Planning ahead for weed control in RR soybeans
* 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.