First off, have you checked for fleas? Even one can make them scratch and bite. It is Soring and lots of potential allergens out there. My own two small dogs have been biting and scratching. First suspect is likely food but both have been on the same food for some time and it would be odd they both start havng a problem. Saw no fleas or flea dirt. Bathed both in a medicated shampoo and conditioner. The biting and scratching has markedly decreased. My guess is the grass.