OP, when you have a chance, take a look at the following link so that you know what Hill's z/d is. Then, if you might be interested, talk with your vet to see what he thinks:
z/dŽ Canine ULTRA Allergen-Free - Dry
Some information from the link:
Food allergy and intolerance are potentially serious problems that can result in sensitive or irritated skin, coat and ear conditions or even gastrointestinal upset. Dogs that have a food allergy or intolerance, also called adverse reactions to food, typically experience an abnormal response to the protein in the food. If your dog suffers from adverse reaction to food, it is even more important to feed the right food.
At Hill's, nutritionists and veterinarians have developed clinical nutrition especially formulated to help manage your dog's dietary sensitivity and help reduce the possibility of adverse reactions to food.