So-called green revolution
Those Western agricultural techniques gave big yields at first, but they require ever-increasing inputs of agricultural chemicals to maintain those yields. They deplete the organic matter in the soil, making it less able to retain water and nutrients and more prone to compaction. In years with drought or excessive rain, yields can be worse even when plenty of chemical fertilizer is used. The soil also erodes more easily.
Where those methods took hold, after a number of year farmers had worn-out soil and were broke - plus, they had more mouths to feed. There was more to it than you are seeing, the "green revolution" farming methods are simply not sustainable in the third world. Not sustainable here, either, for that matter; the whole system is on the verge of collapse.
