It's what I do, but I don't rely on pasture as the main feed. Greens are good for vitamin A but the protein (as others have said) is very variable.
I move my chicks fairly early onto the sprout diet with wheat, corn, peas and sunflower (all sprouted), alfalfa meal (or lucerne chaff as we call it, soaked in molasses water), sweet lupins, meat meal, yeast, a little seaweed meal, and probiotics (kefir whey). This is their basis, then they get to eat whatever's under their feet until the next tractor move.
Last group of chicks (only 12) were on this diet from day 2, but I also included a little cracked corn and dry millet early on until their gizzards were able to grind the harder bigger seeds.
I feel it's best to provide a ration with the appropriate protein level and if I find they're filling up on greens instead then assume they probably know best... You can judge how much ration to put out by what they ate yesterday. Whereas if the protein isn't there in either the ration or the grass, it might only become obvious when the birds fail to thrive.
Good luck with whatever system you choose, mine seems to work well for me.