This is a sad thing to happen to the horses. Shame on those owners who abandon them. If the vets of the USA would lower their prices to help the owners put their horses down in a humane way, perhaps more folks would take advantage of this service. Putting the horses on Rescue Ranches is another good idea. But how many can these places support. I read online almost everyday that these rescues are begging for money to feed and house these horses.
My only suggestions is Immunocontraception aka Birth Control. I know there are MANY people who disagree with me on Birth Control for horses, but there are many other people who also think that birth control for horses is a good idea. If there weren't people breeding and breeding the horses, there wouldn't be such a problem and the rescue places could help take care of them.