Through most of the world's history, people have saved their own seeds and replanted them. Now we have a multitude of hybrid seed to choose from. These hybrid seed have a lot of advantages-crops are very uniform, so they are easily harvested, and all mature at the same time. The crops generally have a very good yield, especially when adequate fertilizer is used, sufficient water is available (often through irrigation), and pests are kept under control.

But over the very long term, it is not entirely clear that we will be able to keep up this system. We likely won't have the commercial fertilizer, nearly as much irrigation, and probably not chemical pesticides and herbicides. We are likely not to be able to store the food as well, either, or to transport it long distances.