Postemergence herbicide options for weed control in soybeans are limited, particularly where glyphosate-resistant weeds are a problem.
Six weed species in Nebraska have now been confirmed resistant to glyphosate, and their populations are widespread. Summer annual weed species like common waterhemp, Palmer amaranth, velvetleaf and foxtail emerge at the same time as soybeans and can be troublesome in some parts of the state.
In addition, many populations of these weed species in Nebraska are glyphosate-resistant. Weed management programs using other herbicides in addition to glyphosate are needed to manage these populations.
Using an early-season control is suggested. Preemergence, residual herbicides mitigate yield loss due to weed competition, provide a longer time for soybeans to establish and reduce the selection pressure for herbicide resistance to postemergence herbicides, with fewer weeds being exposed these herbicides.
In addition, including preemergence herbicides can reduce postemergence herbicide applications and protect against early-season weed competition when weather or busy schedules prohibit a timely postemergence application.
Applying a preemergence herbicide with multiple sites of action is particularly important in fields where herbicide-resistant weeds are present or suspected. Several new herbicides registered in soybeans are a premix of herbicides with different sites of action.
An effective preemergence application can go a long way toward controlling a problem weed population, especially when postemergence herbicide options are limited or not as effective in soybeans.
University of Nebraska-Lincoln research has shown that adding a residual herbicide helps keep fields free of yield-robbing weeds longer and improves yields and resistance management practices. A two-year university study demonstrated season-long control of glyphosate-resistant common waterhemp when a preemergence herbicide with multiple sites of action was applied and followed by a postemergence herbicide (see photo above).
Yet with several products on the market today, it is important for farmers to know what to look for in a residual herbicide. Selection should be based on one or more of the following:
• weed control spectrum
• length of residual activity
• herbicide site of action
• application timing flexibility
• ability to control herbicide-resistant weeds
• crop rotational flexibility
Complete weed control sought
Roundup Ready 2 Xtend soybeans, also known as Xtend soybeans, first became commercially available for the 2017 growing season. If you are planting Xtend soybeans this year, you still need to consider applying a preemergence herbicide. Do not rely only on dicamba (XtendiMax, FeXapan or Engenia) for weed control in Xtend soybeans and restrict application to a single application, possibly preplant or early postemergence. Relying only on a postemergence herbicide may result in only partial weed control.
Remember that preemergence, residual herbicides labeled in Roundup Ready soybeans also can be applied in Xtend soybeans. Relying only on a postemergence herbicide may result in reduced weed control and more selection pressure on weeds. In short, application of preemergence, residual herbicide within two to three days after planting any type of soybeans is the foundation of effective weed control.
Jhala is a Nebraska Extension weed management specialist. This report comes from UNL CropWatch.