[ccozan]

If they choose to put down a horse and decide to used for human consumption, then they will certainly use a different method ( shot, hammer, etc ).

[hackbinary]

No, the problem is that there are other drugs that might have been administered therapeutically, so animals that are in the food chain have to be managed differently than ones that are for companionship or racing. For example Bute is commonly used in horses, but is toxic for humans.

This in part was why the horse meat scandle was such a problem in the UK.

http://www.bbc.co.uk/news/uk-21455419