I don't understand- if you look at performatrin ultra's website, there's no corn in the food... I know a few people here have fed it to their dogs without any trouble..
http://www.performatrinultra.com/cat/cdi_chicken.htm
The bad ingredients in that chicken one are brewer's yeast (by-product) and Menadione Sodium Bisulfite Complex (not for human consumption), but I don't see corn.