On a cow that is not handleable... maybe. But any horse that will stand for a shot, they'll stand for the shot.
The cow was really sick and couldn't do much of anything. It was just really sad to watch the vet carving up that poor cow, trying to find the vein.
The horses I saw put to sleep went down with the tranq, but with the overdose, they went nuts. Peddling their feet, trying to get up, etc. I've never witnessed one that went easily.