By Lizabeth Stahl, Extension Educator in Crops
Does glyphosate perform as well today as it did when you first used it? When producers were asked this question at University of Minnesota Private Pesticide Applicator Training sessions across southern Minnesota in 2014, 87% of the respondents said "No". This percentage is up significantly from 2009, when 55% of respondents answered "No" to this question. Increasing issues with resistance to glyphosate is likely, at least in part, behind reported reductions in weed control. To address issues of reduced weed control with glyphosate, diversification is key.
Currently, resistance to glyphosate in Minnesota has been found in populations of waterhemp, giant ragweed, common ragweed, and kochia. Herbicide resistance, however, is not unique to glyphosate. In addition to glyphosate, resistance has been confirmed to date with three additional herbicide sites of action in MN. Herbicide resistance is not a new issue either. The first official confirmation of herbicide resistance in MN was to atrazine in common lambsquarters in 1982.
Further complicating weed control is the reality that resistance traits can stack up. For example, resistance to both ALS herbicides and glyphosate has been found in populations of waterhemp, giant ragweed, and common ragweed in MN. Also, research demonstrates there is often no "fitness cost" to these resistance traits in weeds. This means that once resistance to a particular herbicide develops in a population, the trait remains in the population regardless of future herbicide use, rendering the particular chemistry ineffective into the foreseeable future. These biological factors can significantly limit the number of tools available to combat weed populations into the future.
Starting with a pre-emergence herbicide is a key step in helping reduce the risk of developing herbicide resistant weed populations. Pre-emergence herbicides can also help in the management of weed populations already resistant to a particular herbicide. Growers are seeing the value of pre-emergence herbicides: According to surveys conducted at U of MN Private Pesticide Applicator training sessions across southern MN in 2014, 78% of growers surveyed plan to use a pre-emergence herbicide in soybean in 2014, and 80% plan to in corn. This is up dramatically from 2009, when 60% and 50% of survey respondents indicated they used a "Postemergence only" program in soybeans and corn, respectively.
Which pre-emergence herbicide is the best fit for you? The "Corn and Soybean Herbicide Diversification Tools" publication developed by U of MN and North Dakota State University, available at http://z.umn.edu/herboptions, can help determine which pre-emergence herbicide is the best match for your needs. Weed control ratings are listed for key weed species including waterhemp, giant ragweed, common ragweed, lambsquarters, and kochia. Rotation restrictions are listed as well. This resource also compares non-glyphosate, postemergence herbicide options in corn and soybean.
Weed scientists across the Corn Belt also worked with the United Soybean Board to develop a number of useful tools to help growers and ag professionals in developing more diversified weed management programs. These tools can be accessed through the "Take Action" link on the U of MN Extension Crops website at http://www.extension.umn.edu/crops. Of particular note is the "Site of Action Chart", which lists the site of action for herbicides used in corn and soybean.
Want to learn more about how herbicide resistance occurs and the various sites of action for common herbicide chemistries? Check out the U of MN Extension Crops YouTube videos on herbicide resistance, available on the main U of MN Extension Crops website under "Social Media" (http://www.extension.umn.edu/crops).