With the introduction and use of new herbicide-resistant technologies in soybeans, it will be important to utilize an integrated weed management system that includes soil-applied residual herbicides to optimize weed control and sustain the technology.
Broadly speaking, there are many good reasons to use a soil-applied residual herbicide for soybeans, including:
- Get early-season control of weeds and grasses to minimize early-season weed competition.
- Provide more flexibility with postemergence treatment timing.
- Provide additional herbicide sites of action to help manage and slow the development of herbicide resistant weeds.
- Help reduce the weed seed-bank over time.
There are a number of good soil-applied residual herbicide options for soybeans depending on the primary target weeds.
Pigweeds (including waterhemp and Palmer amaranth). Glyphosate-resistant waterhemp and Palmer amaranth are now fairly common in many fields throughout Kansas. Pigweed emergence will generally start in April but the greatest amount of emergence will occur in May and June. Preemergence or burndown-plus-residual herbicide applications will need to be targeted before pigweed has emerged or while it is still at small growth stages.
Figure 1. Palmer amaranth in soybeans. Photo courtesy of K-State Research and Extension.
For early-season pigweed control, the Valor-based herbicides (Valor SX, Valor XLT, Rowel, Encompass, Outflank, Panther, Fierce, Fierce XLT, Gangster, Surveil, Trivence, Afforia, Envive, and Enlite) and Authority-based herbicides (Authority First, Sonic, Authority Assist, Authority MTZ, Authority Maxx, Authority Elite, Blanket, Broadaxe XC, Spartan, and Spartan Elite) can all provide very good to excellent control to supplement a postemergence program. If glyphosate-resistant pigweed is suspected, higher use rates may be required to give adequate residual control.
Prefix, Zidua, Zidua Pro, and Anthem, are other excellent “foundation” herbicides for residual pigweed control in soybeans. Metribuzin, Warrant, Dual, Boundary, Outlook, and Prowl products can also provide some early-season pigweed control, but may not provide as much residual control as those previously mentioned products. Split applications of overlapping residual herbicides -- early preplant and at-planting or early postemergence -- may be the best approach to manage glyphosate-resistant pigweed in no-till systems.
Marestail. Marestail is probably the most widespread glyphosate-resistant weed in Kansas. Marestail control in soybeans should begin in early spring by controlling fall-germinated seedlings and rosettes before they start to bolt. 2,4-D and dicamba can be used in early spring, but the proper preplant intervals need to be followed. The preplant intervals for 2,4-D LV4 are 1 week for up to 1 pt/acre and 30 days for 1 to 2 pt/acre. The preplant interval for Clarity is 14 days following an application rate up to 8 oz/acre and accumulation of 1 inch of rainfall. Dicamba has generally provided better marestail control than 2,4-D. Xtendimax, FeXapan, and Engenia can be utilized ahead of Xtend soybeans without a preplant waiting interval.
Figure 2. Marestail. Photo by Dallas Peterson, K-State Research and Extension.
The Kixor-containing products Sharpen, OpTill, Zidua Pro, and Verdict can be used any time before soybean emergence (cracking), but are most effective if applied before plants get too big. To optimize marestail control with Kixor products, use an adequate spray volume to insure good spray coverage and apply in combination with a methylated seed oil.
Liberty herbicide may be the best option as a rescue treatment to burn down bolted marestail prior to planting. There is no waiting interval required between a Liberty application and planting soybeans, but it will not provide any residual marestail control. Other preplant herbicides that can help with burndown and provide residual marestail control include FirstRate-based herbicides, such as Authority First, Sonic, Gangster, or Surveil in combination with glyphosate.
Velvetleaf. Glyphosate is not always entirely effective on velvetleaf. To assist in velvetleaf control, the Valor-based and FirstRate-based herbicides (Valor SX, Valor XLT, Rowel, Encompass, Outflank, Panther, Fierce, Fierce XLT, Gangster, Surveil, Authority First, and Sonic, Trivence, Afforia, Envive, and Enlite) are some of the most effective preplant and preemergence herbicides you can use.
Cocklebur. The most effective preplant and preemergence herbicides to aid in cocklebur control are those that contain First Rate, Classic, or Scepter. Such products would include Authority First, Sonic, Authority XL, Authority Maxx, Gangster, Surveil, Envive, Fierce XLT, and Valor XLT. Pursuit or Pursuit-containing products such as Zidua Pro, OpTill, and Authority Assist can also be used as a preplant treatment in Roundup Ready soybeans to provide residual cocklebur control. Extreme which contains glyphosate and Pursuit can be used either preplant or postemerge for some additional residual control in Roundup Ready soybean.
Morningglory. Glyphosate sometimes has trouble controlling morningglory. To help get better control, you can use either Authority-based or Valor-based herbicides preplant or preemergence. OpTill and Zidua Pro can also provide good early-season morningglory control.
Kochia. Kochia is a major weed problem in western areas and historically has been difficult to control with glyphosate, especially as it gets bigger. In addition, much of the kochia in western Kansas is now glyphosate-resistant. A majority of kochia will probably have emerged prior to soybean planting, so controlling that kochia before planting is critical.
Research by K-State the last couple of years indicates that Authority-based products have provided the best residual kochia control in soybeans. Metribuzin can also provide good kochia control, but soil pH and texture label guidelines need to be followed. The Kixor-containing products, such as Sharpen, OpTill, Zidua Pro, and Verdict, may help with kochia burndown and early-season kochia control, but may not provide very much residual control.
Xtendimax, FeXapan, and Engenia can be utilized ahead of Xtend soybeans for burndown and early-season residual control of kochia. ALS-inhibiting herbicides may or may not provide kochia control because of the occurrence of ALS-resistant kochia.
Crabgrass and other small-seeded grasses. Glyphosate usually gives good control of most grasses, but producers may want to apply a foundation herbicide to control grasses early, followed by a postemergence grass control herbicide. Fierce, Fierce XLT, Prefix, Zidua, Zidua Pro, Anthem, Dual II Magnum, Outlook, Warrant, and Prowl H2O can all provide early season grass and pigweed control ahead of postemergence treatments. Of these, Fierce, Fierce XLT, Prefix, and Zidua, Zidua Pro generally provide the best pigweed control, and Prowl H20 the least.
Dallas Peterson, Weed Management Specialist
Doug Shoup, Southeast Area Crops and Soils Specialist