Horses tail falling out?

[Hormonal Filly]

Advice please?
My mares tail fell out last spring and its happening this spring. It definitely isn't her itching, and I'm 100% sure it isn't because the hair is always flat and she never itches really, and literally just falls out from the centre/top of her tail. It isn't another horse and isn't any rugs etc.
I've tried sudocrem, no rub creams and no itch creams.. also Aloe Vera which didn't do much!

The hair grew back perfect last winter, but because her skin is black and tail is white, from a mile away it looks awful
Any ideas? or remedies I can try?

 

[Sugar_and_Spice]

Could it be an allergy to something that causes it?

 

[IAmCrazyForHorses]

How often do you brush her tail? Because if you brush their tails every day it can make their hair fall out. Just separate the big tangles in her tail with your fingers. If its not this then maybe you can ask the vets?

 

[twiggy2]

a horse i used to look after did this every year-the top 4 inches of his dock used to go bald-he did not rub, was not being rugged, lived out at night, and was not brushed and groomed every day. he was pink skinned so once it started falling out we had to apply sun screen when he was in the sun.
looked terrible but did not bother him so owner just continued as normal

 

[BlackRider]

Check you aren't feeding too much selenium.