Monitor soil temperature when planning fall herbicide applications

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ST. PAUL, Minn. – As Minnesota farmers work long hours to harvest their crops this month, the Minnesota Department of Agriculture (MDA) is reminding them of the narrow window of opportunity for fall applications of pre-emergence herbicides such as Dual, Outlook, Parallel and Verdict.

Dual, Outlook, Parallel and Verdict product labels prohibit fall applications until soil temperatures at a 4‑inch depth are sustained at less than 55 degrees Fahrenheit. The labels also prohibit applications after soil freezes. Reviewing 10 years of climate data, MDA Pesticide Management Scientist Ron Struss found the dates when soil temperature dropped below this label threshold ranged from mid-October to early November. On the other end, dates of soil freeze up ranged from late November to mid-December. With the application window pinched between these two limits, Struss said the timeframe for proper application can be as narrow as 35 days.

“There are both economic and environmental reasons for farmers to keep these limitations in mind as they plan their fall field work,” Struss said. “Delaying fall herbicide applications until soil temperatures drop below 55 degrees reduces the amount of herbicide lost to degradation before spring, and ending fall application once soils freeze reduces the chance of product runoff during thaws.”

Label requirements vary from one product to another. In all cases, Struss said, applicators should take the time to carefully read and follow all label requirements for herbicides they plan to apply.

Two websites are now available to help applicators time their herbicide applications. The University of Minnesota and Minnesota Department of Natural Resources provide maximum and minimum soil temperature data at 4-inch depth for eight locations statewide at http://climate.umn.edu/cawap/soilpan/soilpan.asp.  The MDA provides soil temperature data at 6‑inch depth for nine locations around the state at http://gis.mda.state.mn.us/maps/csgsoil.htm.

Soil temperature can be taken in the field using a soil thermometer. Soil temperature should be measured when it is likely at its maximum; from 3 p.m. to 4 p.m. on sites with good sun exposure.  Soil thermometers should be left in the soil for five minutes before a reading is taken.



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