UACES Facebook Controlling Palmer amaranth with metribuzin tolerant soybean varieties
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Controlling Palmer amaranth with metribuzin tolerant soybean varieties

By Sarah Cato
U of A System Division of Agriculture
May 10, 2019 

Fast facts:

  • Palmer amaranth top pest in Arkansas soybeans
  • Weed control programs with metribuzin shown to be most effective
  • Division of Ag researchers screening for metribuzin-tolerant soybean varieties 

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FAYETTEVILLE, Ark. – When it comes to weed control in Arkansas, farmers need every tool available in order to maintain profit. 

8-10-2016 Norsworthy-Barber

Palmer amaranth, commonly known as pigweed, has been a longtime foe of Arkansas soybean farmers. Jason Norsworthy, weed scientist for the University of Arkansas System Division of Agriculture, said it is the No. 1 weed affecting Arkansas soybean production. 

However, Palmer amaranth has become resistant to many different herbicides. In Arkansas alone, it has become resistant to six sites of action. Because of this, Norsworthy has been field testing weed control programs that involve mixtures of two to three herbicide actions, and one product stands out above the rest: metribuzin. 

“We’ve tested a wide assortment of herbicides from different groups or modes of action over the last several years. Those that contain metribuzin tend to perform best on Palmer amaranth in these field trials,” Norsworthy said. “I’m not saying metribuzin by itself, but metribuzin in combination with some other herbicides is needed on most Arkansas soybean acres.” 

Metribuzin uses the same mode of action as the chemical atrazine, which is used in corn. Atrazine is typically mixed with an HPPD herbicide, which inhibits the 4-hydroxyphenylpyruvate dioxygenase enzyme in the weed. Previously, HPPD herbicides could not be used in soybeans because of their sensitivity to the chemistry. However, that may be changing.

 “There’s some new technology coming forward in soybean called the LibertyLink GT27 that is now available to growers,” Norsworthy said. “It allows for use of glyphosate and glufosinate over the top of the crop and isoxaflutole as a pre-emergence herbicide as soon as the EPA approves use of the latter herbicide.” 

Glyphosate is most familiar as Roundup; glufosinate as Liberty and Interline; isoxaflutole is marketed as Alite27.  

“We see the benefits of taking an HPPD herbicide like Alite27 and mixing it with metribuzin, and we see extended residual weed control,” he said. 

“Farmers need to remember that soybean varieties are sensitive to metribuzin,” he said. 

Norsworthy and his team are screening for metribuzin tolerance in Arkansas Official Variety Trial for soybeans as well as a large portion of the germplasm that’s available today within the industry. 

“There’s been a major focus on trying to breed for greater tolerance to metribuzin over the past eight to 10 years,” Norsworthy said. “I think the industry as a whole understands the need for using metribuzin as a viable component of our weed control programs.” 

“When we combined metribuzin with other herbicides, those were the most robust residual programs that we have,” he said. “Hence the need to continue screening soybean germplasm, making sure the Arkansas grower is making an informed decision when they choose a variety.” 

To read more about Norsworthy’s research, visit For more information on weed control in Arkansas, visit  


About the Division of Agriculture

The University of Arkansas System Division of Agriculture’s mission is to strengthen agriculture, communities, and families by connecting trusted research to the adoption of best practices. Through the Agricultural Experiment Station and the Cooperative Extension Service, the Division of Agriculture conducts research and extension work within the nation’s historic land grant education system. 

The Division of Agriculture is one of 20 entities within the University of Arkansas System. It has offices in all 75 counties in Arkansas and faculty on five system campuses.  

The University of Arkansas System Division of Agriculture offers all its Extension and Research programs to all eligible persons without regard to race, color, sex, gender identity, sexual orientation, national origin, religion, age, disability, marital or veteran status, genetic information, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer. 

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Media Contact: Mary Hightower
Dir. of Communication Services
U of A System Division of Agriculture
Cooperative Extension Service
(501) 671-2126