Malatya Haber Herbicide resistance problems continue
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Herbicide resistance problems continue

By Jennifer Carrico

RESISTANCE—Iowa State University Extension Weed Specialist Mike Owen discussed how weeds are becoming resistant to chemicals at the Respect the Rotation event near Ellsworth, Iowa, recently. (Journal photo by Jennifer Carrico.)

Herbicide resistance continues to be a problem throughout the Midwest, according to Mike Owen, Extension weed specialist with Iowa State University.

"So many acres are affected by waterhemp that is herbicide resistant. Horseweed and giant ragweed are becoming more of a problem too," he said.

The growing problem with herbicide resistant waterhemp is that it is now competing for yield.

"Our first grower assessment of glyphosate-resistant weeds was in 2005. That was at the beginning of the problem. Very little has been done about it and now we have a bigger problem," said Owen. "We need to manage these weeds better."

Associate Professor of Weed Science at the University of Arkansas Jason Norsworthy said, "Glyphosate-resistant marestail costs soybean growers an added $11.50 per acre. Glyphosate-resistant Palmer pigweed costs cotton growers $19.45 per acre. Resistance is impacting land values, conservation tillage and more. The system, the way it is currently set up, is not sustainable."

Owen said it is very important to use effective modes of action for each particular weed problem. Knowing what the problem is in each field can help in managing the field properly.

"Farmers need to consider the impact of herbicide rotation and crop trait rotation when planning for the following year's crop. Without proper planning and proper rotations, the possibility is that resistance to both herbicide and traits will evolve," he said.

Technological advancements help provide opportunities for farmers to manage soil, rotations, herbicides and fertilizers not only from field to field, but also within each field.

"Farmers need to begin to segment fields when spraying for weeds just like we do for planting and fertilizing," Owen added.

The challenge that many have now is with how to kill the problem weeds with so much resistance to the different modes already available.

The Iowa Soybean Association and Iowa State University conducted research on waterhemp's resistance to herbicide. The results showed all the waterhemp in Iowa is resistant to one or more of the most common herbicides--ALS, glyphosate or atrazine. Resistance varies from field to field and therefore should be managed that way as well.

"If you have Palmer amaranth, if you have waterhemp in the Midwest, and it is not resistant, make every effort possible to prevent those weeds from going to seed," said Norsworthy. "And manage specifically for those weeds because they are definitely high-risk weeds."

One Palmer amaranth or waterhemp plant can produce one million seeds. "All of the sudden you go from one plant to a three- or four-acre infestation because this weed is spreading seed everywhere. Definitely, at the end of the day, it is going to cost you much more to manage on 1,000 acres than it would be to catch it and manage it early on one acre," he said.

Norsworthy and Owen both said it is important to use a diversity of tactics to control these problem weeds in order to stay ahead of weed resistance and raise profitable crops.

Jennifer Carrico can be reached by phone at 515-833-2120, or by email at

Date: 10/8/2012


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