Dakota Farmer

Did rain wash away my preemerge?

Lots of spring rains might leave questions for preemerge herbicide efficacy.

Sarah McNaughton, Editor, Dakota Farmer

June 4, 2024

2 Min Read
 Evidence of excessive rainfall in a Midwest field
RAINY DAYS: Excessive rainfall in parts of the Midwest has left some farmers hoping to finish planting in time and others wondering if their weed program is still present in their fields. Kamil Trynda/Getty Images

Growers in the Midwest have received plentiful rainfall this spring. Aside from some delayed or muddy planting conditions, the wet weather has left some wondering if the preemerge herbicide is still present in the soil.

Tom Peters, Extension sugarbeet agronomist and weed control specialist at North Dakota State University and the University of Minnesota, says herbicides like ethofumesate are still there. This herbicide is considered one of the best soil residual herbicide options for sugarbeet growers.

With rain in most places coming slow and steady this spring, herbicide didn’t runoff with excessive rainfall but was instead incorporated into the soil. As the rain continues, growers wonder what the state of their preemerge looks like.

Preemerge herbicides must be present within the soil profile when weed seeds begin to germinate to have maximum effectiveness, as they are inactive when on the soil surface. Generally, a half-inch of rain is needed to move herbicides into the soil profile.

Herbicides including ethofumesate are metabolized by soil microbes and more water-insoluble than chloroacetamide herbicides. Soil microbes need both water and heat to metabolize. While the moisture is present this season, the temperatures have remained mostly cool.

Minimize performance failure

As weed management is a holistic approach, herbicides will likely be applied in sequence. Even if early-season weed control programs are working well, Peters encourages growers to stick to their program. The big challenge might be timing applications between rainfall patterns.

Iowa State University Extension says that use of rotary hoes or harrows can move some of the herbicide off the surface and, most importantly, control weeds that have already germinated and escaped the preemerge.

Some fields might require an earlier postemerge herbicide application to control weeds that potentially escaped the preemerge. This would be considered a layered residual with these applications due to the longer intervals between postemerge applications and canopy closure.

Since it is impossible to determine whether significant herbicide was lost, ISU discourages growers from blindly trying to supplement potential losses. Scout to determine when to make post applications, and brush up on weed management plans for layered residuals.

ISU encourages systematically scouting fields beginning 14 days after planting, making sure to cover the entire field to account for potential preemerge performance issues. Contact your local Extension agent or agronomist for guidance on early-season weed management.

NDSU Extension and ISU Extension contributed to this article.

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Herbicide

About the Author(s)

Sarah McNaughton

Editor, Dakota Farmer, Farm Progress

Sarah McNaughton of Bismarck, N.D., has been editor of Dakota Farmer since 2021. Before working at Farm Progress, she was an NDSU 4-H Extension agent in Cass County, N.D. Prior to that, she was a farm and ranch reporter at KFGO Radio in Fargo.

McNaughton is a graduate of North Dakota State University, with a bachelor’s degree in ag communications and a master’s in Extension education and youth development.

She is involved in agriculture in both her professional and personal life, as a member of North Dakota Agri-Women, Agriculture Communicators Network Sigma Alpha Professional Agriculture Sorority Alumni and Professional Women in Agri-business. As a life-long 4-H’er, she is a regular volunteer for North Dakota 4-H programs and events.

In her free time, she is an avid backpacker and hiker, and can be found most summer weekends at rodeos around the Midwest.

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