Grass fed sucks tho.
I take it you've never lived outside the "city limits"...
Generally speaking, the beef at a supermarket is "feedlot" beef. Meaning, basically its cattle that have done nothing but stand in their own muddy crap for most of their lives and have eaten little but dry grasses and the cheapest of grains.
Farm, pasture raised beef is also fed grain typically. However, they graze on fresh grass at their leisure while being fed higher quality grains as well to help "fatten them up" to provide the tasty marbling.
HTH