How Do Horses Thermoregulate When It Is Cold? Part I

I just got back from a trip to England and it is colder here in California than it was there!  I don’t know about where you are but it is pretty chilly here.  Okay it isn’t as cold as say Minnesota but relatively speaking, it’s cold.  While I was in England I took a walk up onto Dartmoor, home of the native Dartmoor pony.  Although beautiful, it can be a pretty bleak landscape when the weather does blow in.  I took the picture of the Dartmoor pony on the right while I was on my walk. These ponies are out in all weather, never wear blankets and eat whatever they can find.  So how do they stay warm in the winter?  How does your horse stay warm in the winter?  Read on to find out.

 

Horses are mammals and as such are warm-blooded just like humans and so when the air around them is colder than their body temperature, heat transfers from them to the environment and they get colder. To survive they must regulate this heat loss, however such heat loss is not always detrimental, for example if the horse is too hot and needs to cool down.  This is why horses are in danger of overheating when worked in hot conditions.  Their body temperature rises due to energy released from exercise (or when it is very hot, heat absorbed from their environment) and they need to move that heat to their surrounding environment.  They cannot do that though if the surrounding environment is as warm or warmer than they are.  So the environmental temperature and body temperature determine the extent to which heat must be conserved.

If the horse remains in its comfort zone or “thermo neutral zone” little needs to be done to regulate temperature.  Once at the bottom of this comfort zone the horse reaches its critical temperature and the body speeds up chemical reactions within the body in order to burn more calories and to create more body heat.   This requires an increase in dietary energy intake, if there are not enough calories in the diet to meet the additional needs for maintaining body temperature the horse will utilize its body energy reserves (fat).  If this deficit continues for too long body condition will be compromised and the horse will lose weight. (See previous blog posts Understanding Energy Balance and Body Condition Scoring). Exercise produces heat from energy burned by muscles so moving is another way the horse has to stay warm but the energy for movement has to come from somewhere, either the diet or body energy reserves.  This may be one reason why horses seem to run around more when the weather is cold.  Muscle contractions don’t just occur though as a result of the horse physically changing locations they also occur as a result of shivering.  The energy produced from these muscle activities raises the horse’s core temperature. Other sources of heat that don’t require feed or body energy reserves are the sun and such things as heat lamps.

There are of course also ways of conserving energy and heat in order to stay warm.  Just like us, horses can reduce the blood flow to their extremities such as their ears, muzzle and legs.  This is why it is often said that to tell if a horse is cold one should touch its ears.  If you think you might use this as a barometer I suggest feeling them when the horse is not cold so you can tell the difference.  Trying to figure out from the ears whether a horse is cold when the whole horse is wet from standing in the rain for several hours is not as easy as it sounds, everything just feels damp and often you wonder whether you can’t feel any heat because you are cold as well!

the hill was the location for part of the filming of the movie War Horse

Also like us horses can make their hair stand-up, which is called piloerection (think of goose bumps), which acts to increase their hair depth and traps air next to their bodies creating an insulating layer.    It is because of this function that you might hear people say that well cared for horses are quite alright out in the cold as long as it is dry.  Once their coats get wet the hair is unable to stand up and create this insulating layer.  They then rely on the oils in their coat to prevent their skin from getting wet, which is why you should not bathe a horse that lives out in the winter or use a body brush which drags the oils through the coat, as they need the oils to stay near their skin to act as a protective barrier.  Horses living outside need to have access to adequate shelter such as a 3-sided shed as such shelter has been shown to reduce heat loss by 20% not only because it allows their hair to stay dry but it also reduces heat loss from wind chill.

Before we get to wind chill I want to mention that piloerection is also why some people do not believe in using blankets and actually think blankets can cause a horse to be colder.  This is certainly true if the blankets do not contain adequate insulation for weather conditions.  A blanket flattens the horse’s hair and prevents piloerection.  If in turn the blanket is not thick enough to adequately insulate or it leaks, the horse it will be cold and will not be able to use piloerection to stay warm.  This is not to say that blankets should not be used, if you have a horse who does not carry much weight, with a thin hair coat or decide to clip your horse because it is in heavy work, a blanket will be necessary.

A quick word of caution against thinking that by bringing your horse into a stable (box stall) during cold dry weather that it will be warmer, this may be true but also consider that in a stable there is limited space for movement, there are no other horses to huddle up with, air is often still and cold, and there is less heat from sunlight available.  In my experience during such dry cold conditions, stables (box stalls) are often colder than outside.  Often the coldest part of the night is around 6am, as the sun comes up areas reached by sunlight warm up quickly compared to those areas still in shade such as the inside of stall, so horses in stalls are subjected to cold for far longer than those horses that can get out into the sun.  This can cause a real conundrum in spring and autumn when your stabled horses are blanketed at night and you need to take their blankets off early in the morning before you go to work because later in the day they will be hot.  In these instances you have to know your horse and know whether it is better for them as an individual to be too hot or too cold.  The hard keeper who is lean, gets cold and is stressed easily would probably be better left with the blanket on versus the horse carrying more condition who won’t be at any great detriment if he is a little chilly for a couple of hours.

So how can you support your horse nutritionally during cold weather?  We will cover this in part II.

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How Do Horses Thermoregulate (Maintain Body Temperature) When It Is Cold? Part I by Dr. Clair Thunes and Summit Equine Nutrition LLC is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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