When I have time I will address the differences between Temple Grandin and 'United Horsemen', the group she had agreed to speak to before she changed her mind.
It was almost 2 years later when she was contacted by the AVMA and flatly stated that the slaughter process could be made humane.
It might be noted that they have processed horses for centuries in Iceland and raise them for that purpose as well as use the processing plants for other horses that are no longer useful or wanted. People that have visited these plants say that they are calm, quiet and very efficient in their methods. The horses are not stressed and just calmly walk through the whole process. They use the 'captive bolt' method that is approved by the AVMA and the AAEP. 'Stun guns' are not approved. Captive Bolts kill the horse outright and do not just stun it. It is the same as a bullet without the loose projectile. A bullet is far better in my experience than poison chemical euthanasia.