You make mention of "failure" or "set backs", how would you, using only "positive reinforcement", train a dog to not go up to a rattlesnake? In South Texas, Western Diamondbacks are very prevalent, it is essential to the life of the dog that they do not put their face (or other body part) in a rattlesnake's strike zone. The only way I have ever effectively seen a dog learn to flee from a rattlesnake is with the use of an e-collar and negative reinforcement. Now I am not going to say that I have seen more dogs trained than you, because well that would be arrogant of me, but I will say that I have seen many many dogs trained to avoid snakes, as well as I have trained many dogs to avoid snakes. The fact is, the only effective way to "snake break" a dog is with the use of an e-collar. A rattlesnake will very rarely give a dog a second chance.