12 minute read
Weather & Horses
and
HORSES
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Story and Photos By Alessandra Deerinck
The natural conditions
of the weather, related to the seasons are something that is beyond human control. Temperature and humidity changes happen throughout the year, but can also deeply change in a day, and affect the metabolism of any living animal. Animals that have a choice in how to cope with the weather changes, have developed different ways to do so, but when they live in a domestic environment, they are deeply influenced by how we keep them.
The equine species has a very efficient ability of adapting to where it lives.
Horses are warm-blooded animals, and their core body temperature stays within a certain interval even when there are big variations in the environmental temperature. . A sudden exposure to cold causes catecholamines to rapidly increase metabolism, while thyroid hormones cause the long-term cold adaptation. The body heat comes from the metabolism in all the body tissues. Animals with a higher basal metabolism, like growing or working animals, and pregnant or lactating mares, have a higher basal heat production. At rest, the heat production takes place mainly in the heart, liver, kidneys and brain, but during exercise the muscles produce a very high quantity of heat. Surplus body heat is dissipated to the cooler surroundings, mainly over the skin surface. The body size also matters, a large horse has more mass to produce heat, but a relatively smaller surface to dissipate it, compared to a similarly built but smaller horse. The equine limbs have a “radiator system”, where the arteries and the veins run close together in such a way that the warm arterial blood coming from the body core warms up the cooler blood in the veins. This situation reduces the temperature of the distal limbs, and also reduces the total heat loss.
Horses cope with temperature changes by seasonally changing their hair coat. The day-length regulates shedding and hair regrowth, and the coat quality is influenced by the local climate conditions and management factors. In most of the USA the heavy winter coat will start to grow sometime in late August. The hair coat, skin thickness, and subcutaneous adipose tissues have an important function on insulation properties and heat loss, and are related to natural or artificial selection. In fact, there are genetic differences among individual horses and breeds in their ability to grow a thick winter coat.
A horse’s coat has guard hairs which are longer and stiffer, and an undercoat that is soft and fluffy. The guard hairs protect the undercoat from dirt and water. The undercoat traps warm air and acts like a layer of insulation for the horse. There are natural oils that coat the hair and make them water resistant. The mechanism that creates the insulation works through piloerector muscles connected to each hair, that make the hair stay up or lay flat. When they stand up a layer of warm air remains trapped between them above the surface of the skin. When hair lies down the warm air is released, cooling the area above the surface of the skin. Horses can tolerate temperatures down to 5°F before their body temperature drops, but certain conditions like rain and wind can challenge a horse’s ability to keep warm. Rain flattens the hair coat, preventing the undercoat from providing the insulating layer of warm air. Wind blows the warm air away from the surface of the skin. Interestingly, snow is less of a problem, because it collects on the horse’s coat without wetting it and acts as a layer of insulation.
Blankets can be a quick remedy in case of wind or rain. When a horse has a natural winter coat, is healthy and in good body weight, the weather is not too windy or raining, with a temperature above 5°F, blanketing is not needed. If a horse is clipped, therefore deprived of his natural physiologic defense against cold temperature, we must use a rug and be very attentive about the material of the rug and when to apply it or remove it.
Another instance when blanketing a horse can be helpful is when he is sick or injured, older, with weight issues, has difficulty moving around, or has been recently moved to a colder climate. Horses can adapt to moving to a different climate, but need at least two to three weeks to do so. In order to use a blanket, to keep a horse warm and safe, correct fitting is very important. When blankets are too small can cause sores and do not produce enough warmth. When a blanket is too big it can allow too much cold air underneath, or be too loose and the horse can get tangled up, or even manage to slide out of it.
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Actually, blanketing can be worse than not blanketing, because the blanket will flatten the horse’s coat, and will not allow the horse to create the insulating layer of warm air that naturally allows them to cope with cold temperatures. Another variable that we should keep in mind is that a horse cannot remove a blanket when he feels it is not needed, and that a blanket does not cover the whole body and most of all creates a different body temperature in the limbs, which can be a problem for the horse.
Along with the changes of hair coat, nutrition naturally plays a very important role in thermal regulation. In hot weather, horse’s appetite will diminish, while the need for increased thermogenesis in cold environments is the reason for the evident rise in appetite and feed intake. Horses that are given adequate nutrition will start to put on a layer of fat in late summer and early fall. It is important to increase the quantity of hay we feed to horses in the winter, because the fermentation of hay in the horse’s hindgut produces a large amount of heat. Basically, horses have a naturally built-in heating system fueled by hay. The texture of the feed has an influence on the heat production, which is higher when horses consume roughage compared to concentrates. A possible explanation can be that when digesting long fibered feed there is production of heat from the longer time spent in chewing and in contractions by the smooth muscles in the gut.
Cold-blood breeds are more adapted to cold climates than warmblood breed types that are more slender horses. Cold blooded horses are more heavily built, have more protective hair in the mane and fetlock, and a thicker skin. In the winter they develop a thicker hair coat, and body fat reserves when feed is adequate, and have a more even distribution of the subcutaneous adipose tissue. More...
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Another way horses naturally warm up is by increasing their activity by moving around to generate heat, but in order to do so they need to be kept in enclosures that allow that. Horses exchange heat with their surroundings through conduction, convection, radiation and evaporation. The process is also influenced by wind speed, precipitation, humidity, and solar radiation.
Heat exchange by conduction happens through direct contact between the horse and a cooler surface. The speed of the process depends upon the conductive property of the surface and the temperature difference. The heat loss will be higher on a wet surface because water is a good conductor while heat loss to air by conduction is insignificant.
Heat exchange by convection happens by moving air or water across the skin. Horses can tolerate temperatures down to 5°F before their body temperature drops. Heat loss by convection happens also through the inner body surfaces, like in the respiratory system, where cold air is inhaled, it gets warmed up when passing through the airways, before it gets exhaled, and new cold air is inhaled.
Heat can be exchanged from a warm body to a cooler surface like the clear sky during night, or a cold surface indoors by radiation through electromagnetic waves Even an unheated shelter with four walls and a roof can reduce the climatic energy demand. The areas of the body with less insulation because of thinner hair coat, have a higher overall surface temperature, which indicates a larger sensible heat loss. Horses left in the sunshine at high ambient temperatures experience increased skin and core body temperature and show elevated cortisol levels, indicating heat stress. In winter time, horses can actually gain heat from solar radiation. A dark hair coat absorbs solar radiation more than light color that reflects solar radiation, and is more common in horse breeds from areas with a warm climate.
Heat loss by evaporation occurs when heat is taken from the body to evaporate sweat, or water from the wet skin after precipitation, as well as water vapor in expired air. The most effective physiological mechanism in horses to get rid of excess heat is evaporation of sweat, but it becomes less efficient when air humidity is high as it happens in the winter because the sweat production overrides the rate at which sweat evaporates. The benefit of evaporative heat loss is then lost and sweat starts dripping.
When the environmental temperature is cold, the nonevaporative heat loss by conduction, convection and radiation is predominant. The animal body increases the metabolism to produce more heat in order to maintain the core body temperature using natural physiological mechanisms. The animals can even get to shiver in order to elevate the core temperature. The core body temperature will drop when the heat producing capacity is exceeded by the heat loss, disturbing mental and physical functions and even consciousness. When the ambient temperature is too high the heat loss by evaporation dominates. The core body temperature will rise causing thermal stress to mobilize energy reserves, to sweat in order to keep a steady core body temperature. A homeothermic animal can only survive a core body temperature a few degrees above its normal body temperature.
Thermoregulation results from biological processes influenced by many factors, not just temperature. The factors and mechanisms that affect thermoregulation in horses are both environmental (temperature, humidity, precipitation, wind, solar radiation) and anatomical, physiological and behavioral related to the horse’s age, breed, physiological status, adaptation, coat quality, feeding, and body condition. The knowledge of mechanisms and factors that can have an influence on the horse’s thermoregulation definitely can help to improve the equine management and welfare in a domestic environment.
Creating an environment that can help horses self-adjust and cope with thermal challenges is very important and can be done by managing the resources we give them, such as space to allow movement, shelter to manage sunshine, precipitation and wind, dry bedding, proper feeding and body covers. The goal is to give the horse a choice so they can use their natural physiological and behavioral resources to gain thermal comfort. Horses are social animals, and when kept in a group, social dynamics may influence individual horses’ actual freedom of choice. If there is shortage of a resource, the higher-ranking animals place themselves in the middle of a huddle to get sheltered from the other horses when standing in the wind. The horses with higher rank spent relatively more time eating hay which has a more concentrated nutrition and causes them to gain more weight during winter, while the lowranking horses drop in body condition. Social rank seems to be less important in groups of young horses. It is important to prevent valuable resources being monopolized by a few horses in the group by providing food piles more numerous than the horses present in an enclosure, which can also avoid confrontation in terms of territory.
Feeling too warm or too cold will give a horse a reason to pursue thermal comfort just like hunger motivates them to look for food and consume it. When horses feel too cold, they may stand down-wind with a tucked tail, aggregate with other individuals, or start running if they do not have access to a shelter. If they can find shelter from wind and precipitation, they will use it. During winter time on cold days horses will try to expose the largest possible area of the skin towards the sun using solar radiation as a natural resource for thermal comfort. Horses do make use of human made shelters. In a cold climate a large enclosure permits horses to move to increase the heat production. The horse can choose to stay inside or outside a shelter as the weather changes from rainy to sunny conditions. When feeling too warm, horses can still use shelters to seek shade, or protection from insects. When there is no access to a shelter they may drink cold water, or move to a windy place.
CAN HORSES
TAKE CARE OF THEIR COAT?
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