Well, actually their low wage itinerant workers do the weeding.
The problem is the resistance to Glyphosate chemicals that are being widely used in most of the huge monocrop farming operations today. Monsanto has known about the resistance issues since the 90's, but with the cheap cost and availability-along with a dose of fancy advertising, that detail was overlooked. Monsanto did recommend to farmers that they use Round up along with other herbicides, but as Brett Lorenzen, a legal analyst at Environmental Working Group is quoted at saying:
"It's hard to read a 54-page booklet," Lorenzen said. "Monsanto has been saying don't just use glyphosate, but farmers don't have time to read the label."
Now, farmers are having to resort to combining other old fashioned chemicals, like Atrazine and Really Old Fashioned Style - weeding. Don't worry however. AgriNews reports that Atrizine is perfectly safe and there is no connection between it's use and cancer in pesticide applicators. Phew!