RoundUp Ready® (RR) crops and other herbicide-resistant GM crops have increased the use of no-till agriculture, which conserves topsoil and soil structure. Since weeds can be sprayed after RR crops are planted, farmers do not need to plow before planting saving labor, energy and topsoil. These benefits may not last. If a field is sprayed every season, or repeatedly within a season, with the same herbicide, weeds that are slightly less susceptible to the herbicide may survive and possibly breed with other survivors.
The development of weeds resistant to glyphosate and insects resistant to Bt has not involved the movement of genes from crop plants to weeds or insects; instead, pests have naturally evolved their own resistance traits. This is a common phenomenon – many pests have evolved resistance to many pesticides over the years. For example, weeds have developed resistance to 22 of the 25 known sites of herbicides action and to 155 different herbicides including glyphosate.
Our current evidence suggests that RR and Bt crops have had a positive overall ecological impact. RR crops have resulted in greater use of RoundUp®, but there has been a reduction in the use of more toxic herbicides, and soils have benefitted from reduced tillage. Bt crops have resulted in large reductions in insecticide use in cotton and corn. At the same time, some pest resistance has developed as use of these crops has expanded during the past two decades. In future bulletins, we will return to the question of whether these two common GM technologies demonstrate other non-target impacts.
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