Research Highlights

Monitoring Herbicide-Resistant Weeds to Improve Soybean Management

By Barb Baylor Anderson

While the status of herbicide-resistant weed infestations may have changed some since the 2018 growing season when Bryan Young, Purdue University weed scientist, completed research as principal investigator for this soybean checkoff-funded study, he says the overall strategy for best management practices for herbicide-resistant weeds remains the same.

Young, with funding from the Indiana Soybean Alliance, was charged with looking for ways to improve weed management by characterizing new and expanding types of herbicide-resistant weeds across the Indiana landscape. He also focused on managing tough weeds in the Roundup Ready Xtend soybean system to define best practices for Indiana soybean producers.

“Investing checkoff dollars in crop production research is one of the best things farmers can do for themselves,” says C.J. Chalfant, Indiana Soybean Alliance board member who farms near Hartford City. “In the past 20 years, soybean yields in Indiana and across the Midwest have seriously improved. Most of the improvement has come through better practices and technologies that have come through crop production research. I think we’ve only scratched the surface of what is possible. I believe the more research we do, the more bushels we will find.”

“Overall, our research did provide some opportunities to improve weed management, but it also highlighted the fact that we are not staying ahead of weeds in terms of management and resistance evolution,” says Young. “We desperately need to diversify our management tactics to include non-chemical or cultural practices along with diverse, effective herbicide options that still remain in non-GMO and herbicide-resistant soybean systems.”

Young says the most significant obstacle in realizing effective, cost-efficient weed management is clearly the prominence of weed resistance to herbicides. Waterhemp, Palmer amaranth, giant ragweed and horseweed continue to evolve in Indiana – and in other states – with new resistance mechanisms and multiple resistance to several herbicide mode of action groups. 

“Commercialization of soybeans with dicamba resistance, or Roundup Ready Xtend, made a dramatic impact on soybean production,” he reports. “Unfortunately, the beneficial impact of improved management of problematic weeds was marred by extensive off-target drift of dicamba to sensitive plants. We learned quickly that we needed greater attention on reducing the risk for off-target dicamba movement.”

During the 2018 season, Young says new target site mutations that allowed for resistance to PPO-inhibiting herbicides in both waterhemp and Palmer amaranth were identified. He also found metabolism-based waterhemp resistance to ALS-inhibiting herbicides as seen in Illinois.

“Most of the waterhemp samples were confirmed as resistant to PPO-inhibiting herbicides, with additional resistance to glyphosate and ALS-inhibiting herbicides,” says Young. “No giant ragweed samples had been confirmed as PPO-resistant, so our conclusion for the herbicide failure was these populations were too tall at time of herbicide application. There was an increase in frequency of giant ragweed populations being confirmed with resistance to glyphosate.”

Young says the progressive increase in multiple resistance in waterhemp shifted more soybean production into the Liberty Link and Roundup Ready Xtend systems and also now Enlist. 

But that also creates new concerns.

“In addition to all of the previous herbicide resistance reported for Palmer amaranth and waterhemp, there have been recent confirmations of Palmer amaranth populations resistant to dicamba and glufosinate and waterhemp populations resistant to dicamba,” says Young. “Growers should not go into 2021 with a primary focus on those two herbicides to control the most problematic weed species. Strong soil residual herbicides and appropriate non-chemical weed control practices should be mandatory to reduce the control and economic risk of herbicide-resistant weeds interfering with short- and long-term soybean farmer profitability.”

Published: May 10, 2021