There are many good reasons to use a soil-applied residual herbicide for soybeans. Producers may want to:
- Get early-season control of weeds and grasses to minimize early-season weed competition and provide more flexibility with postemergence treatment timing.
- Provide some residual weed control before and following the postemergence 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 postemergence 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 has been confirmed across the eastern part of Kansas. Glyphosate-resistant Palmer amaranth has not yet been confirmed in Kansas, but is a major problem in the southeastern U.S. For early-season pigweed control, Valor-based herbicides (Valor SX, Valor XLT, Gangster, Envive, and Enlite) and Authority-based herbicides (Authority First, Sonic, Authority Assist, Authority MTZ, and Spartan) can provide very good to excellent control to supplement a postemergence glyphosate 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. Intrro, Dual, Outlook, and Prowl products also provide some early-season pigweed control, but generally are not as effective 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. A majority of kochia will probably have emerged prior to soybean planting, so controlling that kochia before planting is critical. In addition, glyphosate-resistant kochia populations have now been confirmed across western Kansas. Research by K-State the last couple of years indicates that Authority-based products have provided the best residual kochia control in soybeans. 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, Gangster, Authority First, and Sonic) 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, Gangster, Envive, and Valor XLT. Extreme, which is a premix of glyphosate and Pursuit, can also be used as a preplant or postemergence 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 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.
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 insure good spray coverage and apply in combination with a methylated seed oil. Ignite herbicide may be the best option as a rescue treatment to burndown bolted marestail prior to planting. No waiting interval required between an Ignite application and planting soybeans, but it will not provide any residual marestail control. Other residual preplant herbicides that can help with burndown and residual marestail control include FirstRate-based herbicides, such as Authority First, Sonic, or Gangster.
- 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.
- Crabgrass and other small seed 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 postemergence glyphosate application later. Prefix, Intrro, Dual II Magnum, Outlook, and Prowl H2O can all provide good early season grass and pigweed control ahead of Roundup Ready soybeans. Of these, Prefix generally provides the best pigweed control, and Prowl H20 the least. Warrant is a new herbicide from Monsanto that can be applied as a postemergence tank-mix partner with glyphosate for residual grass and pigweed control. However, Warrant will not control emerged weeds, so it would not be effective for postemergence control of glyphosate-resistant waterhemp or Palmer amaranth.