when my horse sheds his winter hair, he loses it in patches leaving him with bald spots all over his body. is he lacking in a vitamin?

is he actually loosing ALL of his hair or just his long thick hair? If its just his winter hair, dont worry. one of my horses does that too.

he loses all the hair in patches, completely bald over most of his body.

id call the vet

Ick, I would definitely let your vet know about it. Yes sounds like he is certainly deficient in a vitamin and more! Is he getting enough forage to eat? Ribs showing, rough textured coat? Is he getting unlimited water, how about a salt block? This sounds alot like just malnutrition to me.

If you want to help regrow the hair and condition the skin topically as well, try extra virgin olive oil (organic or cold pressed is in a more original basic form and is best to use) massage it into his skin on all of the bald patches every few days. Hair should start growing back in no time.
Viodine is a medicated shampoo for rain-rot and other skin problems if you want to look that up at your local co-op or Tractor supply. JeffersEquine.com also has it.

Definitely talk to your vet about it though, that really sounds rough!

Well, I can’t say anything about that.

Well, I think you took your horse to doctor, and ask him or her why my horse loses hairs. He should advice you better treatment.

The main cause for hair loss are environment,temperature or it may be caused by a serious fungus that invades the hair.So, try to consult a doctor and will give the medicine.

I think Your horse can't take food properly. The other reason is environment, water and many more. So first you have to find out that which reason is behind the hair loss of your horse and then you can give the treatment. I think you have to take your horse to the doctor.

Yeah, I agree with the others. It is best to check with the vet first - self-diagnosing could risk your horse to more trouble later on. Make sure that it is not brought on by other conditions because you focus on addressing hair loss or other hair problems.

There are also other types of feed supplement for horses that are known to improve the hair and coat quality of horses, so make sure to look for them.

My horse lost all the hair on her face and I tried to find out why just like you. the reason was due to stress, she had moved yards and had found it traumatic. If your horse is a nervous type then this could be why he has lost his hair.
I left my horses face alone and in time all the hair grew back just as normal. Just give it time and spend a lot of time reassuring your horse, if he is new to you then he may have found the move stressful, like starting a new school or job! blue chip feed is excellent for improving condition.
Hope this will help you!!

I'm not sure about the hair loss, but MTG (mane tail groom) is an oily sort or product that can help the hair grow back very quickly!

Great sharing.
There are also other types of feed supplement for horses
that are known to improve the hair and coat quality of horses,
so make sure to look for them.