Source: Kansas State University Research and Extension

There are many good reasons to use a soil-applied residual herbicide for soybeans. No-tillers may want to:

• Get early-season control of weeds and grasses to minimize early-season weed competition and provide more flexibility with post-emergence treatment timing.
• Provide some residual weed control before and following the post-emergence glyphosate.
• Provide some assistance to glyphosate in controlling certain hard-to-control or glyphosate-resistant weeds.
• Use a second herbicide mode of action to prevent or delay the development of glyphosate-resistant weeds.

In developing an alternative to the exclusive use of post-emergence glyphosate treatments on Roundup Ready soybeans, it is useful to know what weeds or grasses are being targeted. Some good options for the most common weed and grass problems include:

Pigweeds (including waterhemp and Palmer amaranth). Glyphosate-resistant waterhemp is now fairly common across the eastern part of Kansas. Glyphosate-resistant Palmer amaranth was first confirmed in Kansas in 2011 and seemed to explode across much of the central and western parts of the state this past summer. Pigweed emergence will generally start in April but the greatest amount of emergence will occur in May and June. Pre-emergence or burndown plus residual herbicide applications will need to be targeted in these months before pigweed has emerged or while it is still at small growth stages.

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, Broadaxe XC, Spartan and Spartan Elite) can all provide very good to excellent control to supplement a post-emergence program. 

If glyphosate-resistant pigweed is suspected, higher use rates may be required to give adequate residual control. Prefix is another excellent “foundation” herbicide for residual pigweed control in soybeans. Metribuzin, Zidua, Anthem, 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.

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 and OpTill, may help with kochia burndown and early-season kochia control, but may not provide very much residual control. ALS-inhibiting herbicides may or may not provide kochia control because of the occurrence of ALS-resistant kochia.  

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, Sonic, Trivence, Afforia, Envive and Enlite) are some of the most effective pre-plant and pre-emergence herbicides you can use.

Cocklebur. The most effective pre-plant and pre-emergence 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. Extreme, which is a premix of glyphosate and Pursuit, can also be used as a pre-plant or post-emergence treatment in Roundup Ready soybeans to provide residual cocklebur control.

Marestail. Marestail is probably the most widespread glyphosate-resistant weed in Kansas. Marestail control in Roundup Ready soybeans should begin in early spring by controlling fall-germinated seedlings and rosettes before they start to bolt. 2,4-D and Clarity can be used in early spring, but the proper pre-plant intervals need to be followed. The pre-plant intervals for 2,4-D LV4 are one week for up to 1 pint per acre and 30 days for 1-2 pints per acre. The pre-plant interval for Clarity is 14 days following an application rate up to 8 ounces per acre and accumulation of 1 inch of rainfall. Clarity has generally provided better marestail control than 2,4-D.

The Kixor-containing products Sharpen and OpTill 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 Sharpen and OpTill, use an adequate spray volume to ensure 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 pre-plant herbicides that can help with burndown and provide residual marestail control include FirstRate-based herbicides, such as Authority First, Sonic, Gangster or Surveil.

Morningglory. Glyphosate sometimes has trouble controlling morningglory. To help get better control, you can use either Authority-based or Valor-based herbicides pre-plant or pre-emergence.  OpTill and OpTill Plus can also provide good early season morningglory control.

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, then make just one post-emergence glyphosate application later. Fierce, Fierce XLT, Prefix, Zidua, Anthem, Dual II Magnum, Outlook, Warrant and Prowl H2O can all provide early season grass and pigweed control ahead of Roundup Ready soybeans. Of these, Fierce, Fierce XLT and Prefix generally provide the best pigweed control, and Prowl H20 the least.