Actually, most beef in the grocery store is not from cows that have been fattened up by high-grain diets before slaughter. The “low end” of beef is from cows, bulls, etc that have not been fed grain, but also some Holstein steers that have. The “high end” is from beef breed steers or heifers about a year to a year and a half old that have been grass-fed except for the last 80-120 days of their lives. I’ll grant that for that last period, they are fed grain. But you have to remember that cattle eat “grain” even when they’re grass fed in a way. Grain is just seeds. Cattle on pasture will eat seeds of all kinds of things; grass, clover, lespedeza, whatever’s there. They’ll eat acorns too.
But I’ll grant that pushing grain at the end does make the meat fattier than it would be if they were “grass-fed” from beginning to end. Aussies, I’m told, prefer grass-fed. I’ve butchered totally grass fed, and it’s a bit less juicy, but it’s otherwise really good. My family prefers it. I’m a bit leery of heavy marbling myself.
Back to acorns. Every fall I turn cattle into the woods to eat the white oak acorns. (They’re not bad…not bitter like red or black oak acorns) That really fattens them up, which is good for the mother cows for winter. I’m told it’s a different kind of fat, though from fat from grain.