Fall harvest is a good time for producers to assess the weed problems they had in soybean fields this year, to plan a good weed control program for next year. In many cases, it is no longer possible to rely strictly on glyphosate for broadleaf weed control in Roundup Ready soybeans. Before deciding which other herbicides should be used in addition to glyphosate, it is necessary to know what weeds are being targeted. Some good options for the most common broadleaf weed and grass problems include:
* Pigweeds (including waterhemp and Palmer amaranth). Glyphosate-resistant waterhemp appears to have exploded across eastern Kansas during the last two years. 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, the Valor-based herbicides (Valor SX, Valor XLT, Fierce, Gangster, Envive, and Enlite) and Authority-based herbicides (Authority XL, Authority First, Sonic, Authority Assist, Authority MTZ, and Spartan) can all provide very good to excellent control to supplement a postemergence glyphosate program. Prefix is another excellent “foundation” herbicide for residual pigweed control in soybeans. Intrro, Dual, Outlook, and Prowl products can also provide some early-season pigweed control, but generally are not as effective as those previously mentioned products. Although some of these herbicides can be applied in the fall or early spring, those treatments generally will not persist long enough to give good pigweed control into the soybean growing season. So, if pigweed is the primary target to control, treatments will be most effective if applied no more than two weeks prior to planting.
* 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, Fierce, 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. However, all of these herbicides are ALS-inhibiting herbicides, and ALS-resistant cocklebur may be present in some fields.
* Marestail. Marestail is probably the most widespread glyphosate-resistant weed in Kansas. Marestail control in fields going to soybeans should begin with fall or early spring herbicide treatments that include 2,4-D, Clarity, or an ALS-inhibiting herbicide such as Canopy EX. Fall treatments should be delayed until November when most of the fall-germinating marestail has emerged. Be aware of the intervals required between application of these herbicides and planting soybeans. A couple of relatively new options for marestail control in soybeans without a preplant waiting interval are the Kixor-containing products, Sharpen and OpTill. Sharpen is Kixor alone, while OpTill is a premix of Kixor and Pursuit. Both products can be used for burndown control of marestail anytime before soybean emergence (cracking). To optimize marestail control with Sharpen and OpTill, spray before marestail gets too big, use an adequate spray volume to insure good spray coverage, and apply in combination with a methylated seed oil. The Kixor rates that can be used in soybeans will not provide very much residual control of marestail. Other residual preplant herbicides that can help with burndown and residual marestail control include FirstRate-based herbicides, such as Authority First, Sonic, or Gangster. Marestail is best controlled before soybean planting and before the marestail begins to bolt. FirstRate would probably be the most effective tank-mix partner with glyphosate for postemergence marestail control in Roundup Ready soybeans. Ignite is one of the better herbicides to control marestail that has started to bolt in the spring. Ignite can be used as a burndown treatment prior to emergence of any soybeans, or as a postemergence treatment in Liberty Link soybeans.
* 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. Ignite can also provide good control of soybeans in Liberty Link soybeans.
* Kochia. Kochia is a major weed problem in western areas and has often been difficult to control with glyphosate, especially as it gets bigger. In addition, glyphosate-resistant kochia is becoming a major problem in western Kansas. Since much of the kochia emerges before soybean planting, one of the keys to managing kochia in soybeans is to control it early in the spring before soybean planting. Research by K-State the last couple of years indicates that several preemergence herbicides can help provide control of glyphosate-resistant kochia, especially the Authority-based products listed above. The Kixor-containing products Sharpen and Optill may help with kochia burndown, but the Kixor rates that can be used in soybeans will not provide very much residual control. ALS-inhibiting herbicides may or may not provide kochia control because of the occurrence of ALS-resistant kochia.
* Crabgrass and small-seeded broadleaf weeds. 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, Fierce, 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 and Fierce generally provide 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.