Figure 1. Graduate student Navjot Singh (on the left) and Extension Weed Scientist Debalin Sarangi (on the right) are investigating a soybean field in southern Minnesota with waterhemp control failure from postemergence herbicide application.
Summary Content
Waterhemp is the most troublesome weed for soybean farmers in Minnesota, capable of reducing yields by up to 70%. Controlling it is difficult because many waterhemp populations have become resistant to multiple herbicides. Farmers often rely on glufosinate to manage resistant waterhemp, but there are growing concerns about its longterm effectiveness—one farmer in Dodge County reported that it stopped working after several years of use. To address this, researchers at the University of Minnesota are studying how resistant waterhemp survives glufosinate and what causes this resistance. Understanding these factors will help develop better strategies to control waterhemp and protect this herbicide option for the soybean farmers.
At the University of Minnesota, researchers are investigating a waterhemp population from Dodge County, MN, suspected to be resistant to glufosinate. In greenhouse experiments, young plants were treated with different doses of glufosinate to determine how much is needed to damage or kill them. To see if this population is also resistant to other herbicides, the plants were tested with several commonly used herbicides. Researchers are additionally studying how these plants survive glufosinate applications. These studies aim to reveal how resistance develops and are expected to be completed by fall 2025.
Findings
We found that the waterhemp suspected to be resistant needs about 3 times more glufosinate than the sensitive populations for effective control. When treated with 32 fl oz/A of Liberty 280 SL, more than half of the plants in this population survived. We also found that more than 80% plants in this population survived when treated with three times the field dose of other commonly used herbicides like atrazine, imazamox, and glyphosate.
These findings alert Minnesota farmers to the growing risk of herbicide-resistant waterhemp and emphasize the importance of proactive, integrated weed management. By understanding that some populations can survive standard herbicide rates—even multiple products—farmers can adjust strategies, such as rotating herbicides, using multiple modes of action, and incorporating cultural practices like cover crops or narrow row spacing, to protect yields and reduce the spread of resistant weeds.
