Grass can be up to 20% of a chicken's nutrition but I suspect that slag includes grass seed and not just the green portion although my hens eat a LOT of green grass and weeds and the more they do, the more golden the yolks. And if it's not cut often, other beneficial weeds will grow that they eat. The bugs and microbiota of the soils, itself, will also provide a huge portion of their diet. That's not grass "alone" obviously, but grass should's be discounted as not nutritious for chickens.Depends on the quantity and quality of the land. A quarter acre of grass will provide next to no nutrition