Careful Weed Control Choices are Critical to Avoid Additional Resistance Issues

Dr. Chisty L. Sprague, Michigan State University Professor and Weed Extension Specialist

Glyphosate- and multiple-herbicide resistant weeds have made the days of uncomplicated soybean weed control a distant memory. No longer can soybean farmers expect perfect weed control with one or two applications of glyphosate in Roundup Ready soybeans. Successful weed management strategies now need to include at least two herbicide applications with multiple sites of action. This can be more complicated in non-GMO and Roundup Ready soybeans due to limited effective herbicide options to manage herbicideresistant weeds. Planting soybean varieties with newer herbicide resistant traits (i.e., Enlist E3 and XtendFlex) provides farmers with additional herbicide options. However, without proper stewardship, additional herbicide-resistance issues can evolve, and we could lose the usefulness of these products. In fact, some states have reported Palmer amaranth and waterhemp populations that are resistant to 2,4-D and/or dicamba. Often these resistance issues appear in weed populations that are already resistant to multiple other herbicides. More recently, two Palmer amaranth populations have been reported resistant to Liberty (glufosinate) in Arkansas and North Carolina. This is extremely troubling since 2,4-D, dicamba, and Liberty are the three herbicides that have helped us manage many of the glyphosate- and multiple-resistant weeds that we have been battling in Michigan. Below are several tips that can be used to slow the development of additional herbicide resistances and help preserve the ever-dwindling herbicide options in soybean.

Multiple Herbicide Sites of Action

Repeated use of the same herbicide or herbicides with the same site of action can increase the selection pressure for herbicide-resistant weeds. One way to break this cycle is to create weed management programs that use herbicides with multiple different sites of action. The key to adequate control is to make sure we are using at least two herbicide sites of action that are effective on the particular target weed. The more herbicide sites of action used in a particular weed management program increases, so do the chances for effective weed control. For example, many waterhemp populations in Michigan are resistant to glyphosate (Group 9) and the ALS-inhibiting (Group 2) herbicides. To effectively manage these populations, a soil-applied herbicide containing an effective Group 14 and/or Group 15 herbicide followed by a postemergence application of glufosinate (Group 10) and/or Enlist One (2,4-D) (Group 4) in an Enlist soybean system would provide up to four different effective herbicide sites of action. For more information about herbicide sites of action, consult the 2023 MSU Weed Control Guide for Field Crops ( https://www.canr.msu.edu/ weeds/2023-weed-control-guide ), or the color-coded Take Action Herbicide Classification Chart found at www.IWillTakeAction.com (pictured at right).

Overlapping Residual Herbicides

In addition to herbicide-resistance issues, many of the weed species that are more difficult to control emerge over several months of the growing season. For example, waterhemp and Palmer amaranth can emerge well into July, past the time when postemergence herbicide applications are an option. One way to improve the management of these species, and some other small-seeded weeds, is to include a residual herbicide in with the postemergence herbicide application.

In soybeans, the Group 15 herbicides of Dual II Magnum, Warrant, Outlook or Zidua are the most common options. Including one of these Group 15 herbicides with an effective postemergence herbicide provides control of weeds that have not emerged yet. The overlapping residual herbicide concept can be extremely helpful for weed control in earlier-planted soybeans.

Labeled Herbicide Rates and Weed Sizes

To help preserve our current herbicide options, it is extremely important to use the maximum labeled herbicide rate for the soil type for soil-applied (PRE) herbicides or the weed heights for postemergence herbicides. Cutting or reducing herbicide rates increases the chances for the development of herbicide-resistant weeds, particularly due to metabolic resistance. Weeds that survive these lower herbicide rates produce offspring that can have higher levels of resistance. In many cases, metabolic resistance confers resistance to several herbicide sites of action. For example, metabolic resistance was found in a six-way resistant Kansas Palmer amaranth population. This population survived recommended field rates of ALS-inhibitors, 2,4-D, atrazine, glyphosate, PPO-inhibitors, and HPPDinhibitors (Groups 2, 4, 5, 9, 14, and 27).

Cultural Practices

Implementing cultural practices can help reduce the selection pressure of different herbicides on certain weeds. These cultural practices include crop rotation, tillage, narrowing soybean row widths and integrating cover crops into weed management systems. Including winter wheat or forages into a crop rotation changes opportunities for emergence of different weed species. For example, summer annual weeds generally are not as much of a problem in winter wheat or forages, especially if the wheat or forage stands are competitive. In fields where small-seeded weeds have been an issue, tillage has been shown to reduce emergence of some of these species. For example, horseweed (marestail) seed needs to be on the soil surface or less than 0.25-inch deep to germinate and tillage operations that bury this seed will reduce these populations. Planting soybeans in narrow rows can also reduce