Liz Bosak, Outreach Specialist, Department of Agronomy, University of Wisconsin-Madison
Pre-emergence herbicides can lower the risk of developing herbicide resistance and add flexibility to the post-emergence application timing. For an excellent discussion of how a two-pass herbicide program can reduce resistance risk, please read Boerboom et al.’s “An Equation for Trouble? Glyphosate x Weeds = Resistance” article, http://fyi.uwex.edu/grain/files/2015/03/Boerboom_An_Equation_for_Trouble.pdf . To see the utility of a residual herbicide applied at planting in less than 90 seconds, watch Purdue University’s time lapse video of Palmer amaranth growth in two plots, with and without a pre-emergence herbicide application, https://youtu.be/VDvh0hhHNEE . When choosing a pre-emergence herbicide, there are a few factors to consider in the decision making process:
1. Select a PRE herbicide with a different site-of-action than the POST herbicide that you will be using. Download the TakeAction Herbicide Classification chart, http://wcws.cals.wisc.edu/wp-content/uploads/sites/4/2013/03/FactSheet_TakeAction_HerbicideClass_2013.pdf , or use their lookup tool on your android phone, iphone, or ipad http://takeactiononweeds.com/understanding-herbicides/site-of-action-lookup/ . For instructions, see this webpage http://takeactiononweeds.com/wp-content/uploads/52618-6_HRM-Bookmark_HR1-2.pdf .
2. Match the PRE herbicide effectiveness with the weed species on your farm. Efficacy information is available in summary tables located in A3646 Pest Management in Wisconsin Field Crops, http://learningstore.uwex.edu/Pest-Management-in-Wisconsin-Field-Crops2015-P155.aspx (scroll down to download the pdf, soybean pp. 143-144, corn pp. 23-37 ) or in yearly research reports published online, http://wcws.cals.wisc.edu/research/herbicide-evaluation-program/ .
Remember to apply the full labeled rate at the appropriate time. Once the application has been made, scout the field after crop emergence to determine the effectiveness of the pre-emergence application and to plan for the post-emergence application. For more information on herbicide resistance management, please consult the following fact sheet http://wcws.cals.wisc.edu/wp-content/uploads/sites/4/2013/03/WCWS_205_herbicide_resistance_management_WEB.pdf .