Researchers explore herbicide resistance in wheat

Kansas wheat producers endured months, if not years, of persistent drought conditions this growing season. But when the rain finally started to fall, it brought another of the biggest challenges to wheat production - weeds. New research from Kansas State University is seeking to help find new solutions to add to farmers’ toolboxes as they battle the consistent and growing threat of post-emergent weeds.

Weeds are a consistent problem with several different types and species that compete with wheat during the cropping system. When uncontrolled, research shows yields can take a 10 to 50 percent hit depending on the density of weeds and how long they are present in the field. In addition to out-competing wheat as it grows, weeds at harvest time can mean significant delays if spraying is required before cutting, challenges in battling weeds taller than the wheat and difficulties at the elevator with dockage and having to dry down green weeds in the bin.

Wheat producers have access to two production systems specific to grass and broadleaf control - Clearfield wheat varieties that pair with Altitude FX 3 herbicide and CoAXium wheat varieties that pair with Aggressor AX herbicide. These two wheat production systems are powerful, but Dr. Mithila Jugulam, who leads the weed physiology laboratory in the K-State Department of Agronomy, noted the importance of researching the next steps in weed control in wheat to continue to provide additional options for wheat producers.

″The availability of resistance to ALS- and ACCase-inhibitor herbicides in wheat is a significant step forward with weed control options targeting both grass and broad-leaved weeds,″ she said. ″Nevertheless, these two technologies alone will never be enough to solve all the post-emergence weed control problems in wheat. Thus, the development of new techniques and tools should be a continuous exercise.″

To find the next addition to the weed control armory, Jugulam teamed up with Dr. Asif Mohmmad, chief scientist for Heartland Plant Innovations, and Dr. Harold Trick, a K-State plant molecular biologist with more than 35 years of experience in plant genetic transformation. The trio and their research teams sought to assess wheat germplasm for tolerance to additional herbicides that can be used for post-emergence weed control in wheat. Their initial focus was on tolerance to the selective herbicides Laudis (tembotrione) and Callisto (mesotrione), both of which have been shown to effectively and efficiently control broadleaf and grass weed species but neither of which are currently registered for use in wheat, either pre- or post-emergence.

Supported by funding from the Kansas Wheat Commission, Kansas Wheat Alliance and Kansas Crop Improvement Association, the three-year research project assessed existing varieties, experimental lines, wild relatives and mutant lines for herbicide tolerance to these two active ingredients. A large collection of germplasm representing winter and spring wheat as well as mutant lines were tested for their response to Laudis and Callisto under greenhouse conditions. Based on the herbicide symptoms and plant biomass accumulation, the least and most sensitive wheat genotypes were identified.

This initial research phase ranked these varieties and identified winter wheat genotypes developed by the Kansas Wheat Alliance with WW-1 and WW-2 being least sensitive and surviving more than six times the field-recommended rates of these herbicides compared to the most sensitive genotype, WW-24. In contrast, the spring wheat was found highly sensitive to these herbicides. The mechanism of differential sensitivity to these herbicides was also tested.

The results indicate that similar to corn (which is naturally tolerant to these herbicides), the WW-1 and WW-2 wheat genotypes can break down these herbicides into non-toxic form by activity of certain herbicide-degrading enzymes. The end goal of this extensive testing regime is to identify exactly which genes help provide tolerance to these herbicides.

Additionally, the team is also exploring CRISPR-based editing systems to pave the way to establish new herbicide-resistant varieties of wheat - the long-term aim of Jugulam’s research.

These potential new lines of herbicide-resistant wheat still have a long time before they would show up in a seed catalog for producers to purchase and plant, but getting the groundwork right will result in even more powerful and targeted tools for producers.

″The availability of diverse herbicide options for weed control is highly warranted, especially after the evolution of herbicide resistance in many major weeds in today’s agriculture,″ Jugulam said. ″Availability of wheat cultivars’ tolerance to these herbicides will help post-emergence control of broad spectrum of weeds throughout the cropping season, thereby reducing the crop loss due to weed competition and increasing the productivity.″