Controlling marestail or pigweeds postemergence in soybeans is always easier when the weeds are small — less than 2 inches tall is preferable for good control, says Dallas Peterson. Once weeds get taller, they are often considerably more difficult to control.

However, conditions are not always conducive to getting optimal postemergence weed control. The wet weather in many areas this spring may cause weeds in some fields to get larger than you intended, says the Kansas State University weed management specialist. The following are some suggestions for controlling these weeds postemergence in soybeans.

Marestail

Marestail tend to be difficult to control even when the plants are small and in the rosette stage, but become even tougher when plants get more than 6 inches tall. That is why fall and early burndown treatments are critical to the long-term management of marestail. Unfortunately, that doesn’t always happen.

In addition, some marestail have developed glyphosate resistance in many areas. However, some marestail populations are still susceptible to glyphosate, and even resistant plants are not completely immune to glyphosate.

Figure 1. Growth stages of marestail from seedling, rosette, to bolting state. Photos by Dallas Peterson, K-State Research and Extension.

The most effective herbicide treatment for controlling marestail in Roundup Ready soybeans is probably a tankmix of glyphosate plus FirstRate. The combination of the two herbicides seems to work better than either herbicide alone, even on resistant plants.

It is important to use the full labeled rates of glyphosate and recommended adjuvants, including ammonium sulfate, to optimize control and help minimize the risk of developing more resistance. Other tankmixes to consider with glyphosate for controlling marestail would include Classic and Synchrony herbicides.

Unfortunately, some marestail may also be ALS resistant, in which case FirstRate, Classic, and Synchrony would also be fairly ineffective. This just further emphasizes the importance of early spring weed control.

If Xtend soybeans are planted, Xtendimax, FeXapan, or Engenia should be some of the most effective herbicides for postemergence control of marestail in soybeans. Remember that Xtendimax, FeXapan and Engenia can only be applied to Xtend soybeans.

Another option to control marestail in soybean is to plant Liberty Link soybeans and use Liberty herbicide. It is important to remember that Liberty can only be applied postemergence on Liberty Link soybeans.

Waterhemp and Palmer amaranth

Figure 2. Glyphosate-resistant Palmer amaranth escapes in soybeans. Photo by Dallas Peterson, K-State Research and Extension.

If preemergence herbicides weren’t applied or didn’t get activated in a timely manner, early-emerging waterhemp or Palmer amaranth may not have been controlled and can grow rapidly.

Flexstar, Cobra, Marvel, and Ultra Blazer can be fairly effective for controlling small pigweed, but are less effective as the pigweed gets larger, especially Palmer amaranth.  

Some waterhemp and Palmer amaranth also may have developed resistance to this class of herbicides, but size still seems to be a factor, even on resistant populations.  These herbicides also provide some residual weed control, so tankmixes of these herbicides with glyphosate should be applied within 3 weeks after planting to optimize performance.

Producers may try to cut the rates of these herbicides to reduce soybean injury. However, lower rates of these burner herbicides still cause similar soybean burn symptoms and weed control is often reduced.

Pursuit and Harmony were once fairly effective for pigweed control and can still provide good control of susceptible populations, but many fields now have ALS-resistant waterhemp and Palmer amaranth.

If Xtend soybeans were planted, the new dicamba products Xtendimax, Engenia, and FeXapan again are an option to help control broadleaf weeds, including the pigweeds. However, just as with other postemergence pigweed treatments, the pigweeds need to be less than 3 to 4 inches tall to achieve optimal control.

Likewise, Liberty herbicide can be used in Liberty Link soybeans to help control small pigweeds. Liberty is also most effective on smaller weeds and generally requires higher spray volumes to achieve good coverage and weed control. A sequential application of Liberty 7 to 10 days after the first application may be required for good control, especially on larger plants.

Residual herbicides such as Zidua, Anthem Maxx, Outlook, Dual Magnum, and Warrant can also be added to any of the previously mentioned postemergence herbicides to provide some extended residual control of pigweeds. This may be especially helpful if a good rate of residual herbicide was not used earlier or with heavy pigweed pressure.