How do you cook grass-fed beef so it is not tough? I recently went to a farmers market and bought some grass-fed beef.  I went with what we normally buy, and got a couple of bone-in thick cut ribeye.  I typically prepare this by liberally applying salt and pepper, letting it come to room temp, grilling (gas) over high heat for a few minutes on each side to about 125-130, then letting it rest for 10 minutes or so.  This gets it to a good medium-rare after resting.  I then slice it across grain, cutting each piece about .25 inch think.

This works great, and loved it every time.  I love the ribeye since it has a good amount of fat (thus flavor), and it not too tough.

The guy we bought the grass-fed beef from warned us that they typically are a little tougher and less fatty so they typically marinate overnight.  I thought this was odd for a ribeye, and prepared it like normal.

Although the flavor was VERY BEEFY (almost too much), way more than the typical high end (probably corn fed) beef we buy, it was SO tough.  It was like leather.  I guess this is not uncommon for grass-fed beef, but what should I do next time to make this more enjoyable?

Marinate?  Emulate dry aging by cooking low temp for a while, then grilling?  Any other tips?
This is total nonsense: grass fed ribeye should absolutely not be tough. Toughness is, however, affected by the cow's breed, it's age, by how stressed it was when it was slaughtered, by ageing of the meat, and I'm sure a bunch of other variables too. Something made the meat tough, but it wasn't grass feeding - the combination of strong beefy flavour and toughness might suggest an old cow, for instance. Next time choose grass-fed beef raised by a different farmer.