When the weed killer Roundup was launched in the seventies, it proved it could kill almost any plant yet still be less dangerous than a number of other herbicides, and it helped farmers to give up harsher chemical compounds and lower tilling which could promote erosion. But 24 years later, a couple of sturdy types of weed immune to Roundup have developed, driving farmers to go back to a number of the less environmentally safe methods they left behind many years ago. The situation is the most severe within the South, in which a number of farmers now walk fields using hoes, eliminating weeds in ways their great-grandfathers were happy to leave behind.