I suspect it will depend on the supplier. In my experience, both in Argentina and Uruguay, in megaurban areas, most of the beef is grain finished, and some is fully grain fed. I have limited experience with Uruguay however.
That's not to say there is no truly grassfed beef, but it's hard to come by and you should know who you're buying from.
Another thing you should consider is that much of what is called "grass-fed" or "pasture-fed" while technically may not be feedlot grain finished, might still be grain/legume fed right in the fields. For example, I know of plenty of producers in Argentina that raise export quality beef, that is usually not sold here, and they call it pasture fed. Much of this receives complemental soy and other grains along with the grasses.
Most of the grasses/pastures in many of these operations are sown however, and are often GMO and sprayed to keep out competing "weeds".
Even if they're not GMO and sprayed, it's often not grass at all, but legumes like alfalfa. Keep in mind cattle fed solely on raw green legumes, is at severe risk of dying from overproduction of foam in one of their stomachs, so they often must be treated with antibiotics and other chemicals, or have on-site emergency surgery to keep it from dying. Being fed legumes however makes them grow and put on weight faster, since legumes have a much higher protein content.
There is a chance that grain finished might be superior quality to some of these "grass fed" / "pasture fed" since most of the grain finished beef spends the first 5-6 months on truly 100% wild grasses, before being sent to the feedlot for bulking up for the last 3 months or so.
