It sounds like you are going to actually do the experiment so you can find out for yourself. But grass fed does not mean they eat only grass. Grass fed means that they get to eat grass. While they are eating grass they are also eating dirt, weeds, creepy crawlies, and many other things. How well they do on a grass fed diet depends on the overall quality of the entire forage, not just the grass.
I don’t know how you plan to set up your experiment or for how long you plan to run it, but you will get totally different results if you mow your lawn and feed them that cut grass rather than allow them to forage in an area that has different varieties of grasses and weeds, grass and weed seeds, and creepy crawlies, plus access to dirt.
Pure grass, especially one variety, is not a balanced diet. If they can forage in a high quality forage environment, they will do extremely well just on what they forage.