Fun fact, that gene is only about whether you can smell the compound in the piss, not whether your body processes asparagus into that smell.
They tested this by having people smell other people's urine, and found that the people who can smell it in their own piss can also smell it in the piss of everyone who eats asparagus, even of the people who claim not to produce that smell.
If you're accommodating another group of people you should produce enough to always feed them, too, not just sometimes in surplus years. The whole point is that you've gotta plan for a surplus, otherwise you risk starvation in bad years (and it doesn't make it any better, morally, if the people who bear the risk of starving are "another group or people").
how does waste prevent a shortage from becoming a famine ?
Making the expected production a higher number than the expected need will give the headroom necessary to deal with a shortage without people starving.
If you're aiming to produce food for a population of 100,000, but have the capacity to make food for 200,000, then you can afford to waste half of your food without starvation. You can also accommodate a 50% drop in production without starvation.
So that buffer is expected waste, but it's also starvation resistance.
Hmmmmmm