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Latest Research on Herbicide Resistance

by Wyoming Livestock Roundup

By Jeremiah Vardiman

Is crop management hindering or benefitting herbicide resistance? Is changing herbicide chemistry the only weapon available to combat herbicide resistance? 

The latest research from the University of Wyoming’s Plant Sciences Department provides valuable insights into cultural and mechanical weed control effectiveness to combat herbicide resistant weeds, particularly acetolactate synthase (ALS) resistant kochia. 

The large study was conducted over four years under field conditions in four locations – Lingle, Powell, Scottsbluff, Neb. and Huntley, Mont. The research focused on kochia management since kochia is a problematic weed to the major crops grown in Wyoming, such as sugarbeet, corn and dry beans. 

Herbicide resistance is the inherited ability of a weed to survive herbicide applications to which the original weed population was susceptible. ALS herbicides are among the most widely used herbicides in the world and control weeds by inhibiting the enzyme acetolactate synthase.

Kochia has also developed herbicide resistance to four different groups of herbicides, including ALS herbicide, with some populations exhibiting resistance to multiple herbicide groups. 

The research indicated crops with dense canopies and tighter row spacing treated with non-ALS-herbicides significantly reduced kochia emergence. Also, more diverse crop rotations treated with ALS-herbicide mixtures did the best at reducing kochia density, seed production per plant and in turn, decreased the weed seedbank affecting the next year’s crop.

 In addition, a four-crop rotation of corn, dry beans, small grains and sugarbeets increased yields and returns on variable and labor costs compared to a monoculture of continuous corn, a two-crop rotation of corn and sugarbeets and a three-crop rotation of corn, dry beans, corn and sugarbeets. 

Overall results of this study found there was little to no influence on the herbicide resistance within the weed population over time, no matter the control methods, which included less and more diverse cultural practices, mechanical control methods and chemical weed management programs.

The field locations were selected based on low initial kochia densities, so native kochia populations would not skew results. Each site was planted with a five percent ALS-resistant kochia seed blend to provide a uniform kochia population within the field and a known percentage of resistant population to monitor over the course of the study. 

Effects of crop canopy were assessed on corn, dry beans, spring wheat and sugarbeets. The crop rotations studied were continuous corn, a corn and sugarbeet rotation, a corn, dry bean, corn and sugarbeet rotation and a corn, dry bean, small grain and sugarbeet rotation. 

Tillage treatments overlaid on the study were heavy tillage that completely inverted the soil and minimum tillage, which only disturbed the soil with harvest practices such as with sugarbeets.

The takeaway from this study is long-term management for herbicide-resistant kochia is possible when implementing a proactive herbicide resistant weed management program. This program needs to focus on more than just herbicide selection and mixtures – it needs to include all the other aspects of the field to be successful. 

The discussion and details of the various components of this study are too large to discuss in this article alone. Watch for part two, which will focus on the effects of crop canopy and herbicide application.

Jeremiah Vardiman is a University of Wyoming agriculture and horticulture Extension Educator. He can be reached at jvardima@ 

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