Hi, Glad to hear that your girls are enjoying the summer. Yes, if you want them to lay their best they will need some good yellow corn in their diet. Either ground up in their feed or fed as whole grain corn. Wheat does not supply chickens with all the nutrients that they need so while it is ok, though usually too expensive, as a supplement they still need good yellow corn also. While letting your girls out to roam will allow them to add some additional bug proteins and greenery to their diet, they will actually use most of the energy they gain in the process of scratching for the food. You should expect fewer eggs but they will tend to taste better and may be larger with strong yellow yolks if they are fed lots of corn as well as what they find on their outings.
Good Luck.