Improved Weed Control Using Soil-applied Herbicides Starts With Maps
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New tools to improve soil-applied residual control of herbicide-resistant weeds may involve a combination of soil maps and historical satellite imagery.
The current buzz focuses on post-applied technology that senses weeds and then sprays them, yet another approach involves reducing the number of these weed escapes before they emerge.
A layered residual herbicide approach — from burndown/early preplant through postapplication — can help if herbicide groups are altered annually. However, these applications treat every acre the same, which can limit control in field areas where herbicide-resistant weeds like to congregate heavily.
Since innovative growers apply variable-rate nutrients to precision-feed a crop, why not reduce blanket applications of soil-applied residual herbicides in favor of variable rates tied to soil properties and weed intensity?
That’s the thinking that led Purdue University weed scientist Bryan Young and his colleagues on this quest. They are working on basic research to eventually build prescription maps that will apply higher rates of soil-applied herbicides where weeds are located before they break through the soil.
“We think there’s opportunity to do better than a single soil-applied dose that’s uniformly applied across an entire field,” Young says. “We talk a lot about optimizing our foliar application postemergence sprays, but no one is giving attention to the possibilities of soil-applied residual weed management.”
Initial research by Young and colleagues focused on using soil organic matter and texture maps to apply a correspondingly higher or lower prescription herbicide rate instead of a blanket rate. However, this soil-only map prescription lacked consistent control.
After two years of soil-mapping applications and two years of actual on-farm variable-rate soil residual herbicide applications, their best success occurred when encountering weed patches.
Weed Map Critical
“We realized the need to add weed map data along with our organic matter, soil texture and electrical conductivity (EC) maps,” Young says. “But detailed weed seedbank maps don’t exist.”
Researchers have tried building weed maps by using obvious locations like end rows, field entryways, and along grass waterways and stream buffers. They attempted to find correlations between soil attributes and weed seed concentrations. Researchers even tried soil-sampling and lab grow-outs to determine seedbank estimations. None of the efforts produced the detail they sought.
“I think the best weed mapping method is perhaps incorporating an actual ‘as applied’ postemergence application map on emerged weeds from a previous season using a ‘sense and spray’ technology from Deere, CNH, Greeneye Technology, One Smart Spray or other such company,” Young says. He believes such technology can confidently develop weed pressure maps, especially with several years of data in similar crops.
Young understands that such weed escape pattern maps may not truly indicate where weeds will emerge the following year. “While we don’t have data that shows if we only apply a residual soil-applied herbicide on these previous escaped-weed locations, would we be too far off base? It’s worth testing, especially using variable rates in combination with soil maps,” he says.
Satellite and Drone Imagery
Until “sense and spray” maps become more common and proven for such use, Young believes satellite and drone imagery can help build weed maps for soil-applied use.
He is working with a Canadian company that’s using high-resolution satellite imagery to detect weed escapes. Such technology can identify and provide a historical map that locates green weeds in a drying yellow crop before harvest.
“They’re using this imagery for a weed mapping strategy to apply residual soil-applied herbicides in the fall that reduce herbicide-resistant wild oat patches in cereal crops,” Young says. “We’re currently awaiting reports to proceed with our tests on giant ragweed.”
Young believes drones will be a more useful component in estimating how much post-spraying is needed and creating historical maps of resistant-weed locations.
The left half of the slide is a map of the different soil types in a single field that would influence the labeled application rates for common soil residual herbicides. The right half of the slide is from DTN small plots in the same field, but the heat map shows the density of burcucumber that emerged in the field. The main point is that weeds are random and patchy in fields. Combining the weed density information with soil factors can help optimize soil residual herbicide applications. Similar to the principle of Integrated Pest Management (IPM), you only apply a pesticide when warranted; however, that is hard to do with a soil residual herbicide that has to be applied prior to weed emergence. Historical weed maps may inform where these soil residual herbicides are indeed justified.
Drone Weed Maps
Commercial companies like Sentera (now owned by Deere), Agristry and Agremo can turnkey precision spraying from drone scouting imagery into weed maps and postemergence spray prescriptions.
“Quick turnaround is needed because a two-day delay with a Palmer amaranth weed problem could mean the difference between an on-label versus an off-label post-spray timing,” Young says.
As more farmers and retailers understand that postemergence herbicides won’t always save the day, Young sees a day when historical weed maps drive the success of prescription variable-rate soil-applied herbicides.
“It gets very exciting to think about what we might be able to do from a soil-applied weed management standpoint and the herbicide molecules that might come to market,” Young says. “More optimal placement of herbicides, whether soil residual or postemergence applications, can reduce potential pesticide waste and enable more robust weed management strategies.”
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