Dry conditions with scattered, variable rainfall have been the story in northern Illinois over the past month. With this pattern expected to continue, some soybean chemical programs are changing. 

Photo – Seth Wiley

A common question I’ve been fielding for the past couple weeks is: 

“We have beans planted that didn’t get the pre-emerge on before the rain, and there is no rain in the forecast. Do we spray the pre-emerge before the beans come up or do we switch to a row-and-go type program?” 

Before I can answer that, there are two things I need to know: 

• Are there weeds currently growing in the field or is it clean? 

• What bean trait is planted? E3? Non-GMO? 

Weed pressure dictates the urgency of a spray schedule. My answer will always be to spray if weeds are present – small weeds are easier to control than big weeds. Another situation where spraying is essential is with non-GMO soybeans. Due to limited post control chemical options in non-GMO beans, you really need to prioritize getting a pre-emerge residual product before they emerge. 

This leads to the next question: 

“If we need to spray, are we wasting our money putting the residual (group 15) in the tank with no rain in the forecast, or should we just scout and spray the weeds as they emerge?” 

From personal experience, I compare pulling the residual herbicide out of the tank to cutting hay: If you want it to rain, pull the residual out of the tank or go cut hay. Mother Nature will make it rain.  

There are four commonly used post applied residual (Group 15) products used in soybean programs today: 

• Acetochlor (Warrant)  

• Dimethenamid (Outlook) 

• Pyroxasulfone (Zidua) 

• S-Metolachlor (Dual II Magnum) 

Acetochlor and Dimethenamid require the least amount of rainfall to activate – approximately ¼ inch – but they have shorter half-life persistence in the soil. Pyroxasulfone and S-Metolachlor require more rainfall for activation; ¾ inch for pyroxasulfone and at least 1 inch for S-metolachlor. However, both have longer half-lives and will persist in the soil longer. Pyroxasulfone also has a better photodegradation rating.  After 10 days of being on the soil surface without rainfall, it will have 80% of the active still available to hold off weeds. Whereas S-Metolachlor, Acetochlor, and Dimethenamid will have approximately 40-50% active still available to hold off weeds after 10 days without rainfall. As always, read and follow herbicide labels. 

Staying in tune with weed pressures on your farm will help guide the decision-making process to your chemical program. “Start clean and stay clean” are words to strive for this time of year! 

Sources: 

Iowa State University Extension and Outreach. “Absorption of Soil-Applied Herbicides.” Iowa State University Integrated Crop Management, https://crops.extension.iastate.edu/encyclopedia/absorption-soil-applied-herbicides.

Hefty Seed Company. “Which Group 15 Herbicide Is Right for You?” Agronomy Answers Yield, February 11, 2022. https://heftyseed.com/aayfebmar2022-which-group-15-herbicide-is-right-for-you.

Jeremy J Hoskey, Chem Rep, BASF Northern Illinois, jeremy.hoskey@basf.com