Approximately 37% of the 37,000,000,000 acres is already classified as farm or grazeland, with any sort of monocropping producing far more food than would be by grazeland due to the laws of physics and the 10% rule. So any replacement by additional graze land at the expense of existing agriculture of any kind would be a net reduction in total yield of food. I.e., you aren't going to be growing less than 705,000 calories out of 3 acres of land (1,137 calories per lb of beef). At a yield of 32,000 lbs of celery per acre and 64 calories per lb, that's 6,144,000 miserable calories of celery, almost 9 times as many. Now, stop and think... Remember that 10% rule? It isn't magic, I deliberately chose the most favorable comparison I could, and there it is. 90/10. Because a cow MUST use energy to survive, there is some finite amount of energy coming in from the sun to the plants, and here we are with less (90% less!) per level of the food chain. You cannot replace existing agriculture with grazing and end up with more food.