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Keeping Horses Warm This Winter: An Insight into Thermoregulation and The Horse's Requirements

Keeping Horses Warm This Winter: An Insight into Thermoregulation and The Horse's Requirements

Rebecca Allan


Winter conditions vary dramatically between various regions, as does the tolerance of individual horses to changes in temperature, precipitation, and wind. However, horses are built for survival, being warm-blooded animals, they are able to maintain their internal core temperature within narrow limits despite the changing environmental conditions. They are able to do this due to being endothermic, meaning they can produce heat through metabolic processes in their body, enabling them to maintain their average body temperature (Frappell et al., 2008). This article aims to improve the understanding of thermal regulation  in  horses so owners can help their horses achieve thermal comfort under winter conditions, with a specific highlight on nutrition, the use of rugs, clipping, and shelter.


Thermoregulation is the process through which horses control their internal core temperature through physiological responses, achieving the balance between heat loss and heat production in response to environmental temperature changes (Guthrie et al., 1998). In winter, conserving heat is the horse's priority. The two primary physiological responses to decreased temperatures are cutaneous vasoconstriction which reduces blood flow to the skin surface and therefore aids heat retention, and through piloerection where the horse’s hair stands up to create an insulating layer of air to maintain heat close to the horse's surface (Hines, 2018). Sudden temperature changes also trigger behavioural responses leading horses to seek shelter from  the  cold  and  huddle  together  to  stay warm  (Hetem et al., 2007).
Under certain temperatures, horses can maintain warmth. The norm, known as the thermoneutral zone (TNZ), defines the range in temperature where the horse regulates its body temperature with little or no energy expenditure (Christopherson et al., 1986). It achieves the balance between heat loss and generation through physiological responses only. A mature horse's average TNZ ranges from 5℃ to 25℃ for horses living in mild climates (Morgan, 1998). However, as every horse is an individual, TNZ will be influenced by the temperatures the horse is acclimatised to. Of note, horses can reach complete acclimatisation in 21 days (McCutcheon et al.,1999).
One extreme of the TNZ is the lower critical temperature (LCT), known as the lowest temperature in the TNZ (Martin-Rosset et al., 2015). The horse must increase metabolic heat production to maintain its average body temperature as heat loss exceeds heat production. The LCT limit in adult horses is usually 5℃ for those in temperate climates and extends to -15℃ for those adapted to northern continental environments with natural unclipped coats (McBride et al., 1985; Morgan, 1998). Increased demands for metabolic heat production will incur energy costs. Additional energy costs for horses to remain in thermal comfort have been studied, with comparative results achieved. McBride et al. (1985) indicated an overall increase in maintenance energy requirements for horses suddenly exposed to cold environments (0℃ to -40℃).  Mature horses required an additional 3.4kJ digestible energy (DE) per kilogram of body weight (BW) per every degree Celsius drop below the horse's LCT. Similarly, Cymbaluk et al. (1990) established yearlings’ DE intake requirements to increase from 110kJ per kg/BW to 145kJ per kg/BW when exposed to warm (10.9℃) and cold (-5.2℃) ambient temperatures, respectively. This study estimates a 1.3% DE increase for yearling horses per degree Celsius below 0℃. For weight gain, an additional 0.7% DE intake per degree Celsius below 0℃ was calculated. These studies show the importance of providing enough energy for horses to keep warm, and for those that are growing, enough energy to also achieve optimum growth rate. 
Although air temperature defines TNZ and LCT ranges, it is important to highlight how wind and rain increases thermoregulation requirements. As mentioned, the horses' coat traps air creating an insulating layer. However, wind will continuously remove the warm air close to the skin aiding heat loss. Similarly, rain causes the coat to be continuously flushed with water again contributing to heat loss. Mejdell et al. (2005) performed a 23-day study on Icelandic horses acclimatised to living outdoors with access to forage and shelter, to investigate their responses to cold weather. Results showed the average incidence of horses remaining outdoors was 70%. The study recorded air temperatures of up to -31℃, however, shivering was only observed once, on the day where air temperature was 5℃ and was in combination with heavy rain. A strong tendency for increased shelter use in response to wind and rain was also noted. Similarly, Jørgenses et al. (2016) studied horses in paddocks in Northern coastal climates, who had access to two shelter compartments. One of the compartments had infrared lighting increasing warmth. It was reported that horses spent more time inside shelters on rainy days and changes from unheated to heated shelter occurred on wet and windy days. These two studies highlight the increase in thermoregulation requirements for horses when faced with wind and rain, as well as the importance of shelter to be able to deal with these conditions.  
Other factors affecting thermoregulation are breed, age, body condition, size, and health condition. Each horse is unique and must be treated as an individual. For example, young, thin, and older horses are less tolerant of cold environmental conditions than mature horses in good condition. Increased DE intakes can be counteracted through correct nutritional management by feeding appropriate amounts of forage. Therefore, it is essential to make  dietary changes for vulnerable horses much sooner than for mature horses in good condition when temperatures fall to ensure thermal comfort.


Body temperature is maintained by metabolic processes, including the heat provided from nutrient digestion. Feeding fibre is the best way to keep your horses warm during winter months. The primary source of high fibre feed comes from forages such as grass, hay, or haylage. The equine digestive tract is unique because enzymatic digestion occurs in the foregut and fermentation occurs in the hindgut. The hindgut houses a vast population of microbes which breaks down feed that was not digested in the small intestine, particularly fibrous feeds such as hay or pasture. Fibre is converted by microbial fermentation into volatile fatty acids which can then be absorbed and utilised by the horse’s body (Dougal et al., 2013, Merritt et al., 2013). As a by-product of fermentation, metabolic heat is generated helping warm the horse from the inside out and supporting thermal comfort (Santos et al., 2011). Grain feeds on the other hand, are ultimately the reverse of forage and have a high starch and low fibre content. Starch is enzymatically digested in the foregut, particularly in the small intestine, by the enzyme amylase, where it is broken down into glucose for absorption by the horse’s body. However, this process does not produce metabolic heat as a by-product (Merritt et al. 2013).  Overall, bacterial fermentation produces more heat than enzymatic digestion. Due to this, fibrous feeds such as hay have a higher heat increment than low-fibre feeds such as grain (Julliand et al. 2004). This highlights the importance of forage consumption for your horses to stay warm during winter months. Nevertheless, it should be mentioned that out of the grains, oats, despite being low in fibre, have a higher heat increment than other grains due to their fibrous outer hull (Lindberg, 2013). 
Regarding consumption, horses are recommended to consume 2 - 2.5% of their BW per day. Maintenance diets should involve around 2% of a horse’s BW per day, whereas older or growing horses should consume 2.5% of their BW on a daily basis. Forage should be the basis of any horse´s diet as it supports their trickle feeding behaviour and keeps their gut healthy. Good quality forage contains a lot of nutrients and may be capable of providing enough energy  for healthy horses in maintenance or light work. Also of note is that  fibre provides slow-release energy (Hervik et al. 2019), so can be ideal for horses that can become over-excited.  
Harper et al. (2004) estimated an increase of 10-15% good quality forage to be fed when temperatures drop below freezing level (0℃). Ideally, an unlimited (ad libitum) supply of forage fed at ground level is the best way to meet your horse's behavioural and nutritional requirements during winter when there is an increased demand for heat production.  Nevertheless, this system may not be appropriate for all horses. For example, an overweight horse would benefit from a restricted intake due to weight loss; in this case, you can ration their daily requirement intake and spread it out throughout the day. On the other hand, for young, old, thin, or performance horses who are more vulnerable to winter temperatures or have a high energy output due to exercise, forage alone, although it should be the baseline of their diet, may not be sufficient to keep them in top condition as they cannot consume enough forage to meet their energy requirements. Therefore, it is recommended to increase calorie intake, through commercially available feeds that contain higher energy grains and/or fats to provide the necessary energy. Selecting  which  feeds  are  most  suitable  will  depend on your individual horse’s requirements (see Feeding programme considerations). 


Figure 1. forage is an excellent way of keeping your horses warm this winter. 


Overall, a winter-feeding programme should be based on pasture availability and good-quality forage, supplemented with appropriate sources of energy, protein, vitamins, and minerals. The amount of grain fed should be calculated based on your horse's condition and workload and supplemented properly with good quality forage before grain addition. A diet high in fibre should meet the increased heat demands for your horses during winter, whereas low fibre diets would not fully support metabolic heat increment requirements.
As crucial as forage consumption is during the winter months, water intake is equally important. When temperatures decrease, dehydration is probably the last thing on your mind. However, it is imperative to know that horses can become dehydrated at any time of the year, even at low temperatures during winter months.


Horses lose water from their body through faeces, urine, sweat and even through exhaling air, as dry cold air increases water loss from the respiratory tract and lungs. On top of this, forage consumption in winter increases meaning extra water is required for fibre digestion in the hindgut.  The accepted water requirement for horses is around 60ml/kg/day (Freeman, 2021). Thus, an average horse (500kg) would require approximately 30L of water per day as a minimum requirement. In addition, summer grass has moisture levels of approximately 60-80%, contributing to the horse’s water intake. In contrast, in winter, hay and grain feeds are fed more abundantly due to reduced grass availability. These feedstuffs contain less than 15% moisture and so poorly contribute to the horse’s daily water intake (Cymbaluk, 2013). Water is necessary for maintaining moisture levels in the horse’s gastrointestinal system therefore decreased water consumption causes ingesta  to  dry  up,  increasing  the  risk of  impaction colic or intestinal blockage (Bihonegn, 2018). Equids tend to drink less water throughout the winter months, therefore it is essential to observe their water intake and encourage them to drink if necessary to maintain hydration and reduce the  risks  of  colic  (see  Equine  Colic  and  its  associated  risk  factors).
Kristula et al. (1994) performed a study consisting of two trials to compare ab libitum consumption of ambient near-freezing water (ranged from 0℃ to 1℃) and warm water (46℃ to 49℃ when provided) of 14 ponies. The study concluded that on average, 40% of the ponies drank more warm water compared to ambient, near-freezing water. Warm water refills occurred at 46℃ to 49℃ however, most consumption happened when the water was 20℃ to 35℃. Based on this study, providing warm water for horses may encourage them to drink and help maintain hydration levels.  
Another factor to consider is that low temperatures encourage water to freeze, therefore through correct management water troughs/ buckets should be checked at least twice a day to break and remove any ice if water supply has frozen. Supplying warm water may minimise this risk, although this may not be practical for many owners.
A common theme associated with winter is the concept of rugging and clipping horses. Through many years of evolution, horses have developed a thick winter coat, composed of two hair types: long stiff guard hairs and fine  hairs as an undercoat (Legnani et al., 2017). The long guard hairs help to shed rain whilst the fine  hairs help trap warm air and act as an insulation layer for your horse (Davies, 2018). Additionally, natural oils in horses' coats have water resistance properties (Langlois, 1994). The hairs are also connected to piloerector muscles which allow the hair to stand up, magnifying the insulating layer or allowing the hairs to lie flat, acting as a cooling mechanism. Thus, horses are naturally well adapted to cope with winter conditions. So, the question arises, to rug or not to rug?


Mejdell et al. (2016) trained warm-blooded and cold-blooded horses to use simple “sign language” to communicate their preferences for wearing rugs. Mejdell et al. (2019) tested 23 horses, under differing weather conditions, for their rug preference. Results suggested that no horse preferred to wear a rug in sunny spring and summer weather, although most asked for a rug in cold, wet, and windy conditions. By air temperature, 80-90% of the horses preferred to have rugs on at temperatures under -10℃. As temperatures increased, the horses’ preference for rugs also decreased, and at 20℃, they did not want rugs. On  the  other  hand,  100%  of  horses  chose  to  wear  a  rug  in windy, rainy, and chilly conditions. This  study  showed  horses  preferred  a  rug  in  wind  and  rain  combined  with  low  temperature  conditions  due  to  increased  thermal discomfort. 
Theoretically, a horse with a good body condition, a natural coat, and a good supply of forage would not require a rug under moderate temperature conditions (5℃) as its own body would be able to generate sufficient heat to maintain an average body temperature. However, as per the previous studies, rain and wind cause thermal discomfort; therefore, providing access to shelter with the possibility of a rug would be the best option to challenge these fluctuations in climate. 
Clipping nowadays is a common practice in winter, it minimises sweating and allows horses to dry off quicker after exercise, preventing the development of a chill. However, workload is not the only deciding factor when deciding whether to clip or not. An individual horse’s coat also determines whether they need clipping. Generally, a horse in light work would not need clipping, however, a horse with an extremely fluffy coat may benefit from a lower half body clip (trace clip) as sometimes hacking only once a week makes them sweat excessively. On the other hand, a horse in medium/hard work which would normally benefit from being clipped may not require it if the horse is thin skinned and doesn´t grow very much of a winter coat. Overall, every horse is an individual and the combination of workload and coat thickness would determine clipping necessities. However, as clipping removes the horse’s natural winter coat, supplementary rugs for protection would be required. 
Another factor to consider is weight loss. In natural situations, horses lose excess fat during winter, so the horse starts off lean in spring to cope better with the new spring grass. Rugging an unclipped overweight horse will prevent weight loss as they will not move around and burn fat to stay warm. If your horse is obese and weight loss is desired, consider a trace clip as it will increase the amount of energy your horse needs to keep warm encouraging heat generation through movement and burning excess fat. 


In addition to rugging and clipping, shelter is imperative and must be accessible whether your horse has a full coat or is rugged or clipped. The shelter can naturally occur through vegetation such as trees or hedges, or be provided as manufactured enclosures such as field shelters and stables. These man-made options provide a more permanent management solution for when the weather changes rapidly and enables horses to cope with sudden wind and rain.
Overall, it is safe to say, an unclipped horse with access to shelter in good body condition with plenty of forage and water would not require a rug, as metabolic heat alone produced by digestion will suffice. However, rugging the unclipped horse depends on rain, wind, sunshine, and each horse’s cold weather threshold as well as owner requirements. Horses needing to be ridden after being out in the rain would benefit from a rain sheet to keep the saddle area dry. At the same time, fully clipped horses will need rugs in winter no matter the circumstance due to not having their natural winter coat for protection.  


Horses will experience differing thermal challenges during the winter months.  Lower temperature combined with wind and rain are very demanding, and the management of horses throughout this period must be based on thermoregulation principles, facilitating horses to maintain their internal core temperature. Once the LCT limit is reached, energy demands increase, and human intervention to meet feeding requirements for each horse is required. Providing horses with the necessary forage is fundamental to support their internal heating mechanism and ensuring water intake requirements are met is crucial to prevent dehydration and colic. Additionally, a rug is a helpful supplement for more vulnerable horses and clipped horses as it provides extra protection from cold temperatures. However, it is imperative for horses to have access to shelter during winter months to be able to cope with sudden wind and rain.



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