The SI joint and the pelvis are major anatomical players in a pregnancy. I would not want to subject a mare to more possible pain. It her problem is undiagnosed, or not healed, how can the vet know that a pregnancy will not cause her serious pain?
It might be more Economical for you db however I only breed 1 mare every few years and I have no experience with stallions. Plus all the studs around here have terrible conformation and are not the discipline I want. I live amid ranches and cow horses but I'd have to travel nearly 4 hours to find a stud worth breeding her too then the question of IF they live cover comes up. Also I don't have fencing for a stallion. So a.i is a much better choice. I'm hoping this baby comes out a colt though because In 1 1/2 years I will be out of college and working with a trainer and hope to get a colt worth keeping a stud.
I'm assuming owner does not have a stallion so unless they are breeding locally they will bein the same boat as the rest of us A.Iers.
OP, have you had a chiro look at your mare? They might be able to figure out and possibly fix whatever is ailing her in her hind end.
As it is, with a mystery lameness, I would be extremely hesitant to breed her regardless of her bloodlines, conformation, temperament, etc. First and foremost, her health and well being is the most important thing.
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