In my neck of the woods, the vast majority of the beef we eat is grass fed for most of their lives, but then grain finished. They only eat grain for the last month (out of 8 or so), but they put on most of their weight in that last month.

Here in the UK, pretty much 100% of cattle are grass fed. In the winter, when there isn't enough grass, they're fed on silage (which is basically just grass cut and baled while still green, which turns it into kind of grass sauerkraut, which smells exactly like you'd expect) and "draff" or "spent grains" (depending on where you are) which is the stuff left over from brewing beer or the pot ale that goes to make whisky.

It's all a pretty delicate balance, but ultimately what happens is you end up growing a bunch of things humans can't eat so that cows can shit solid gold all over the fields and chop it into the soil with their hooves.

We eat because there's six inches of earth on the ground, it rains, clover grows, and cows (and pigs) shit solid gold.

The vast majority is beef is finished on grain, but start on grass.