For those interested in the problems with horse racing I've located a documentary-style piece that has some facts and figures which are truly disturbing.
The sad fact is that this happens not only to horses, but greyhound dogs are also given the boot once their race ability declines. I doubt that they end up being sold for their meat and by products, but those not adopted out are mostly in mass graves.
Do these animals a favor. Don't bet on cruelty.