Firstly, stating that famine has never happened in the US? Simply not true. For example, St Lawrence Island in Alaska suffered an absolutely brutal famine in the late 1870s Yes, this happened a long time ago - but again, constitutional amendments are supposed to last forever.
But on a broader scale - making exemptions for war, famine, disaster relief, financial crises, and any time the government feels it needs to do so means that these budgetary limitations basically never apply. Or when they do apply they won't be needed! So I'd suggest you really need to think of what your goals are and what the criteria should be before settling on a specific proposal like this.















