1. Check the fit of your horse’s blanket. Mane growth can be stunted from something as small as the fit of their blanket.
First, remeasure your horse, and make sure that their blanket fits properly (not too tight or too loose). Second, double check that your horse's blanket is not rubbing the section of the mane above the withers (this also may cause rubbing)
2. If you have a hood, we recommend that you use it. Using a hood is a great way to protect your horse's mane. Hoods help your horse avoid rubbing, and the less your horse rubs out its mane, the longer the mane will be
Give these two tips a try and let us know how they work for you!