We try to give ours a balanced diet based on the fact that the pellets should have everything in them the birds need. So fruit and veg are limited to a treat basis Stereo. We have never seen the effects of a fruit and veg only diet. They need protein and it would naturally come from grubs. Without it they stop laying.
Our Orchard used to have apples which we stored for Winter treats. We grew Savoy cabbage which we either hung in the runs or fed them sliced leaves -makes the yolks a golden colour. Also grew sweetcorn and fed slices to the hens that were not good for freezing or eating by us. Most of them were on grass anyway. They liked brussel sprouts and hated carrots. They never had any potato scraps, which must be cooked anyway. Ours also had marrow offcuts and the seeds, but not all of them ate it. We've had two chickens that appeared 'drunk' after gorging on fallen pears and one had an impacted gizzard after eating damsons whole (she survived after a month of tlc and the trees were felled). Cherries were OK, but they got juice stuck to their heads which then got covered in dust and looked like a case of 'blackhead'.
We were hoping to grow our own sunflowers at some stage, but it's whether it's worth the trouble when that's a primary crop here.