Part 2, Section 1: Pest Management
Pest Management
WEED SPECIES SHIFTS
Expectations of Future Shifts in Roundup Ready Cropping Systems
It is difficult to predict precisely what weed shifts will occur in Roundup Ready cropping systems. These shifts should not be a cause for great concern if they are well managed. Farmers will adjust their weed management programs, just as they have in response to previous weed shifts.
Some species are already resistant to glyphosate. Glyphosate resistant horseweed or marestail is a problem from the Northeast into the Midwest and southern states in Roundup Ready soybean and cotton. Glyphoate resistant pigweed, ragweed and possibly lambsquarters are also a problem on some farms. In addition to resistant weeds, shifts will likely occur in species that are currently more difficult to control at commonly used or reduced rates. Examples include morning glory species, nightshade species, velvetleaf, crabgrass, and members of the lily family (such as dayflower or wild garlic and onion). Shifts to later-emerging species that can grow under or elongate through a crop canopy (for instance black nightshade) and avoid herbicide application would be expected. For each of the previously listed weed characteristics, there are a few species with this trait to which a weed community may shift. If Roundup Ready crops increase the adoption of reduced tillage systems, shifts normally associated with these systems (described previously) could occur. In each region, the actual shifts that occur will be different as the local weed communities adjust to the Roundup Ready cropping systems.
Volunteer Roundup Ready crops in a subsequent crop (for example, Roundup Ready corn as a weed in Roundup Ready soybean) will also occur if not managed. Roundup Ready soybean are frequently treated with a grass specific herbicide to control Roundup Ready volunteer corn. New weed problems, from either minor background weeds increasing in prevalence or introductions of new species from other areas, will likely occur over time if prevention is not practiced. Finally, glyphosate-resistant weeds could evolve within Roundup Ready cropping systems. Although weed resistance to glyphosate is still relatively rare, there are several examples of resistance occurring. Glyphosate-resistant annual ryegrass (Lolium sp.), horseweed (marestail), the waterhemps and palmer pigweed, giant and common ragweed, and other weeds are becoming more common every year in all parts of the U.S.