MTG works wonders on hair growth! My mare fell on her knees last summer and completely skinned them. Someone recommended it and the hair grew back completely within a couple of weeks! It smells disgusting though! Google it! It is about £20 but lasts forever!
I used to rub corcucrescine into rug rubs and it grows back soooo well. Some people say this is because of the product, while others believe it's the rubbing action itself
Yes it's wonderful stuff, however the general health of your horse will also affect how fast the regrowth of hair is, if she's a healthy coat and you apply twice a day, you will see the difference in a week.