As temperatures begin to warm and rain falls, wheat producers managing their crops for yield and grain quality may turn to herbicides to get a handle on grass and broadleaf weeds.
“With herbicide applications, it’s always a balancing act. We obviously want to kill the weeds, but we don’t want to harm our crop,” said Misha Manuchehri, Oklahoma State University Cooperative Extension small grains weed specialist.
At this stage of the growing season, weed control involves three important factors: weed size and health at the time of herbicide application, tank-mix partners and the presence or absence of herbicide resistant weeds.
Weed size at time of application is critical for most postemergence herbicides to be effective. This is especially true for growth regulating herbicides, or auxins, such as 2,4-D, dicamba and MCPA, in which weed control is greatly reduced for weeds 4 or more inches tall.
“It’s better to target winter annual weeds in wheat in the fall when they’re small, actively growing and haven’t yet experienced dormancy. Once daytime high temperatures dip below 60 degrees F and plants begin to go semi-dormant, weed control measures just aren’t as effective,” Manuchehri said.
Producers who did miss the fall application window do have another shot now, in the late winter and early spring when winter annuals are growing again or spring weeds are emerging. However, the timing is crucial.
“Producers should look for signs of new leaf emergence and consistently warm daytime temperatures,” Manuchehri said. “Since postemergence wheat herbicides move in the living tissues of weeds, having actively growing and healthy weeds at the time of application is extremely important. Stressed plants, whether weeds or crops, can lead to poor weed control and crop injury.”
In determining the right herbicides to apply, producers should think about premixes and tank mixtures of various herbicide modes of action as they can broaden the spectrum of weeds controlled and help with herbicide resistance management.
“Liquid fertilizers are often used as the carrier for herbicides in the spring,” Manuchehri said. “If a herbicide is approved to be applied with liquid fertilizer, be aware the fertilizer doesn’t replace required adjuvants such as NIS, COC or MSO. Always consult the herbicide label for required adjuvants and proper mixing and be sure the products you’re mixing are compatible.”
Finally, producers should always assess the effectiveness of their herbicide application. Was the level of weed control satisfactory? Was the crop injured? Is a herbicide that was previously effective now failing?
Herbicide resistant weeds are an increasing concern in Oklahoma as well as the nation and the world. Producers who suspect they have an herbicide resistant weed in their crop can send a sample to OSU for confirmation.
For more details on this free service, producers can download a free OSU Fact Sheet on the topic, PSS-2779, “Diagnostic service to test herbicide-resistant weeds in Oklahoma,” at facts.okstate.edu.
Extension personnel also can help producers develop new weed management plans to replace systems that are no longer effective.
For more information about weed management strategies, contact the nearest county Extension office.
