I'd get rid of the corn, it is not adding to anything and not giving them anything useful. Makes them full and gives them sugar, but it's not healthy in the longrun. Sometimes the corn can have mold associated with it that, over time, can build up in their system and kill them. Those toxins from the mold are called Aflatoxins/mycotoxins. They also make them more predisposed to cancer, from what I've experienced.

For healthier options, put more pasta in, brown rice, slow-cooking oatmeal, barley, dried fruit/nuts, cheerios(or generic), generic Total cereal, in-shell roasted unsalted peanuts(good protein and fatty acids source). You can also try a good brand of dog food(makes up about 10-15% of the diet), such as Nutro ULTRA(only this type is good, Natural and the other kind are both crap), Wellness Senior, Flint River Ranch,etc. Basically a good food with less than 18% protein, no fillers, artificial preservatives or by products.

And if you are using ANYTHING with the preservative Ethoxyquin, get rid of it. It WILL cause cancer and is proven to do so. Some lab blocks, such as the Kaytee crapola, have that preservative as the last ingredient. Just something to watch out for.