Ongoing Evaluation Key to Controlling Weeds, Improving Yield in Soybeans

Soybeans weeds
A preemergence first strike against yield-limiting weeds, such as waterhemp and Palmer amaranth, can give soybeans the early season boost needed to improve yield potential.

As the sun bears down on record U.S. soybean acres this August, farmers keep their eyes on their fields to evaluate weed control decisions made earlier in the year. Ongoing scouting and peer discussions near harvest can help farmers fight back against persistent weeds the following season.

Every field potentially has unique weed management challenges depending on weather, soil type and weed control methods, which is why it’s hard to make decisions solely based on neighbors’ experiences. In addition to conversations with fellow farmers, Dow AgroSciences field scientist Dave Ruen recommends consulting with trusted retailers, herbicide and seed suppliers, and Extension weed scientists for additional support.

“At this point in the season, farmers can see weed control performance firsthand in their own fields and ask specific questions to improve their programs for the following season,” Ruen says. “Herbicide-resistant weeds, particularly Palmer amaranth and waterhemp, are top of mind for many Midwest farmers this year.”

Ruen offers advice on farmers’ three most common weed control questions this season:

Q: In areas where Palmer amaranth is not yet herbicide-resistant, what steps can farmers take to keep it from becoming resistant?
A: Palmer amaranth differs from other pigweed and waterhemp species due to its rapid growth. Farmers cannot let this weed get ahead of the crop.

With any potentially resistant weed, frequent and timely scouting helps farmers identify troublesome areas and formulate a timely postemergence herbicide plan. After postemergence applications, scouting allows farmers to identify at-risk fields where herbicide-resistant weeds may already exist and decide if it’s necessary to make a sequential herbicide application with an effective mode of action.

After applying full rates of broad-spectrum preemergence herbicides in the spring, farmers should alternate postemergence herbicides with mode-of-action-tolerant seed traits to avoid repetitive use of one active ingredient and to expand the mode-of-action family of options.

Another way farmers can battle herbicide-resistant weeds is to stay current on newly identified resistance in their geography and keep detailed notes of specific weed pressure in their fields. Electronic files are easy to customize for individual fields based on crops and known weeds, plus they can be used as visual aids to build an inventory or cropping mix, and program solutions for the 2018 season.

Q: How does rainfall affect the residual control of soybean herbicides?
A: Rainfall is needed for any herbicide to take effect in the soil for controlling weeds. As farmers look at the variability over the course of multiple seasons, the timeliness of needed rainfall partly depends on the soil temperature following application.

When soil conditions are cooler, weeds aren’t germinating as quickly, which provides a slightly larger window to receive rainfall. Under normal conditions, receiving rainfall within seven to 10 days after a herbicide application is ideal. If it’s extremely warm and it’s a late-spring application, an activating rain within three to four days will provide optimum performance of Sonic herbicide and other soybean herbicides. Sonic is proven to perform in a variety of soil types, including fields with slightly delayed activating rains.

Q: What is the best plan to control waterhemp in soybeans?
A: Ranked as one of the most troublesome and most common weeds of 2017 by the Weed Science Society of America, waterhemp can cause devastating yield loss if left uncontrolled. Research from the University of Illinois has shown waterhemp can cause up to 40 percent yield loss in soybeans.

A preemergence first strike against waterhemp can give soybeans the early season boost needed to improve yield potential. The best strategy to control waterhemp includes a program approach using preemergence and postemergence herbicides with multiple modes of action.

Commonly known as a summer annual, waterhemp germination and emergence also can extend late in the season. Waterhemp management needs to begin early with a preemergence residual herbicide that will provide long-lasting control. Sonic can be applied in the fall, preplant incorporated, preplant or preemergence up to three days after planting, before soybeans emerge. With two nonglyphosate modes of action, Sonic controls many broadleaf weeds resistant to glyphosate, including waterhemp, Palmer amaranth, giant ragweed and common ragweed.

Early season scouting can help farmers determine the appropriate window of application for a postemergence program.
To learn more about stopping hard-to-control weeds before they steal yield, visit


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