Updated July 4, 2018:

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Soybean PRE Herbicide Activation
Mike Ostlie and Greg Endres
A pre-emerge herbicide program is very important to managing glyphosate-resistant weeds in soybeans. A study was conducted over the 2016 and 2017 growing season to determine the consequences of delayed herbicide activation of soybean herbicides (aka no rain). The study was conducted under a center pivot irrigation system to supply 0.5” of water to some of the treatments. The weeds present were redroot pigweed, common lambsquarters and kochia (2017 only). The herbicide treatments were metribuzin, Fierce, and Spartan, representing some of the most commonly used active ingredients in soybeans. Activation strategies included watering within 24 of herbicide treatment, watering 7 days after herbicide treatment, not watering at all, or using a rotary hoe to mechanically activate the products 7 days after application. In both years, there was sufficient delay in natural rainfall to evaluate treatment differences. In 2016 there was a 16 day absence of activating rain and 22 day delay in 2017. In both seasons, there were small intermittent rainfall events prior to the activation rainfall.
A common understanding about PRE activation is that smaller rainfall events totaling ~0.25” or more are not sufficient to activate a herbicide. This study provides further evidence that this is the case. In 2017, there was a rain event 6 and 7 days after herbicide application that totaled 0.19”, to go along with an additional 0.09” 7 days later. This did not activate the herbicide treatments even though the total was 0.28”. The products were not activated until the 0.79” rainfall 22 days after herbicide application.
There was no decrease in herbicide activity with a 7 day delay in activation with any product (Table 1). When averaged across herbicides, there was also no difference between mechanical incorporation and no activation strategy, except that mechanical incorporation decreased redroot pigweed control.
There are a few important contrasts to point out. With redroot pigweed and common lambsquarters, Fierce and Spartan were consistent performers except that Spartan control dropped after 7 days with no activation. Control of kochia dropped off with both products after 7 days without activation as well. Rotary hoeing typically did not improve weed control, except that it appeared to improve metribuzin’s effect on kochia.
Overall, metribuzin performance decreased the quickest with delayed activation. This data suggests a need to revisit any recommendation relating to mechanical incorporation of herbicides, with special attention to specific weed species and soil moisture status/precipitation.