I work at a farm where they raise pastured Cornish cross in 8 weeks. they raise 2,500 every summer.  They feed a specialized antibiotic-free, high protein diet, and figure they get 15 percent of their diet from eating grass and bugs.  they move the chickens mobile pens every day so they can get fresh grass and it keeps them healthier. 
(Greyfield, my chickens eat blades of grass all the time, and if they're penned up and I put a pile of grass in their house its gone in short order. I heard that the best eggs come from grass-eating chickens because their diet has more nutrients from the soil and their yolks are much darker orange).  Anyone else?