It would not be like a juice fast. Liquid diets more or less provide full nutrition in very easily assimilated form, without any fibre or solid food at all. Wheat grass fits that descr*iption but on it's own would not provide full nutrition. It is highly nutritious in it's own right and no doubt you could survive on it for a long time, but not indefinitely as seems to be the case with juice fasting. And if you aren't providing your body with full nutrition you shouldn't give it anything, otherwise you are just interferring with the bodies efforts to provide it's own nutrition from it's stores. Wheat grass juice is great but not as part of a fast.