Herbicide Resistance Risk Calculator Developed by University Scientists/Specialists
Modern crop production is increasingly being plagued by weed species evolving to biotypes that are herbicide-resistant (HR). Weed species such as Palmer amaranth that have developed resistance to multiple herbicide modes-of-action have created a major production problem throughout the soybean-producing portion of the U.S. This is exacerbated by the lack of new herbicide chemistries becoming available. Thus, producers must rely on current herbicides and crop and herbicide rotation techniques that can be used to control weeds that are already HR, and to minimize their further development.
Weed Scientists/Specialists from the Universities of Nebraska, Idaho, and Wyoming have developed The Herbicide Resistance Risk Calculator (UNL CropWatch), an interactive web-based tool that can be used to estimate how a particular cropping system and the herbicides used in it will affect the potential for HR weed development in that system. Click here to access the tool. The following are pertinent points from the article and about the tool.
• The tool was developed by Dr. Nevin Lawrence, Integrated Weed Management Specialist, Univ. of Nebraska; Dr. Albert Adjesiwor, Extension Specialist, Univ. of Idaho; and Dr. Andrew Kniss, Professor and Dept. Head, Univ. of Wyoming.
• The tool was developed on the premise that one of the most successful strategies for managing HR weeds is applying herbicides that target different sites of action (SOA) or herbicide mixtures that target multiple SOA’s.
• The tool allows a producer to choose a problem weed, enter a four-year crop rotation (can be the same crop each year), select herbicides for each crop/year in the rotation, and indicate herbicide resistance [Group 2 (ALS), Group 4 (synthetic auxins), Group 5 (PSII), Group 9 (glyphosate), Group 14 (PPO), and Group 27 (HPPD)], if any, that already exists in the selected weed species.
• The program assesses how well the chosen herbicide(s) control the problem weed based on 1) how many times a herbicide is used in the 4 years, and 2) whether or not effective mixtures are used.
• Once herbicides are chosen for each year of the rotation and a weed species is selected from the list of those in the calculator, a herbicide risk score for each herbicide SOA that was chosen will be calculated.
• Risk scores are estimated on a 0 to 4 scale, with lower risk scores indicating less likelihood that the weed population will develop resistance. The goal when selecting herbicides to use during the 4 years is to keep the 4-year risk score below 1.0. Using efficacious herbicides with different SOA’s across the 4 years will result in lower scores.
• Risk scores will appear for each SOA after a weed and herbicide(s) are selected for all 4 years. Risk scores less than 1 indicate a SOA was never used without a second effective SOA, and therefore the risk of resistance is relatively low.
• Herbicide choices for each year of the 4-year rotation are left to the producer. Thus, the tool should be considered a supplement to current weed management resources such as herbicide labels and weed control guidelines published annually by individual states. Labels for herbicides can be accessed here, and weed control guidelines for Midsouth states can be accessed here.
• The tool does not account for herbicide rotation restrictions that should be considered if diverse crops are included in the rotation scheme. The goal is to add this, along with other crops and weeds, to future versions of the calculator.
• The tool assumes that all selected herbicides, tank-mixtures, and sequential applications will be applied at full labeled rates.
• Even though this tool was developed for Midwestern U.S. cropping systems, it does include soybean and corn crops and Palmer amaranth and cocklebur weeds which are common in the Midsouth.
• At the present time, risk scores are not meant to be quantitative. Rather, the current model simply estimates the level of risk of HR weed development based on the use of herbicide SOA’s over the 4 years.
In the above UNL CropWatch publication, example calculations for four rotations–three with monocropped corn using different herbicide programs, and one with corn rotated with dry beans and sugar beets–are shown (each example in the publication can be enlarged by clicking on the individual screen shots). The examples show how the inputs to the tool are recorded, and the results from those inputs.
This tool is just that–a tool. It should be used along with other resources that are available to provide guidance in selecting a weed management program for the varied crops and cropping systems that are adopted by producers. As this tool is further developed and refined to cover more crops and weeds, it can provide valuable insight into how the selection of currently available herbicides and their mixtures can be used to thwart the development of HR weeds.
Composed by Larry G. Heatherly, Dec. 2020, larryheatherly@bellsouth.net