Managing to Overcome Stubborn Weeds

Resistance to herbicides has been a management challenge as long as herbicides have been used for weed control. The introduction of crops with engineered resistance to glyphosate in 1996 prompted most growers to adopt a technology that was easy and efficient.

Now, as with other herbicides, some weeds are growing resistant to glyphosate. To date, glyphosate resistance has been confirmed in 24 weed species worldwide, including 15 in North America.

According to Mark Jeschke, agronomy research manager for DuPont Pioneer, some of these weeds had previously developed resistance to other herbicides. In the Midwest, for example, common waterhemp is a real problem. Other weeds that are showing resistance include horseweed (marestail), common and giant ragweed and Palmer amaranth.

Jeschke provides the following guidelines to help manage resistant weeds.

• Use multiple herbicide modes of action. The first step is to use a soil-applied herbicide, which many growers are doing already. In places where resistance is already an issue, look at other herbicides, your tank mixes and other actions you can take.
• Follow the principles of good weed management. Growers will have to be more active and diligent managers, hitting the weeds at the right size. There aren't many herbicides out there as forgiving as glyphosate.
• Look out for potential problems. Things can go from barely noticeable to infested fields within a few seasons, especially with prolific seed-producers such as waterhemp.
• Be sure machinery is clean. In areas where resistance is established, it's especially important to clean machinery between fields. Combines, in particular, can spread seed from resistant weed populations.