Pretty much what the title says.
I've heard that decreasing daylight encourages winter coat growth. Others say that rugging will prevent winter coat growth.
Does rugging horses interfere with the grow of the winter coat?
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In two studies it proves that winter/summer hair growth can be controlled and stimulated by photo period - or daylight length, by using artificial light sources. This is independent of rugging.
The colts and fillies in the EP treatment group changed from winter to summer coats (molting of winter coats), whereas those in the control group did not.
The use of an extended lighting program is an important management tool for horsemen who need to get horses to slip winter hair or keep hair short throughout the year.
The difference a rug will make is it will flatten the hair, so it doesn't look as woolly. This can also inhibit the horse's ability to stay warm, as it is the extended hair that creates a barrier between the cold air and the horses skin.
.../ 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.
This article from the RSPCA, discusses rugging horses in winter:
What about rugs in winter?
In cold wet weather a good quality and well fitting rug can help the horse to maintain condition, as a cold wet horse will burn a lot of energy keeping warm. Keep in mind though that if your horse is young and healthy but tends to get fat, rugs will actually help him or her to maintain that fat. In a natural situation excess body fat is burned off through the winter.
The coat of an unrugged horse stands up in cold weather to trap air and warm the horse. If you decide to rug you have to compensate for this mechanism as a rug will stop the hair from being able to do its job. In some circumstances a rugged horse is actually colder than an unrugged horse if it is a badly fitting thin rug that flattens the hair and reduces the movement of the horse without providing any real warmth.
Rugs need to be checked regularly (at least twice a day) to make sure that the straps have not broken and the rug slipped, which can cause injury to your horse. Hoods can be very dangerous for horses and should only be used if the horse is being checked very frequently (much more than twice a day) because if a hood slips it can cover the eyes and rub the eyeball (which can cause an ulcer on the eye – a very serious condition for a horse requiring immediate veterinary attention). Horses with slipped hoods (due to not being able to see) have been known to fall into dams (and drown), injure their eyes on branches or other protrusions etc. or panic and run through fences.
A rug should be removed regularly to make sure it is not rubbing, letting in water etc. and to make sure the horse hasn’t lost weight or gained too much weight. Rugs prevent horses from exfoliating their skin properly (by rolling and mutual grooming etc.) so a rugged horse must be groomed thoroughly and frequently to get rid of the build up of dead skin and hair (see the article Why do I need to groom my horse?).
There's many other factors to consider when deciding whether to rug a horse, including living conditions, diet and grooming which is outside the scope of this question. This is purely about whether rugging prevents winter coat growth.
My father was a thoroughbred horse trainer he always taught me to let your horse be natural no blanket because they must get used to the cold and a blanket will cause the horses coat not to grow in properly ...but he would allow blankets if it was well below zero only during the coldest part of the day ...or night ...so we always followed his orders and my horse is now 30 years old her winter coat is sooo THICK she looks like a wooly mammoth haha lol its going to take a lot of curry combing to help her shed that coat in the spring.
We have the horses in as barn at night and paddock with outbarn during the day so they have protection from wind snd weather at all times but if its snowing that old mare wants to be out in the snow she loves the snow she actually chooses to go in the snow instead of protection of a roof and walls And she's 30 years old....