A few weeds developed immunity to the herbicides atrazine and simazine within 10 years of the chemicals' first commercial use in the 1950s. Now, hundreds of weed strains are resistant.
Weeds are even developing resistance to the the kill-all herbicide glyphosate, known by its Monsanto brand name Round-Up. American farmers used to be able to plant crops with genetically engineered immunity to the herbicide, then wipe out all weeds with glyphosate.
However, as weeds are evolving their own defenses to the chemical, farmers must develop new strategies, such as spraying increasingly toxic herbicides or labor-intensive hand pulling of weeds. This increases costs to farmers and could subsequently raise food prices.