Is your crop management hindering or benefiting 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 and Powell, Wyo.; Scottsbluff, Neb.; and Huntley, Mont. The research focused on kochia management, since it is a problematic weed in the major crops grown in Wyoming, such as sugarbeets, 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 — ALS herbicides being one — 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 that would affect the next year’s crop.
In addition, a four-crop rotation (corn/dry bean/small grain/sugarbeet) increased yields and returns on variable and labor costs compared to a monoculture (continuous corn), two (corn/sugarbeet), and three-crop (corn/dry bean/corn/sugarbeet) rotations.
The last 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. Then each site was planted with a 5 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 bean, spring wheat and sugarbeets. The crop rotations studied were continuous corn; corn/sugar beet rotation; corn/dry bean/corn/sugarbeet rotation; and corn/dry bean/small grain/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 is that 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.