Here is a good article about dog food allergies:
http://allergies.about.com/cs/canine/a/aa091800a.htm You might want to just try the Duck and Potato Natural Balance food and see if that takes care of the allergies. It is designed for allergy-prone dogs. An ad says:
"Natural Balance? Potato & Duck Dry Dog Food is our special Allergy Formula, made specifically for dogs prone to allergies. COMPLETE AND BALANCED FOR ALL BREEDS AND LIFE STAGES. "