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Check on rain records and with consultants on chemical carryover

Dry conditions throughout the growing season of 2021 may have resulted in some herbicide carryover issues for 2022.

If farmers suspect carryover, there are things to double-check to put one’s mind at ease.

Some of the environmental characteristics that could lead to carryover include rainfall, temperature, and soil type.

Farm fields that received less than 6 inches of rain from June 1 to Sept. 1 could have herbicide carryover challenges.

That was the message of Tom Peters, associate professor, sugarbeet agronomist for North Dakota State University and University of Minnesota. He spoke via the University of Minnesota Strategic Farming series on weed management.

“If you didn’t measure 6 inches of rain, I think you need to look at some of the products that you used, consider the crops that you’re going to plant in 2022, and investigate if you have to worry about any carryover concerns,” Peters said.

He added that some regions received rain after Sept. 1, but he’s not sure of the amount of herbicide degradation under that scenario.

As far as temperature, Peters said the “best conditions for degradation of products are when the temperatures are between 70-85 degrees.”

He also added that soil type, including soil pH can affect herbicide carryover.

Farmers and their consultants will want to check the herbicides they used in 2021 and find the “absorptive coefficient” – a measurement that describes how each herbicide binds to the soil and correlates with the water solubility of the herbicide.

“Some herbicides want to bind to the soil,” he said. “If they’re bound, they’re going to be in the soil longer than something that’s more available in the water solution and potentially be able to leach out of the soil profile.”

He compared two herbicides, Outlook and Warrant, that are in the same herbicide family.

“Warrant binds to soil harder than Outlook does, so consequently, the water solubility of Warrant is going to be a lot lower than Outlook,” he said. “Outlook’s going to be available in the soil solution much easier than Warrant will be.”

Farmers need to be aware that just because a herbicide was incorporated into the soil – that doesn’t mean that it was available in very dry conditions.

A herbicide in the soil solution is available for uptake by the seeds, shoots, or roots of weed seedlings. A herbicide that is bound to soil particles is temporarily not available.

“The good news is, if we get a rain event, that herbicide will move off the soil colloid, and back into the soil solution where it can work for you,” he said.

He encourages farmers to work with their chemical suppliers now to determine any carryover concerns and how to deal with that situation in 2022.

Peters also wants farmers to ask their retailers about supplies this spring.

“Just a couple words about the availability of products – I think the best message I can give you is to plan ahead, talk to your retailer, give them an indication of what products you intend to use,” Peters said. “Don’t wait till the end. Don’t put all of your eggs in the post-emergence basket, but to use an integrated approach, including soil-applied, soil-residual herbicides.”

Some trait post-emergence herbicides may be in short supply, he added. Looking at traditional post-emergence herbicides could be one way for farmers to protect themselves against herbicides shortages.

Peters also encourages farmers to spend time ahead of planting getting more information on weed biology. Weed management, he suggests, involves learning about each significant weed. That involves identify weeds and their characteristics – its life cycle, when it emerges, what is its response to other weeds, how it reproduces, and seedbank issues.

Recognizing when a weed type will germinate and emerge can go a long way toward accurate herbicide placement for the greatest effectiveness.

“I think it’s important to understand your weed control challenges, how those plants flower and reproduce, and that (often) relates to the seed that they produce,” he said.

Peters pointed out that a plant breeder works diligently to produce a handful of seed from a topnotch cultivar. Mother Nature, on the other hand, spreads seeds by the thousands, hundreds of thousands, millions, and tens of millions.

Weed control is not an area that can be overlooked or ignored. Weeds come charging back as soon as they get the opportunity and are ready to compete with crops.

For more information about this article and to see Tom Peters extended presentation, as well as presentations by fellow Extension agronomists and scientists, please visit Closed captioning is available.

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Take time to get to know your local weed species for better control. Photo by Andrea Johnson at Farmfest 2021.

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