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As temperatures begin to warm and rain falls, wheat producers 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 Extension weed specialist for small grains. 
At this stage of the growing season, weed control involves three important factors: growing conditions 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 synthetic auxin herbicides, such as 2,4-D, dicamba and MCPA, in which weed control is greatly reduced for weeds 4 or more inches tall or in diameter. 
“For many species, 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 plants begin to go semi-dormant or are coming out of dormancy, weed control measures aren’t as effective,” Manuchehri said. 
Producers who missed the fall application window have another shot 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 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 sites of action as they can broaden the spectrum of weeds controlled and help delay the selection for herbicide resistant weed biotypes. 
“Liquid fertilizers are often used as the carrier for herbicides being applied in the spring,” Manuchehri said. “If an 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 an herbicide that was previously effective now failing? 
Herbicide resistant weeds are an increasing concern in Oklahoma and across the nation. 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, download a free OSU fact sheet on the topic, Diagnostic service to test herbicide-resistant weeds in Oklahoma. 
OSU Extension personnel can also 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.

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