Herbicide-Resistant Trait Patented in Conventional Grain Sorghum
Newly patented trait for grain sorghum shows resistance to certain herbicides
By John Lovett – Jan. 23, 2025
FAYETTEVILLE, Ark. — Grain sorghum is widely grown in arid environments as an animal feed grain, but weedy grass control in the crop is a big challenge due to limited post-emergent herbicide options.
A joint effort between the Arkansas Agricultural Experiment Station and Texas A&M AgriLife Research has resulted in development of a trait that was patented and bred into conventional grain sorghum with resistance to herbicides targeting johnsongrass and other troublesome grass weed species. The experiment station is the University of Arkansas System Division of Agriculture’s research arm.
The breeding work began in 2019 with the discovery of johnsongrass on an Arkansas roadside that showed resistance to acetyl-CoA carboxylase-inhibiting herbicide, also known as ACC-ase. ACC-ase affects plant growth by affecting fatty acid synthesis, a key step in membrane formation. The ACC-ase gene in the johnsongrass was bred into a grain sorghum, currently known as ArkTam™.
Jason Norsworthy, Distinguished Professor of weed science with the Arkansas Agricultural Experiment Station, said a grain sorghum that is resistant to ACC-ase herbicides is beneficial because there are fewer ACC-ase chemistries that work on grass weed species than the other major herbicide for the job — Acetolactate synthase, or ALS, herbicides — which target the acetolactate synthase gene in plants to inhibit growth by affecting synthesis of certain amino acids.
Norsworthy, who also holds the Elms Farming Chair of Weed Science, shares inventorship on the patent for the trait used in ArkTam™ with Bill Rooney, Muthukumar Bagavathiannan and George Hodnett at Texas A&M AgriLife Research.
Rooney is the Texas A&M AgriLife Research sorghum breeder, professor and Borlaug-Monsanto Chair for Plant Breeding and International Crop Improvement.
Bagavathiannan is a Texas A&M AgriLife Research weed science and cropping systems agronomist and the Billie Turner Professor of Agronomy.
Hodnett is a senior research associate in the department of soil and crop sciences for Texas A&M AgriLife Research.
“All the work so far looks very promising,” said Parker Cole, associate director of Technology Commercialization for the University of Arkansas System Division of Agriculture. “We are excited about the opportunity to help grain sorghum farmers, including those in Arkansas, by working with Texas A&M in developing this new trait.”
Why ACC-ase?
Only a limited number of herbicides are available to provide post-emergence control of selective grass weeds in grain sorghum crops. And the herbicides currently labeled for use with grain sorghum have strict use restrictions, low efficacy on johnsongrass, or weed resistance issues, Norsworthy said.
“From an herbicide-resistance standpoint, there are more weeds resistant to ALS herbicides worldwide than any other herbicide that we have out there,” Norsworthy added. “And weeds rapidly evolve resistance to that type of herbicide.”
Reasons for that wider resistance to ALS chemistry is due in part to them being used on both grasses and broadleaf weeds like Palmer pigweed, whereas the ACC-ase chemistry is strictly used on grasses, Norsworthy said.
Experiment results
An experiment was conducted at the Milo J. Shult Agricultural Research and Extension Center in Fayetteville to determine the effectiveness of two ALS-inhibiting herbicides and nine ACC-ase herbicides on ArkTam™ sorghum and problematic grass weed species.
The sorghum and weeds — johnsongrass, broadleaf signalgrass, barnyardgrass, and Texas panicum — were sprayed when ArkTam™ sorghum reached the two- to three-leaf growth stage.
The sorghum was tolerant of most ACC-ase herbicides tested, exhibiting less than 10 percent injury at all evaluation timings. The exceptions were clethodim and sethoxydim. Additionally, all ACC-ase inhibitors tested, except diclofop and pinoxaden, controlled all grass weed species tested by more than 91 percent at 28 days after application.
The ALS inhibitors that were tested — imazamox and nicosulfuron — provided more than 81 percent control of broadleaf signalgrass 28 days after application and controlled all other grass weed species tested by more than 95 percent.
Volunteer outcrossing control
ArkTam™ sorghum had no resistance to the ALS-inhibiting herbicides that were evaluated.
“That’s actually a good thing,” Norsworthy said. “Because if you have volunteers, or for some reason you were to have outcrossing, you could come back in a non-sorghum crop and spray the ALS-inhibitor in a broadleaf crop as a means of controlling the sorghum.”
In plant biology, outcrossing refers to the process of a plant fertilizing another plant of the same species. Instead of using its own pollen to produce seeds, which is self-pollination, the plant receives pollen from a different plant by either the wind, insects, or other animals.
The study showed that unwanted “volunteer” ArkTam™ plants can be controlled with cledthodim, sethoxydim, nicosulfuron or imazamox.
Norsworthy said the ALS-inhibiting herbicides imazamox and nicosulfuron are effective options for problematic grass weed control on Igrowth™ and Inzen™ grain sorghum crops, respectively.
How it happened
The ACC-ase herbicide-resistant johnsongrass was found as part of a roadside survey for johnsongrass resistance that Bagavathiannan conducted when working as a postdoctoral research associate in Arkansas under Norsworthy.
A material transfer agreement in 2014 allowed Bagavathiannan to take samples of the johnsongrass to his next post at Texas A&M, where the resistance trait was further refined and characterized through multiple cycles of screening and selection.
The ACC-ase resistance was then transferred from johnsongrass to sorghum using a specific sorghum genotype that facilitates the hybridization of the two species, Rooney explained. After several backcrosses to sorghum and selection for herbicide resistance, sorghum lines were derived that had good agronomic properties and herbicide resistance. These were used for further sorghum breeding efforts.
“Sorghum producers have long wanted post-emergence grass control options for sorghum production,” Rooney said. “This trait provides one such option, and there is great interest in its deployment.”
Mention of product names does not imply endorsement by the University of Arkansas System Division of Agriculture.
To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website. Follow us on 𝕏 at @ArkAgResearch, subscribe to the Food, Farms and Forests podcast and sign up for our monthly newsletter, the Arkansas Agricultural Research Report. To learn more about the Division of Agriculture, visit uada.edu. Follow us on 𝕏 at @AgInArk. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit uaex.uada.edu.
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 three 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.
About the Technology Commercialization Office
The Technology Commercialization Office (TCO) commercializes world-class research to support a lasting knowledge-based economy to benefit Arkansas and the world. We help faculty and research scientists identify, protect, and commercialize intellectual property developed from their research or other university-supported activities. To contact the TCO, please email agritco@uada.edu.
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