Why are there so many pigweeds in soybeans this year?

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More fields of soybeans than usual in Kansas have a problem with pigweeds this year – both Palmer amaranth and waterhemp. Populations of these pigweeds are unusually high, and the weeds have gotten tall and formed seed by now. In these fields, the pigweed seedbank in the soil after harvest will be heavy and could create significant problems for years to come.

Figure 1. Glyphosate-resistant Palmer amaranth in soybeans. Photo by Dallas Peterson, K-State Research and Extension.


Why is pigweed pressure heavier this year than in recent years? Increasing pigweed pressure in soybeans has been the trend in Kansas over the past 10 years or so, increasing a bit overall every year. But this year pigweed pressure has taken a big step up.

There are two main reasons for what is happening this year:

1. Glyphosate resistance is spreading. More populations of Palmer amaranth and waterhemp are now resistant to glyphosate. Waterhemp populations have been resistant to glyphosate for several years. Glyphosate resistance in Palmer amaranth has been a more recent occurrence, and resistant populations are now increasing rapidly within the state.

2. Wet weather in May and early June. The rainy pattern in May and early June delayed planting and caused producers in some areas of Kansas to plant later than expected. As a result of both the wet soils and the delayed planting, the effectiveness of EPP (early preplant) herbicides had worn off by the time the beans were planted. Pigweeds began emerging in some cases before the beans could be planted. Then when the soils dried out enough to plant beans, producers had to hurry their operations and may not have had time to apply burndown or preemergence residual herbicides.

Producers who are still trying to rely primarily on postemergence herbicides to control pigweeds are having an increasingly hard time getting good control. It used to be that glyphosate would provide 95% or more control of both waterhemp and Palmer amaranth even if those weeds were a foot tall or more.  But now, glyphosate provides poor control of pigweeds on many fields in Kansas.

There are other options for postemergence control, but most of those options require that the weeds be less than 3 to 4 inches tall for good control. That means producers have to watch their fields closely early in the season and spray the weeds when they first see them emerging. That’s an entirely different mindset than just a few years ago when glyphosate was more consistently effective on pigweeds. Both waterhemp and Palmer amaranth grow very quickly once they have emerged, and can quickly get too tall for good control with postemergence herbicides – if they are glyphosate-resistant. If these weeds get to be a foot tall or more, postemergence herbicide alternatives to glyphosate often just burn back the tops of the weeds but will not kill them.

Consequently, a good residual herbicide program in the spring will likely be important for pigweed management in the future, regardless of the postemergence program. Where glyphosate-resistant pigweeds have become a problem, producers may want to consider Liberty Link or conventional soybeans. However, even these soybean varieties will need to be part of a planned program that utilizes residual herbicides and timely applications. Timely applications and higher spray volumes that can provide good thorough coverage of the weeds is very important for the postemergence herbicide options currently available for pigweed control in soybeans.

There may be new varieties of soybeans coming in the future with resistance to 2,4-D (Enlist) or dicamba (Xtend) if key export markets get approved. However, these options also work best in a program approach using residual herbicides and timely postemergence applications. In our tests at K-State, a tankmix of glyphosate+2,4-D or glyphosate+dicamba still had problems controlling 6-inch-tall Palmer amaranth this summer. So it will still be important to apply these postemergence herbicides on small weeds to get good control.

The best approach to good pigweed control in no-till is to start with a two-pass program early. Apply EPP residual herbicides at a two-thirds rate in mid- to late-April, then follow up with rest of the residual herbicide at planting. If pigweeds are emerged at planting time, it will be important to include a burndown herbicide to control those weeds as well. If you want to rely strictly on a single EPP treatment, be sure to include an adequate rate of a residual herbicide product in the mix.

Then be ready to apply any needed postemergence herbicides early, before weeds get to be 3 to 4 inches tall. On fields with heavy pigweed pressure, you may want to add additional residual herbicides to the postemergence treatment. 


Dallas Peterson, Weed Management Specialist