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Evaluating Herbicide Incorporation Via Water Quality and Timing Under Different Tillage Strategies
Mike Ostlie, Greg Endres, and Jesper Nielsen

The most important piece to combatting glyphosate-resistant weeds in soybeans is an effective pre-emergent (PRE) herbicide program. Predicting when an activating rainfall will occur is as difficult as ever even though we know that the amount of rain can impact the success of PRE herbicides. The question then becomes, “how will the herbicide be affected if we don’t get enough rain to activate the product?” A field trial was established in 2016 to evaluate weed control using three soybean herbicides affected by delayed rain and if rotary hoeing can substitute for rain activation. These treatments were tested in both tilled and no-till (direct seeded) situations.

The three herbicides chosen, Spartan (sulfentrazone), Fierce (flumioxazin + pyroxasulfone), and metribuzin (aka Sencor), represent the chemistries of a large portion of the rain activated PRE herbicides for soybeans. Each herbicide was compared in corn stubble that was either tilled or left un-tilled. A center pivot irrigation system was used to simulate rainfall (0.5” of water) for the herbicide activation treatments. The herbicides were activated within 1 day of application, 7 days after application, rotary hoed 7 days after application, or received no activation treatment. The first natural activating rainfall occurred 16 days after treatment. Only weeds emerging after application were evaluated.

Fierce was the most resilient product of the three. There was never a decline in performance across the tillage and activation strategies. Spartan was also resilient except for the one instance under tilled conditions where receiving no activation reduced control by nearly 20%. Metribuzin was affected the most by tillage and activation. A one week delay in rain activation did not affect Spartan or Fierce. With metribuzin, the control dropped off dramatically under no-till when no activating rain occurred. Rotary hoeing did not increase metribuzin activity in no-till and actually reduced weed control under tilled conditions. In this case the rotary hoeing may have planted more seeds than it terminated, while not activating the product. Rotary hoeing did increase the weed control from Spartan under tilled conditions. Rotary hoeing had no effect on no-till Spartan nor either tillage strategy with Fierce but control was already very high with those treatments. Based on these results, rotary hoeing would not be recommended with metribuzin.

In 2016 the activating rain came 16 days after the herbicides were applied. One important note for this study is that there was no new weed emergence during the dry period, so the products that were tested were still able to be activated and function correctly. If weeds had emerged prior to herbicide activation, such as with a minor rainfall (ex. 0.05”), more treatments would be at risk for failing.