With the rise in glyphosate-resistant weeds, growers are being forced to use additional or alternative management tools to achieve adequate weed control. A small number of weed species are resistant to multiple herbicides, leaving growers with very limited viable options for control.
The most troublesome multiple-resistant weeds for North American crop production are 2 pigweed species, common waterhemp and Palmer amaranth. Other important multiple-resistant weed species are common and giant ragweed as well as kochia and horseweed (Figure 1).
Figure 1. Confirmed glyphosate-resistant weed populations in North America.
A critical part of managing weed resistance is controlling resistant populations before they get out of hand, which means being aware of potential resistance issues in the area and watching for weeds that have survived a herbicide application.
DuPont Pioneer is working with the University of Illinois to identify glyphosate-resistant plants in fields where resistance is suspected and also to raise awareness that glyphosate-resistant waterhemp is increasingly becoming prevalent in Illinois.
In 2012, 379 plant samples were taken from 88 fields in which waterhemp plants survived a glyphosate treatment. DNA was then extracted from the plant samples, and molecular assays were used to test for resistance to glyphosate and PPO inhibitors.
Results were broken down on a plant basis and a field basis. On a plant basis, 54% of plants were confirmed as glyphosate-resistant, 12% of plants were confirmed resistant to PPO inhibitors, and 6% of plants were confirmed resistant to both glyphosate and PPO inhibitors.
Resistance Prevention and Management
- It is important to remember that development of herbicide resistance is a numbers game – the more plants that are exposed to a herbicide, the greater the likelihood of selecting a resistant individual. These individuals will then proliferate and result in a resistant population.
- In several weed species, resistance to glyphosate originated in continuous monoculture systems where glyphosate was the primary weed management tool used every year.
- Management practices, such as crop rotation and herbicide rotation, can reduce the selection pressure exerted by a given herbicide and prolong its usefulness.
On a field basis, 67% of fields were confirmed as glyphosate resistant, 24% of fields were confirmed resistant to PPO inhibitors, and 14% of fields were confirmed resistant to glyphosate and PPO inhibitors.
A total of 10 new counties in Illinois confirmed glyphosate-resistant waterhemp in 2012 (Figure 2). Although not shown in the figure below, the presence of glyphosate-resistant waterhemp in southern Illinois is well documented. Sampling for glyphosate-resistant weeds in Illinois is continuing in 2013 and will include Palmer amaranth in addition to waterhemp.
Figure 2. Illinois counties (blue) in which resistant waterhemp has been confirmed by molecular assay.