Get an early start on weed control in soybeans
* 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 dicamba, 2,4-D, or an ALS-inhibiting herbicide such as Canopy EX or Autumn Super. Unfortunately, ALS resistant marestail may also be present in some fields, so a tank-mix with dicamba or 2,4-D is still recommended. Dicamba has provided better marestail control than 2,4-D in K-State research the last several years. Fall treatments should be delayed until November when most of the fall-germinating marestail has emerged. With spring applications, 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 (except on coarse soils with 2% organic matter or less) 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 Valor and FirstRate-based herbicides, such as Valor XLT, Envive, Enlite, Authority First, Sonic, or Gangster.
Marestail is best controlled before soybean planting and before the marestail begins to bolt. FirstRate or Synchrony would probably be the most effective tank-mix partner with glyphosate for postemergence marestail control in Roundup Ready soybeans. However, if ALS-resistant marestail are present, these treatments will not be very effective. Liberty is one of the better herbicides to control marestail that has started to bolt in the spring. Liberty can be used as a burndown treatment prior to emergence of any soybeans, or as a postemergence treatment in Liberty Link soybeans.