Winter has started again. Well, at least over here in the Netherlands until now not very convincingly, but we have had some days with frost. It is not possible to foresee what kind of winter it will be. We are not able to do that, but most likely neither are our horses (and ponies! but for the sake of convenience we will call them all horses). Still they prepare themselves. They already start their preparations towards the end of the summer. Influenced by the shortening of the days. they start producing their winter coat. Despite the fact that it is still often very warm in August. The horses start their preparations by producing more hairs. Not immediately longer hairs, but many more than in mid-summer. How that coat will develop further will depend on the circumstances.
Horses that are kept outdoors 24/7 will continue producing more hairs in the autumn. They are stimulated to do so, predominantly by the reduced hours of daylight, and they will continue to do so approximately until the shortest day of the year. How many extra hairs they will grow not only depends on the day length, but also on the outside temperature. The colder it is, the more hair they will grow (up to a maximum). And also, the colder it is, the longer the hairs will grow (up to a maximum as well). The numbers and length of the hairs, apart from day length and temperature, will depend on their genetics (breed, but there is also within breed variation), the shelter options, the amount of precipitation and wind, the health, and the nutritional status.
Apart from the winter coat, horses have other possibilities to protect themselves against the winter weather. Already at the end of the summer their metabolism start to change, so that the energy uptake predominantly will be stored as fat. And especially as subcutaneous fat. Unlike in the summer, this fat will not be stored in patches, but nicely divided over the entire body. This layer of fat serves as insulation. The prioritized fat deposition is ‘switched on’ because the horses will develop some degree of insuline resistance. The elevated insuline concentration in the blood will stimulate the liver to deposit the energy as fat, rather than as glycogen in the muscles. Additional advantage of the elevated insuline levels is that it helps the horse to better digest the feed that in winter is of much lower quality than in summer. Two birds with one stone!
It is getting colder and the temperature falls below that where no action is required to retain a constant body temperature. We will call that the lower critical temperature. How do horses deal with that? They have a number of mechanisms. The temperature regulatory system in the hypothalamus
A wet skin results in a quick cooling down, especially in combination with wind. Fortunately horses will not freeze after every little shower. Their skin produces a substance that, in addition to the fact that the hairs are overlapping each other so that the water can glide from one hair down onto the next, makes their coat water repellent. Water that falls on their coat slips off again, without reaching the skin. Because grooming reduces the water repellent properties of the coat, it is better not to groom horses that are kept outdoors extensively during winter. The fact that snow remains on the horse, and does not melt away, indicates its insulating qualities.
A second mechanism that is initiated by the hypothalamus in case of cold is vasoconstriction (the narrowing) of the veins that are located under the skin. The blood flow through these veins is reduced, which also reduces the amount of heat loss through radiation. Blood is the heat transportation system of the body. It transports heat from the muscles, where it is produced as a by product of the burned energy (muscle activity). The body temperature can be regulated by sending more or less blood past the skin. The ‘surplus’ blood is directed more towards the center of the body, where it is used to maintain body temperature.
Horses at rest have a certain basal metabolism. That is the level of metabolism that is required for the processes of a functioning body at rest. Part of that is maintaining body temperature. In action, approximately 75% of the energy is converted into heat. Most of that disappears from the body through radiation from the skin. Horses at rest that are on the verge of becoming cold will reduce their level of activity. Research on Camargue horses has revealed that they save up to 17% of their daily energy expenditure by moving less when temperatures drop. As soon as horses are cold they will rather increase their activity level to become warm again, provided they have the energy and space to do so.
When the combination of the erection of the coat, vasoconstriction of the veins under their skin, and reducing the activity is not sufficient to remain warm, then the horse can start to shiver. That is also initiated by the hypothalamus. Shivering is the involuntary, rhythmic contraction of muscles, so that heat is produced. This type of muscle action works on aerobe (oxygen demanding) burning of energy, so that no lactic acid is produced and the muscles can maintain their activity for a long time. Horses that shiver often also have a slightly higher breathing frequency, so that sufficient air is taken in to provide the muscles with oxygen. By shivering, the heat production can be increased five to six- fold as compared to at rest.
The idea is that the shivering has a function in the acute phase of the being cold. When that phase is prolonged, horses will have to switch to the system that they develop when getting accustomed to lower temperatures. They then produce the hormone thyroxine, which causes the base metabolic level to increase, so that it produces more heat. In that way, the horse actually ‘turns up the heat’ of their own internal heater. The base metabolism normally works on a combination of aerobe and anaerobe (no oxygen required) burning of energy. The thyroxine stimulates a shift towards the aerobe process, causing the metabolism to shift towards the burning of fat and no lactic acid is produced.
The extent at which the base metabolism is increased depends on temperatures. The increase in the base metabolism results in a decrease in the horse’s lower critical temperature (the temperature below which the base metabolism no longer is sufficient to keep the horse warm). The horse then is better capable to withstand the cold. Also horses with a less thick coat can increase their base metabolism this way, so that they can better deal with the cold. It is all relatively speaking, because of course they are at a disadvantage compared to horses with a thick coat. On the other hand, a horse with similar less thick coat, but that comes from a warm environment, will be less capable to deal with the cold than the horse that has had the opportunity to acclimatize.
Horses obviously can deal with some degree of cold. But we have not discussed the preconditions yet. On the one hand those of the horses, and on the other those of ‘their’ humans. Horses can handle the cold, provided they have had the opportunity to prepare themselves, and provided they have the opportunity to seek shelter from strong wind and precipitation. And, possibly even more important, provided they have sufficient roughage at their disposal. The digestion of roughage in itself produces heat. Apart from that, maintaining body temperature in the cold requires a lot of energy. This is, as we have seen, amongst others due to the higher base metabolism that horses develop when they are exposed to the cold for an extended period of time. It is obvious that horses in nature are able to cope with lower temperatures. They will be cold at times, but as long as their body temperature does not drop below the level at which their body can continue to function, it is not life threatening. They will loose a lot of weight during the winter as a consequence of the burning of the fat reserves, but that is a natural process. Horses that end the winter in a rich condition run a serious risk of developing obesity in the following summer.
Apart from the preconditions for the horses, there are also precondition for the people who own these horses and/or work with them. In general the reason they are involved with the horse is because they want to do something with
him (or her, obviously). Riding, driving, showing, or whatever you decide to do. The preconditions for the horses and their people not necessarily need to be conflicting. But when you really like to work with your horse more intensively it would be convenient if he would be able to easily get rid of the heat that his muscles produce during training. Excessive sweating as a result of the inability to loose the heat through radiation because of a long and dense coat is undesirable. The production of the long and thick coat can partly be controlled by putting a blanket on the horse as soon as the temperature starts to drop and/or keep them indoors in winter. This is because the influence of the low temperature on the wintercoat development is reduced. Shaving is another, additional, option. If you decide to shave your horse it is very important to take precautions so that the horse will not be cold outside the training hours. A shaven horse misses the insulation layer of his coat and, in addition to that, the coat is not longer water repellent because the hairs are too short to overlap.
Apart from the ‘sweat issues’, the type of energy burning plays a role. In the training the horse is expected to respond actively and, especially in sports like jumping, that he is able to come to intense activity quickly and is able to maintain that activity for
some time. In an activity burst the aerobe system is started and that burns a small glycogen reserve in the muscles. After that, the anaerobe system is taking over. Without going into detail on the metabolism of muscles, it is sufficient to realize that the muscles are shifting towards the anaerobe system. The ‘problem’ with the increased base metabolism as a result of the acclimatization to the cold is that it is mainly based on the aerobe burning and that shifting to the anaerobe burning is not easy for these horses. They are much better set up for prolonged exercise, but at a low energy level. No intensive training in, for example, showjumping.
Summing up, the way you keep your horse in winter should be directly related to what you expect from him during winter. The question of whether to use a blanket or a stable or to shave or not depends very much on what your plans are for the winter months. Obviously you can also choose to demand less from your horse (or pony) in winter and pick up training again in the spring. This reflexion will be different for everyone and we will not go into that decision here.
The technical details from this blog are mostly coming from two scientific reviews. No recent reviews, but during the past years no spectacular new insights have become available. Unfortunately, these reviews are not freely accessible, but for those of you who can get access, the references are:
B. Langlois. 1994. Inter-breed variation in the horse with regard to cold adaptation: a review. Livestock Production Science 40: 1-7.
N.F. Cymbaluk. 1994. Thermoregulation of horses in cold, winter weather: a review. Livestock Production Science 40: 65 – 71.