We've already seen the horrific outcome of what happens to horses when there is no outlet for those who are unwanted....people abuse, neglect, leave them to starve, turn them loose to fend for themselves...
I hate that we have come to the place where we have to dispose of so many unwanted horses. But it beats the alternative of having them die slow, agonizing deaths or meet with tragic accidents because their owner could not or would not take care of them.
A lot of people blame the small, back yard breeder, and to an extent they have a part in the horse overpopulation....but I put just as much if not more blame on the shoulders of the huge ranches with 20, 30, 50 or even hundreds of broodmares dropping foals each year. Most of those foals end up flooding auction rings across the country. Only a select few of those foals produced are deemed "high quality" and go on to represent the ranches and stallions who produced them. And what's worse is the major breed associations encourage and reward them with notoriety.
So with so many horses being mass produced, and not enough homes for them all...and with the economy the way it is now, making it difficult to impossible for the average person to properly house and care for a horse, there just is no other humane way to deal with this problem at the moment.
Slaughter is a necessary evil. Sad, yes. But necessary none the less.