Further to Wampus' point, some study figured out that cows need to eat grass for just one week to get the natural immunizations that preclude them needing antibiotics, but that even giving the cows a one-week vacation from the feed lots is more expensive than the cost of medicating them.
We have a 'buy local' movement in the area raising the question of whether cows in our county need to travel to Kansas to be slaughtered instead of cutting and selling locally. (1,000 miles of truck fuel) So, we bought half a cow. I must say that the flavor is surprisingly different and once you learn not to overcook it, the cuts are as or more tender than Costco with a lot more flavor.
When picking up our order, I met the rancher and his story was similar to Cowgirl's in saying that his cows were grass fed until their last couple months as winter approached, when they were grain fed and marbled up.
It's hard to beat. The average cost was $4.05/lb delivered which makes for some expensive (but very tender and flavorful) ground beef, but really inexpensive Loins, T-bones and Rib eyes. We'll do it again!