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Early Season Weed Control: Timing is Everything (05/30/19)

With several rounds of rainfall this past week, many people pushed hard to wrap up planting season where the ground was fit.

With several rounds of rainfall this past week, many people pushed hard to wrap up planting season where the ground was fit. In years with these compacted planting seasons, it is often difficult for sprayers to keep up with planters in order to get preemergence herbicides applied on all fields. Most corn herbicides can be forgiving and allow applications preemergence through an early vegetative growth stage. Herbicides used for soybean weed control tend to have stricter labels. Four popular herbicides that only allow application only prior to crop emergence are sulfentrazone (Authority/Spartan, generics), flumioxazin (Valor/Fierce, generics), saflufenacil (Sharpen-based products), and metribuzin (Sencor, generics). Examining these labels reveals that there is typical label language that says applications “must” or “can be” applied from planting up to three days after planting soybean. This three day-window is typically written into labels because under ideal soil conditions, soybeans will emerge within three days after planting. The reality we are facing in 2019 is that cold soils will delay emergence of soybean. There are many reports of soybean planted two weeks ago that have yet to emerge. The language on each individual label will decide how long we can legally apply each of these products after planting. Some of the products simply say to apply after planting, but prior to crop emergence. Our interpretation is: labels that state “must apply within three days” means you have a three-day window. Applications made after that three-day window may not cause crop injury if the soybeans have not yet emerged, but any liability for crop injury would be on the applicator, not the chemical manufacturer. Remember, as always, the label is the law. These label languages may beg the question, what does happen if I apply these products on emerging soybean? We applied these products at the Carrington REC last year to use as a teachable moment. Attached are pictures of sulfentrazone (Image 1) and flumioxazin (Image 2) applied to VE, or cotyledon soybean. The bottom line is that severe injury, and possible plant death and stand reduction can be expected if these Group 14 products are applied to emerged soybean. These products are important in our ever-lasting fight against herbicide-resistant and other difficult-to-control weeds. If we do not apply these products prior to crop emergence, several of our best herbicides must go back in the tool-box and we have fewer options remaining for weed control. This is not ideal for weed control, but hardly anything has been ideal in 2019. So what is Plan B if we do not get a preemergence herbicide on our soybean acres?

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Plan B for soybean weed control will greatly depend on which soybean technology has been planted. Roundup Ready Xtend, Roundup Ready, Enlist, and Liberty-Link soybean all allow approved herbicides to be applied over-the-top of emerged soybean. These products will control emerged weeds that are not resistant to their active ingredients. There are also herbicides that can be applied on any-traited soybean for control of emerged weeds. These mainly fall into three modes of action: Group 2 (Firstrate, Pursuit, Raptor, generics), Group 14 (Blazer, Cobra, Flexstar, generics), and Group 6 (Basagran, generics). For early vegetative herbicide applications, a herbicide with residual activity should be tank-mixed with the first application. In emerged soybean, this limits us to Group 15 herbicides (Dual, Outlook, Warrant, Zidua, generics) and fomesafen (Flexstar, genercis). For the benefits of applying residual herbicides, please see Dr. Tom Peter’s article from last week’s Crop & Pest Report.

 

Joe Ikley

Extension Weed Specialist

 

 

 

Greg Endres

Extension Cropping Systems Specialist

NDSU Carrington Research Extension Center

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