Puppy is itching due to her food,what food can I switch to?

My puppy is An Australian Cattle dog at 6months old. She is on Puppy Authority for Large breed dogs.There is no fleas,no flea dirt, no dry skin. Everyhing is leading to her feed. What brand of feed can I switch her to that will stop her itchig. I like Blue Buffalo but it's just too expensive for me.