nutritional management of the horse during the winter months.
Its function is not only to supply fibre and energy, but also to contribute to heat production through the fermentation processes that take place in the large intestine.
When ambient temperature falls below the lower critical temperature, the horse must increase metabolic heat production in order to keep its body temperature stable.
This entails an increase in the energy requirement which, in many cases, can be met by increasing the amount of forage provided.
Why does hay help keep the horse warm?
The horse is a monogastric herbivore with a digestive system specialised in the fermentation of plant fibre.
In the caecum and colon, a complex population of micro-organisms breaks down part of the fibrous components of the ration, in particular cellulose and hemicellulose, producing volatile fatty acids, mainly acetate, propionate and butyrate.
These compounds are absorbed through the intestinal mucosa and used by the body as important energy substrates.
Microbial fermentation is also an exothermic process: part of the energy is released as heat.
The heat production associated with fibre fermentation contributes to maintaining body temperature and makes forage particularly useful during exposure to low temperatures.
Compared with a ration rich in starchy concentrates, a ration based predominantly on forage generally results in greater heat production associated with digestive and fermentative processes.
This does not mean that hay can replace every other source of energy, or that all forages produce the same amount of heat.
The amount of energy actually available depends on chemical composition, digestibility and the quantity eaten.
During the winter, increasing hay therefore brings a twofold benefit: it supplies additional energy and supports the heat production linked to intestinal fermentation.
How much should a horse normally eat?
For a healthy adult horse, forage forms the basis of the ration.
In terms of dry matter, a forage intake of at least 1.5% of body weight is a commonly used minimum reference, while many rations provide quantities between 1.5% and 2% or more, depending on individual needs.
In horses kept predominantly on hay, an intake of 2–2.5% of body weight as fed is a practical reference frequently used, provided that forage quality and the overall balance of the ration are adequate.
For example, taking a hay with about 90% dry matter:
| Horse's weight | Daily hay at 2% | Daily hay at 2.5% |
|---|---|---|
| 400 kg | 8 kg | 10 kg |
| 500 kg | 10 kg | 12.5 kg |
| 600 kg | 12 kg | 15 kg |
These quantities are management references, not compulsory rations.
They must be adapted to the level of activity, body condition, the availability of grazing and the other feeds provided.
A horse already receiving large amounts of forage may not need further increases, while one on a restricted ration or with greater metabolic demands may require a more substantial nutritional review.
By how much does the energy requirement rise as temperature falls?
The scientific literature and university publications on equine nutrition report different models for estimating the additional energy needed during exposure to cold.
One of the most frequently used indications is an increase in the maintenance energy requirement of about 1% for every degree Fahrenheit below the lower critical temperature.
Converting this relationship to metric units, the increase corresponds to about 1.8% for every degree Celsius.
This is an indicative coefficient, not a universal physiological law.
The actual response depends on the degree of acclimatisation, weather conditions, coat quality, body condition and the horse's level of activity.
To understand the practical meaning of these figures, let us consider a 500 kg adult horse at maintenance, with a hypothetical lower critical temperature of -10 °C.
Using the indicative coefficient of 1.8% per degree Celsius, we can estimate the additional energy required.
| Ambient temperature | Difference from the LCT | Estimated energy increase |
|---|---|---|
| -10 °C | 0 °C | 0% |
| -15 °C | 5 °C | +9% |
| -20 °C | 10 °C | +18% |
| -25 °C | 15 °C | +27% |
This table is a simulation based on a hypothetical LCT of -10 °C.
It does not mean that every horse must have its ration increased from this temperature onwards.
A well-acclimatised horse might have a lower LCT, whereas a clipped or poorly acclimatised horse might reach it at much higher temperatures.
It is also important to remember that the additional requirement is expressed as energy, not directly as kilograms of hay.
A practical example: how much hay should you add for a 500 kg horse?
Let us consider a 500 kg adult horse at maintenance, with an ordinary energy requirement of about 16.7 Mcal of digestible energy per day.
Let us assume that the hay used supplies 1.8 Mcal/kg of digestible energy, on an as-fed basis.
If the energy requirement rises by 1.8% for every degree Celsius below the LCT, we can calculate the extra energy needed and convert it into a quantity of hay.
| Degrees below the LCT | Energy increase | Additional energy | Additional hay |
|---|---|---|---|
| 5 °C | +9% | 1.5 Mcal | 0.8 kg/day |
| 10 °C | +18% | 3.0 Mcal | 1.7 kg/day |
| 15 °C | +27% | 4.5 Mcal | 2.5 kg/day |
These quantities are calculated on the assumption that the additional energy requirement is met exclusively by adding the hay in question.
They are therefore not amounts to be added automatically to every ration, but they do show how to translate an increased energy requirement into kilograms of feed.
For example, a 500 kg horse that normally eats 10 kg of hay a day might need, under the assumed conditions, about 1.7 kg more of that same hay if the temperature stays consistently 10 °C below its LCT.
Naturally, if the horse also receives other energy feeds, the calculation must take the whole ration into account.
The aim is not to increase the amount of feed indiscriminately, but to compensate for the greater energy expenditure without creating nutritional imbalances.
The quality of the hay matters as much as the quantity
An aspect that is often underestimated concerns the nutritional quality of the forage.
Not all hays supply the same amount of digestible energy.
A hay cut late, heavily lignified and with low digestibility may provide less energy than a forage harvested at a more favourable stage of growth.
When the energy requirement rises appreciably, feeding large quantities of very coarse, poorly digestible hay may not be enough to meet nutritional needs.
In such cases it is preferable to use a good-quality forage with adequate digestibility and a known energy value, ideally established through a nutritional analysis.
If the horse cannot eat enough forage, or has a particularly high requirement, it may be necessary to supplement the ration with other energy sources, selected according to its clinical and metabolic condition.
In older horses with reduced chewing efficiency, part of the long hay can be replaced with pelleted or cubed forage, suitably soaked, provided the overall ration remains balanced.
Why not simply increase the concentrate feed?
Increasing concentrates is not necessarily the best strategy for coping with cold.
A ration excessively rich in starch can alter intestinal fermentation processes and, if managed inappropriately, increase the risk of gastrointestinal disturbance.
Increasing forage, where possible, supports the physiological fermentation of the large intestine and supplies energy without needlessly raising the starch load of the ration.
In horses that need a higher calorie intake but cannot increase their hay consumption further, alternative energy sources may be considered, including highly digestible fibres and dietary fats.
The choice should be made through an individual nutritional assessment.
Water matters too: cold and the risk of colic
During the winter, an increase in forage intake must be accompanied by an adequate supply of water.
Low temperatures can reduce voluntary water intake, especially when the water is very cold or partially frozen.
The combination of high dry-matter intake, reduced water intake and little movement can create conditions that predispose to impaction colic.
It is therefore important to ensure clean water, always accessible and, where necessary, moderately warmed.
Any increase in the amount of hay should also be gradual, avoiding abrupt changes to the ration.
In conclusion: how much extra hay during the winter?
There is no supplementary quantity that is valid for every horse.
In moderately cold conditions, a healthy, unclipped, well-acclimatised horse can maintain its body temperature without needing significant increases to its ration.
When, on the other hand, ambient temperature falls consistently below the lower critical temperature, the energy requirement rises and it may be necessary to increase the forage available.
As a practical reference, some nutritional models assume an increase in the maintenance energy requirement of about 1.8% for every degree Celsius below the LCT.
The additional quantity of hay must be calculated according to the digestible energy of the forage and the composition of the overall ration.
Regular assessment of body condition remains one of the most useful tools for checking whether energy intake is genuinely adequate.
Hay, then, is not only the horse's main feed: during the winter it is also a fundamental means of supporting metabolic heat production and maintaining a sound energy balance.
10. Practical guidance for good winter management
Management that respects equine physiology should follow a few fundamental principles.
- Ensure an adequate supply of forage. In particularly cold periods, especially when ambient temperature falls below the lower critical temperature, the energy requirement can rise. The ration must be adapted to body condition and to individual needs.
- Make sure water is always available. Cold can reduce voluntary water intake, especially when the water is very cold or partially frozen. It is therefore important to check its availability and accessibility every day.
- Provide effective shelter. Protection from wind and precipitation is essential, even for horses that normally live outdoors.
- Assess body condition regularly. Maintaining weight through the winter is an important indicator of an adequate energy balance. A very thick winter coat can hide progressive weight loss: for this reason it is useful to combine observation with palpation of the main anatomical regions used for the Body Condition Score.
- Check rugs every day. It is advisable to verify that they are dry, correctly positioned and that they do not cause sweating, rubbing or restricted movement.
- Consider individual characteristics. Age, health status, clipping, sporting activity and acclimatisation should always guide management decisions.
- Avoid abrupt dietary changes. Even when hay needs to be increased, changes to the ration must be introduced gradually, bearing in mind gastrointestinal function and individual characteristics.
Conclusions
The horse is a species with a remarkable capacity to adapt to climatic variation and, in appropriate conditions, can tolerate very low temperatures without necessarily requiring artificial covering.
The winter coat, peripheral vasoconstriction, metabolic adaptations and the availability of adequate feed resources are fundamental means of maintaining body temperature.
Scientific research shows, however, that air temperature is not the only element to consider: rain, wind, body condition and individual characteristics can profoundly alter thermal comfort.
Clipping and the use of rugs are management interventions that must be assessed case by case, taking account of their effects on the animal's physiology.
Feeding plays a particularly important role.
During exposure to temperatures below the lower critical temperature, the energy requirement can rise significantly. Hay, through its energy contribution and through intestinal fermentation, helps to maintain thermal balance.
Good winter management therefore requires an integrated approach, considering environmental protection, feeding, health status and capacity for acclimatisation together.
The fundamental principle is simple: we should not decide whether a horse is cold on the basis of our own sensations, but through knowledge of its physiology and observation of its real needs.
Protecting a horse from the cold does not necessarily mean covering it more, but giving it the conditions to maintain its own thermal balance effectively.
Bibliography and scientific references
Scientific studies
- Mejdell C.M., Bøe K.E., Jørgensen G.H.M. (2020). Caring for the horse in a cold climate—Reviewing principles for thermoregulation and horse preferences. Applied Animal Behaviour Science, 231, 105071. DOI: 10.1016/j.applanim.2020.105071.
- Mejdell C.M., Jørgensen G.H.M., Buvik T., Torp T., Bøe K.E. (2019). The effect of weather conditions on the preference in horses for wearing blankets. Applied Animal Behaviour Science, 212, 52–57. DOI: 10.1016/j.applanim.2019.02.001.
- Jørgensen G.H.M., Mejdell C.M., Bøe K.E. (2019). The effect of blankets on horse behaviour and preference for shelter in Nordic winter conditions. Applied Animal Behaviour Science, 218, 104822. DOI: 10.1016/j.applanim.2019.06.003.
- Wallsten H., Olsson K., Dahlborn K. (2012). Temperature regulation in horses during exercise and recovery in a cool environment. Acta Veterinaria Scandinavica, 54, 42. DOI: 10.1186/1751-0147-54-42.
- Preference for shelter and additional heat in horses exposed to Nordic winter conditions. Equine Veterinary Journal (2016). PMID: 26509636.
Technical, scientific and nutritional sources
- University of Minnesota Extension. Caring for your horse in the winter. University resource on winter management, the lower critical temperature and the increase in energy requirement. https://extension.umn.edu/agriculture/animals-and-livestock/horse/caring-for-your-horse-in-the-winter
- University of Minnesota Extension. Caring for the underweight horse. A detailed resource on the nutrition of the underweight horse, the role of forage and dietary adaptations in relation to climate. https://extension.umn.edu/agriculture/animals-and-livestock/horse/caring-for-the-underweight-horse
- Ontario Ministry of Agriculture, Food and Agribusiness. Management and feeding of horses in cold weather. Guidance on the nutritional management of adult horses exposed to low temperatures. https://www.ontario.ca/page/management-and-feeding-horses-cold-weather
- Utah State University Extension. Caring for Horses in Cold Weather. A detailed resource on energy requirements and forage quality during the winter. https://extension.usu.edu/equine/research/caring-for-horses-in-cold-weather
- Institut Français du Cheval et de l'Équitation (IFCE). How do horses adapt to the cold? Equipedia. A technical and scientific resource on thermoregulation, acclimatisation and the management of the horse during winter.