I agree, genetics does have a lot to do with it but good care can help a bad situation too. I use MTG on the roots of the mane and tail to help keep the skin healthy. That will help stop the rubbing off of the hair. I use a human hair oil on the ends to keep them from breaking off. I prefer Africa's Best Herbal Oil that you can get at Walmart. It's in the African American hair products section. I leave both the MTG and hair oil in and braid the hair.
I also feed ground flax year round. Not sure it it helps with the hair growth but it sure gives them a pretty coat shine.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France