Actually the corn is probably much more problematic then the beef is. Corn is very difficult for dogs to digest and will likely just add to her dehydration. Updated thinking on kidney problems is to limit the carbohydrates. If you are making your own food then perhaps consider using sweet potato and pureed veggies instead. In earlier stage kidney problems like yours it is much more important to have a good quality protein source then to limit the protein. Here is a good site for you to check out:
and here is an excellent pets.ca thread on kidney disease. It it actually for a cat but much of the advice and theory would also apply to dogs.
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