After much trial and error and very itchy dogs, I discovered that two of mine needed both grain free and chicken free. Interestingly enough, they had been eating a chicken and rice based kibble for years with occasional raw chicken and turkey, but gradually developed allergies and began scratching and chewing on themselves horribly. And then Princess also developed chronic ear infections. Symptoms improved when I removed the chicken, but didn't get completely better until I removed the rice too.
As an aside, I personally eat grain free now. After years and years of being sick almost continually, my health has never been better. I know for a fact that some people don't digest or utilize grains well and it can cause numerous health problems including autoimmune issues. According to my vet, he is seeing more allergies and autoimmune issues than ever in dogs and cats and in his opinion, grains are the culprit for many dogs and feels grains promote diabetes in many cats. Note that I'm not saying he said ALL dogs or cats, just like not ALL people are grain intolerant, but some genuinely are and for that reason, I'm glad there are grain free options for pets and I'm glad more people are trying grain free a lifestyle. I personally know a lot of people who, like myself, feel so much better without grain and have had some lifelong, chronic health issues resolve completely on a grain free diet.