This might work if income were more equally distributed. If food becomes more scarce and the typical American has to eat 20-30% fewer calories, the price of food will spike significantly. People at the bottom will be priced out and will starve without government intervention.

Managing the food supply to make sure there's always a healthy surplus makes a lot of sense to me. The problem with holding land in reserve, as you're suggesting, is that it takes at least an entire growing season to start producing from that land. That might be too long in the case of a famine.