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Got monster weeds? Control options limited

The heat of summer has settled in over the Corn Belt, and that's got everything -- crops and weeds alike -- growing rapidly. That's starting to create some weed control challenges, especially in those acres you weren't able to plant this spring on account of the wet weather.

If you've got acres on which you filed prevented-planting crop insurance claims earlier this year, the weeds on those acres probably aren't facing much competition for water and nutrients in the absence of a crop. Parts of Minnesota, for example, have as many as 30% of the usual acres unplanted, and that's opened the window for a lot of weed pressures, says University of Minnesota Extension agronomist and weed scientist Jeffrey Gunsolus. The implications of those monstrous weeds last well beyond this crop season, too.

"On many of these acres weeds such as giant ragweed, common lambsquarters, and waterhemp are thriving. Although weeds are beneficial from an erosion-control perspective, their rapid growth will make seedbed preparation for planting cover crops very difficult, and weed seed production potential will challenge even the best weed management tactics available in 2014," he says in a university report. "If left untended and without crop competition, giant ragweed can produce approximately 10,000; common waterhemp 70,000; and waterhemp 100,000 seeds, or more, per plant. Such large additions to the weed seed bank make next year's weed management tactics less effective because as weed density increases, herbicide effectiveness decreases."

If you claimed prevented planting and have no crop growing, you've likely also not conducted any other field operations, namely tillage. So, the weeds out there have faced an easy road. That means your weed management options may be limited.

"Fields that have not been tilled this spring now have weeds that have been growing without crop competition and currently are several feet tall and growing rapidly. Therefore, weed management tactics must be implemented very soon. Fortunately, our current string of dry and sunny days does provide a window of opportunity to manage some of the worst fields," Gunsolus says. "Based on weed size and rapid growing conditions, I do not see herbicides as being a viable management option. Broad-spectrum herbicides such as glyphosate, glufosinate, and paraquat are logical choices but due to weed size, effective control is not likely due to incomplete coverage."

You're likely left with tillage as your only option to control monster weeds out in those unplanted fields. But, if your weeds are too big, it will likely require more than just a light pass through the field, especially if there's a cover crop present.

"Tillage with a disk or field cultivator will also lose effectiveness as weed size increases; however, the disk is likely to perform better than a field cultivator for smaller weeds," Gunsolus says. "At this stage in the growing season, mowing or chopping the larger weeds appears to be the most effective recommendation because it will destroy the most plant biomass and it will not expose the soil to wind and water erosion. In some particularly weedy fields, if a cover crop is desired, mowing or chopping will still be necessary before seedbed preparation can begin."

If you all of a sudden find yourself surrounded by weeds in all your fields, you'll need to prioritize and make sure you knock down the weeds that have the greatest potential for carrying over seeds to next year, Gunsolus says.

"Attempting to inhibit weed seed viability by applications of 2,4-D or other systemic growth regulators is not recommended because the risks of off-target movement due to volatilization or drift far exceed their effectiveness in inhibiting seed viability. The extended flowering period and rapid seed maturation of weed seed would imply that multiple treatments would be necessary and at best only a small percentage of the seed would be affected," he says. "It is not realistic to think that all fields in need of weed control will receive treatment. It would be wise to focus your attention on the fields that contain weeds that will be the most difficult to control in next year's crop and have the highest weed densities.  Mapping of field areas that you anticipate to be particularly challenging next year is strongly encouraged."

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