For years glyphosate has been the herbicide of choice for most growers in both corn and soybean. However, when growers were asked during the recent Private Pesticide Applicator Training sessions across southwestern Minnesota if glyphosate performed as well today as the first years they used it, a resounding 87% said “NO”.
Although reduced control with glyphosate doesn’t automatically mean you have a resistant weed population, it is a red flag. In Minnesota, resistance to glyphosate has been confirmed in three weed species to date including giant ragweed, common ragweed, and waterhemp.
Use of a preemergence (PRE) herbicide this spring is a key step in combatting weed resistance. A PRE herbicide application can help enhance control of problem weeds, especially weeds like waterhemp that emerge over an extended period of time during the growing season. Use of a PRE herbicide can also reduce early-season weed competition and enable more timely postemergence applications. Increased weed control can ultimately lead to higher yields at the end of the season. Another benefit of PRE herbicides is that weed escapes can be reduced, which will help reduce weed seed production and seedbank levels in the following years. Including a PRE herbicide will also add diversity to the chemistries used in a field when glyphosate is applied postemergence. This will help reduce the selection pressure on weed populations for resistance to glyphosate.
The University of MN in conjunction with North Dakota State University, has developed a reference sheet to help growers select PRE and postemergence herbicide options based on the key weed species present in their field. This publication, the “PRE and POST Diversification Options for Glyphosate-Resistant Corn and Soybean” is available online. This publication rates the control of common PRE and POST herbicides on waterhemp, lambsquarters, common ragweed, giant ragweed, and kochia.
When using a PRE herbicide, be sure to check the herbicide label for any restrictions regarding crop rotations or replant options, and if there are any restrictions based on soil type, pH, or organic matter. Keep in mind that although the active ingredients may be the same between two products, concentrations may vary, so always be sure to check the label of the product you are using.
In addition to using a PRE herbicide, other weed resistance strategies include utilizing non-herbicide weed control options like cultivation, and having diversity in your weed management program over the long term. Targeting three to four inch weeds when making postemergence applications is also recommended, as smaller weeds are easier to control than larger weeds. Using full labeled rates is also another recommended weed resistance management strategy.
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.