I've got all of my corns and rat snakes switched over to rats. The only time I ever use pinkie mice, is for tiny hatchlings. As soon as possible, I switch them up to rat pups - usually by the time they're three or four months old.
Rats are much more prolific than mice. They're easier to keep. Less smelly. Also, I calculated that it takes at least twice as much food to produce 10 grams of mouse-body compared to 10 grams of rat. A tub of four or five rats requires no more food than a tub of ten mice, despite the fact that the four or five rats outweigh those ten mice by easily two or three times as much.
It certainly won't hurt your snakies to be fed exclusively on rat pups. Some people believe that rats are a bit fattier, and that may be so. I don't think it makes much difference. One thing I've noticed, is that often-times little snakies that are balky about taking pinkie mice, will willingly take rat pups. Also, I see less regurging from the snakes that are on rats.
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tricia