If it takes six months to get your rabbits to what they should weigh by eight weeks, it doesn't sound to me like your rabbits are healthy so much as they are starving. You can get feeds that are not GMO, although all the hysteria about GMO feeds escapes me. If I were in your shoes, I would figure out what nutrients your rabbits are getting vs. what they need for optimum health and then supplement their pasture with the missing nutrients. I don't know what kind of pasture you have, but you can plant clover or alfalfa in it if there isn't any now. Just a suggestion. Of course if you are happy with the status quo, never mind.