Do you have goals beyond not having horns?
I personally do not like Dexters because they rely on breeding animals with a dwarf gene to animals without. You can have some serious problems with them. If you want to breed your mini you might consider a full blood lowline angus. I raised them for a number of years. They do a great job of turning grass into marbled meat but you have to raise them to 30 months to get an 1100 lb animal that has good marbling on grass. They have been by far the best beef I have eaten. I still have a lowline/red angus I breed to my big Murray Grey bull and her calves are fantastic.
Another thing I would consider is how big is your heifer? In my grass fed program we do not breed at 15 months. We shoot for a first calf heifer to to calve between 30 and 36 months in order to get a full size momma and a full size calf. This was especially so with my smaller cattle back in the day.
My biggest beef with minis is the unscrupulous breeding practices of those that push this breed. Every single pen of mini herefords i have seen, all born within days of each other, were all over the map as far as size, musculature, confirmation. They should look like well formed, same size, carbon copies if the breed and the breeder is any good. It just is not so with mini cows in general.
If you love your mini Hereford and you really want to breed her, wait until she is 21 months and AI her to a good lowline bull if you don't have one in your area, keep her as a pet, or let her go. That is my advice. And is truly the only ethical options I can see.