An 8 yr old male neutered lab named Marner owned by Ricky and Marlene presented for numerous large round patches of bloody erosions on his skin, constantly scratched at them, and was covered in huge dandruff flakes some an inch in diam. He was being fed generic dog food from a big box membership store. Diagnosis was Zinc Deficiency and a diet change to Science Diet cured him in 2 weeks.
In general, Wiseman Animal Hospital Doctors recommend the following rule of thumb for selecting a diet. If you have never heard of it, if you can only buy it in one store in town, or on line, if you only found out about it on the internet, if it is trendy, do not feed it. Buy something that you have heard of your whole life. Purina, Science Diet, Alpo, Kibbles and Bits, Iams and so forth.