We don't have to convert cattle farms into cropland, just eat the grain we feed the cows, and if we get sick of that grain, then grow something else where we are growing that grain.
But we don't feed grain to cows, unless it's stuff like brewery mash where it's already been used to make human food.
You cannot necessarily grow different stuff in the same field. Not all crops do well in the same kind of soil. For example, you need totally different soil for potatoes than you do for cereal crops if you want them to actually grow.
We (in the US anyway) feed a massive quantity of unspent (not brewery mash for example) grains when finishing cattle in feedlots. Corn, barley, milo, soybeans etc along with harvested forages like alfalfa and sorghum (which is often fed as silage).
People do feed brewery mash and other byproducts but that isn't most of what goes into feedlot rations nationwide.
