Yeah when I mean a grain mix... I mean whole grains in bulk at the grocery store - intended for human consumption. Not those commercial rat seed mixes found at Walmart and pet stores. I also wanted to point out that feeding your rats dog food doesn't benefit the snake, unless you were to gut load. (feed right before you kill) Feeding them a good quality rat food will always produce the biggest healthiest rats. Check out my big 2lb boy Oliver.. who is just 9 months old.
I have a lot of respect for snake owners that can show some compassion towards their feeders. I totally agree that there is a margin for error with breaking the neck. There will never really be a nice way to pre kill them, if there was a humane way with Co2 that would be great. You have to remember that rats by instinct are great at hiding pain, so you really need to look closely at studies and see how 'distress' was measured. I am part of two large pet rat forums, here is a link to one of our stickies about using Co2 which contain some good links:
The Rat Shack • View topic - Euthanasia using CO2