Are you for or against it? Pros/cons? Personal experiences?
You can't just go off of size though. You need to look at the filly's joints. They don't close until they are over 5 years old. That's why you shouldn't ride young horses, and if they are ridden, it should be very lightly. So you breed a mare and add extra weight to her joints and knees that aren't closed yet? Not only that, all of the nutrition that you are feeding your mare is going to the foal, the mare is barely getting any. Just because her growth isn't stunted, doesn't mean that her body got all the nutrients that is should have.I personally don't think its bad at all breeding a three year old as long as they are a decent size. I've heard breeding a four year old is better for the mare however, because it allows her to finish growing herself without the added strain of carrying a foal.
Yes, it completely depends on their care. Your mare looks like she's very well taken care of. But I feel if your going to breed a mare, you should be responsible and wait until the horse is ready, both physically and mentally. Breeding young horses to me, seems like owners are just trying to make money because they have a mare and the stud fee is cheap.
Most 3 and 4 year olds haven't even hit the show ring yet. Therefore what have they proven themselves to be good at? Horses that breed should prove themselves before hitting the breeding shed, so to speak.
I guess I'm the odd man out. I bred horses for many years, and have bred lots of 3 year olds for foaling at age 4 - never a problem, never a bad back, never a nutrition problem - no problems or issues at all. From my experience, and those of other long time breeders I know and have known, I would dispute these claims as groundless and based upon speculation rather than experience.