So might I assume, a steer, that you're butchering at 1250 lbs live? At 90 days on full feed they would have to be around 1000 lbs coming off grass. The last 200 lbs is the least efficient in ADG. You can't go from roughage only to full feed overnight. You gotta step them up.
Here's what I've seen from tons of data. You can't just create marbling and tenderness. If you short an animal on energy, which you will do on grass, marbling disappears which is the IMF intermuscular fat. Marbling can but doesn't necessarily translate to tenderness.
Grass fed cattle if called that should never see corn or ethanol byproducts. It's vague just like natural and organic.
Bottom line imo and my decades of doctoral experience from the school of hard knocks, 250 days on feed is the best for prime, certified angus beef, and beef grading choice/prime at 90 to 100%.
But to each their own. I cut my meat with a butter knife or my fork...1 1/2 inch thick. Haven't taken a steak knife out of the box yet. Plus don't have to worry about draining grease off of ground beef.
I'll get off my soapbox....sorry