By Lizabeth Stahl, Extension Educator – Crops
Seventy-five percent of growers surveyed at the 2011 University of Minnesota Extension Private Pesticide Applicator Training sessions in southwestern Minnesota responded with a "no" when asked if glyphosate performed as well today as it did the first years they used it.
Several factors are likely contributing to reported reductions in glyphosate performance. One potential way to address glyphosate control issues, however, is to also use a residual herbicide that controls the main weeds of concern.
Using a residual herbicide in a glyphosate system can reap many benefits. For example, preemergence residual herbicides can help protect crops from potentially yield robbing, early-season weed competition. This is particularly important where weed densities are high, or when conditions delay postemergence applications.
Application of a pre residual herbicide can also lengthen the amount of time before a post application should be made. Having a larger window of application can be particularly important in a year when conditions, such as a wet year, delay post applications. If a residual pre herbicide has been applied, weeds should be more uniform in size and less likely to be greater in size or growth stage than is listed on the label compared to a total post program.
Pre herbicides can also boost control of weeds that have been tougher to control with glyphosate alone. Recently, the U-M and North Dakota State University published "Pre and Post Diversification Options for Glyphosate-Resistant Corn and Soybean," available at http://tinyurl.com/y45yopj. This publication lists control ratings for common pre and post herbicides on weeds including waterhemp, lambsquarters, common ragweed, giant ragweed and kochia.
Chemical diversification can also help delay or prevent herbicide-resistant populations from developing in your field. To date, 21 species worldwide have been confirmed to have populations with resistance to glyphosate. In the U.S., confirmed cases have been found in 12 species. In Minnesota, confirmed cases have been found in 3 species: waterhemp, giant ragweed and common ragweed. It is important to note that chemical diversification is a good resistance management strategy in any herbicide program.
Research demonstrates the benefits of using a pre herbicide in a glyphosate program. For example, Mike Owen, weed scientist at Iowa State University, recently reported yields and profits were greater when soil-applied herbicides were used compared to post treatments in a five-year field-scale on-farm trial.
Take control of your weed control options. Including a residual pre herbicide in your weed management program can enhance weed control, help prevent yield losses, enhance profitability and ultimately help ensure the longevity of glyphosate herbicide programs.