Last summer, I saw more fields with weeds in our region than I had ever seen in the past. Weed resistance is a real problem that cannot be overlooked on your farm. The use of preemergence (PRE) herbicides must be in your weed management toolbox this spring.
With the majority of our corn and soybean acres in Roundup Ready technology, glyphosate resistant weeds are today’s primary weed concerns. The glyphosate resistant weeds present in the Dakotas and Minnesota include Horseweed, Kochia, Common and Giant Ragweed, and Waterhemp. Of these, the prevalence of glyphosate resistant Kochia is rising most rapidly. Even if you use multiple modes of action to keep weeds in check, or have not encountered resistance issues on your farm yet, be on the lookout for Kochia, as it can be windblown from neighboring fields. Two or three Kochia plants rolling across your ground in the fall can deposit many seeds that you will need to address the following year.
Valor™, Sharpen®, and the Authority® premix products are the most used PRE’s for soybeans. While all provide good residual control, they each have strengths and weaknesses. Growers must decide which weed is the priority, and refer to the label for control ratings. The window of application for most of these products is short. For example, Valor™ must be applied before the seed is within 1” from the soil surface.
PRE’s in corn have a much wider application window and can be applied from early PRE to V2 or V4 stage corn. Balance® Flexx (not registered in MN), Sharpen®, SureStart®, Verdict®, and Harness® are the most popular PRE’s in corn. Refer to the label for application directions.
Tip: Always use the recommended rates of the PRE’s, as reduced rates will lead to additional resistance issues in the future.
In addition to adding a mode of action to your system, PRE herbicides also allow for wider flexibility of POST herbicides. We have several effective POST options, including Callisto®, Laudis®, Status®, and Capreno®. These can be added with glyphosate for corn, providing additional modes of action and residual control.
There are fewer POST options for soybeans. The herbicides we do have for POST control of weeds in soybeans should be applied to small weeds (under 4” normally). If the glyphosate does not kill these weeds with the first application, the weeds are usually too large to try a rescue treatment of another product. For this reason, I believe it is crucial to use PRE’s for soybeans.
LibertyLink soybeans or corn are a great alternative to glyphosate and are another tool for managing weed resistance. They are not the complete answer though, and I recommend using PRE’s with all of your LibertyLink acres as well. Liberty does not have the same ideal spraying window that Roundup does, but applying a PRE will provide more flexibility for Liberty application.
The battle against resistant weeds is on, and will continue to be a concern for growers in the Northern Plains as more glyphosate resistant weeds move into the region. Increasing the number of modes of action within a field minimizes the chances for resistance to develop. Using all of your weed management tools this spring, beginning with PRE’s, will help you be successful in preventing weed resistance.