I doubt the feed has anything to do with it.
She might just be a mare that is either difficult to get bred, or she may never conceive.
If she was exposed to a stallion, get her checked. You never know what might be hiding. But I think you might want to give up with this particular mare, you've been trying like crazy to get her bred and some mares just aren't cut out for it.
If you were to try anything else to get this mare bred, I personally think it'd be a big waste of money. You've done injections, medications, artificial insemination etc. Bentley covered her.
I think if you really wanted her bred, just turn her out with the stallion and leave them together. If it's going to happen, that's the way it'll probably happen. Let nature take it's course. The stallion and mare know better then you when the mare is ready to conceive.
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."