Blue heeler training/horses

Question
I have a 9 month old neutered male heeler, an outside dog, still on puppy chow.  He is well-behaved (for a pup) knows sit, stay, come, and down, doesn't nip, chase cars, or jump on people.  However I also have horses, and his herding instincts kick in when they're excited or are out of the pasture.  I'm really afraid he (or I) will get hurt.  He completely ignores me in these conditions.  I have some success with commanding him to sit and stay outside the fence when I'm working with the horses, but if they increase their pace he can't contain himself, and chases them until one of them turns on him and then he bolts for the other side of the fence.  I'd like him to be able to come along on trail rides but at this point don't feel it's safe.  Any suggestions?

Answer
Ruthie, you're going to have to enlist the help of someone else. That person should be the one working the horses inside the pen, to begin with. You can't do it by yourself. Put him in a down-stay outside of the pen, with a leash on him to keep him from bolting into the pen. Then, have the other person go into the pen and begin working with one of the horses (preferably one that is least affected by the pup's antics). All you should do is sit outside the pen with the leash in your hand, and immediately correct him when he starts 'zoning in' on the horse. At first, keep the sessions very short, because of his age. You want him to be successful at this, not become so aroused that you have to fight with him.

You may want to enlist the help of a professional trainer who can come out and work with you and him.