It's no wonder the price of beef is going up. If producers would use something cheap like grain or grass instead of spinach, the price might drop.
But in all seriousness, here is a guy explaining how cows need more and more spinach to grow. I realize he was using it to compare human to bovine diets, but still, it's being put out there. About the 1:50 mark if you don't want to watch the whole thing.