Herbicide resistant crops and weeds: Implications for herbicide use and weed management

Since their introduction in the mid 1990s, growers adopted genetically modified (GM), herbicide resistant (HR) crop varieties quickly in the United States and they now account for most of the hectares planted to corn, soybeans, and cotton. Benefits to growers not captured in standard farm profit calculations appear to account for the popularity of HR varieties. HR crops have been credited with encouraging the adoption of conservation tillage and causing substitution to herbicides with lower toxicity and persistence in the environment. Evidence for the effect on conservation tillage is stronger than evidence for herbicide substitution. The latter has relied more on expert opinion surveys that are sometimes, but not always corroborated by careful farm-level studies. Adoption of HR crop varieties led to a dramatic reduction in the diversity of weed control tactics in U.S. agriculture and the predictable evolution of HR weeds. Grower adoption of resistance management strategies has been limited and insufficient to delay resistance. Development of crop varieties resistant to multiple herbicides is being pursued as one strategy to respond to HR weeds. Debates remain over the potential of this approach relative to a more comprehensive integrated weed management strategy to successfully delay resistance.