My father was a rancher in Texas. Most ranchers here sell their calves at auction when they are 450-750 lbs, strait off the cows teat. Depending on when and where (in Texas), most of the cows live on grass during the spring, summer, and fall, and hay during the winter. Grain based feeds are more expensive, and generally used very sparingly if at all. Mineral supplements are pretty common (aka salt licks), abet most these days are not like the salt licks of old. Anyway, I grew up eating "grass fed" calves. I will agree that they taste better than most supermarket beef. That said, my freezer is currently stocked with Buttercup - a calf that was grain fed for almost a year after weening. Buttercup is either primer or darn near it - and her flavor blows grass fed away.
If you buy direct from a rancher, you can probably get some good, healthy meat. That said, if you want near perfect marbling, and the taste and texture of prime - just be prepared to pay. Buttercup was hormone free, lived as happy of life as any Japanese cow - and ate about $1k in feed as we finished her out. Starting with a steer and using hormones gets more efficient weight gain, which in turns allows a commercial feed lot to make a profit and stay in business.
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fwiw, I used to be a food-hippy in Austin (you know, grain feed, organic, raw milk etc...). Technically I still am, just not in Austin anymore (seems food-hippies and traditionalists are pretty much identical). Anyway, there are a lot of like minded people in Austin, and group buy's were fairly common and often (there were a number of different groups, but they tend to work together. Amish, Mormans, food-hippies, traditionalists...) I saw the price they were paying for their "organic" beef, and noticed there was not really a damn thing special about it that was different than common practices of 95% of the ranchers I knew (ok, other than marketing - that was the 1 thing special). I outlined to their forum exactly how the costs works, and how I could get them their beef for less than 1/2 what they were paying - and crickets chirped. Without the marketing, they were not interested AT ALL. No problems for me, I could still keep my freezer full of grass fed beef).