Oh boy, I hate to admit it now, but I also raise, sell, and eat grass fed cattle. While today's breeds are no doubt better at digesting grains than cows of old, I do believe the genetics are in place for cattle to be healthy on grass. Their digestive system compared to something like a rabbit (although rabbits also seem to be geared for grasses because mine tend to run their food through the digestive system twice), pig, horse, or chicken just seems to have evolved for longer fermentation and digestion that is needed by organisms getting their sustenance from grass. I freely admit I may not know what I'm talking about because this is a learning adventure for me.
Now in my case when I say grass I'm simplifying what they actually eat. My fields are moving towards a mix of grass and legumes. The grass is mostly native to Michigan (i.e. what grows) but I did overseed for legumes, in my case a mix of clover and trefoil (I like that trefoil self seeds whereas Alfalfa poisons its young). And I also supplement. For my cattle that is free choice loose mineral and salt. Furthermore, I soil test and periodically add potassium, phosphorous, and a little boron. So far I haven't needed anything else but those minerals also obviously end up in the cows. When other minerals are deficient I'll add those to the soil as well. I have never needed to add calcium and I wonder if enough of that is actually passing through my cattle from their mineral supplement or if my land is naturally high in it. Because we have a lot of legumes we never need to add nitrogen.
I also prefer the taste of grass fed, but then I also like venison which is grass fed in my area. I sell to people who are looking for leaner meats (most noticeable in my ground beef which is unbelievably good, even though its lean). When it comes to sales, managing expectations has been key for me. I am very upfront with what people can expect from my cuts of meat (I sell shares). Nobody walks away expecting a store bought cut.
People also don't expect store bought prices, or better from me. As Nelson points out you won't get to market weight in 15 months on grass. 24 months is a minimum (and I do mean minimum, longer is gooder). Do I sometimes process cattle sooner? Sure, I just sent in two smaller intact males last week which I sell as all ground (well, almost all, we do have the tenderloins, flank steaks and rib steaks, cut out for people). I can also auction off one year olds as feeders.
I have a herd of lowlines which we are growing to 30 cows (13 cows and heifers as of this spring!). I went with lowlines because they were supposed to do better than average on grass. Another reason is I sell mostly to folks in the city and they have smaller freezers. Because I can only sell shares, smaller beef is a plus. I hope to create an efficient grass fed herd by bringing in fresh genetics for bulls ever few years and culling cows that either have a difficult time calving or produce poor performing calves on the grass an supplements I provide. We'll see how it goes. Like a mentioned, its a learning adventure for me.
So I guess there are two of us out here JL...