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Preemergence and postemergence herbicide combination offers best weed control outcome for early planted soybeans: Study

Farmers in most states often try to plant soybeans as early as possible to increase yields. However, planting soybeans sooner than later might also require a corresponding change in herbicide application strategy to garner top results.

According to a research article recently in Weed Technology, a strategy utilizing both preemergence (PRE) and postemergence (POST) herbicide treatments optimized weed control outcomes in early planted soybeans. Researchers conducted their study in 2021 at three locations across Central Illinois.

"The research objective was to examine various herbicide treatments applied at different timings and rates to assess the effect on weed control and yield in early planted soybeans," says Aaron Hager, Ph.D., Department of Crop Sciences, University of Illinois, and corresponding author for the study, and WSSA member scientist. "Overall, we found that treatments comprised of a full rate of PRE followed by a POST resulted in the greatest and most consistent weed control at the final evaluation timing."

The study's PRE treatments consisted of S-metolachlor plus metribuzin premix (Boundary) at either half or full-label recommended rates, applied at planting or just before soybean emergence. The POST treatments consisted of glufosinate (Liberty), glyphosate (Roundup), and ammonium sulfate (AMS), with or without pyroxasulfone (Zidua), either at half or full-label recommended rates, applied when weeds reached 10 cm (about 4 inches) tall.

"The full rate of PRE extended the POST application timing two to three weeks for all treatments at all locations except Urbana," notes Hager. "Full-rate PRE treatments also reduced the time between the POST application and soybean canopy closure. Although pyroxasulfone added to POST treatments did not consistently reduce late-season weed emergence, this may be due to earlier soybean development and canopy closure from early planting, which suppressed weeds naturally."

PRE-only treatments failed to perform as well as either POST-only or PRE plus POST herbicide treatments in early planted soybeans, points out Hager. "For optimal effect on both weed control and yield, the PRE plus POST combo proved best for early planted soybeans," sums up Hager.