Weed control advice in corn and soybean with recent planting delays

Wet conditions and delayed planting will cause challenges for weed control.

Wet conditions this spring 2014 have made it a challenge for growers to plant corn and soybeans. Due to these planting delays, once fields are dry enough, getting the crop into the ground will be the first priority. However, it is important to remember good weed control will be essential to get the most out of these crops.

There are several scenarios that can occur when the planting season is under demanding time constraints. However, one of the most important things to remember is to plant into a weed-free seedbed. This can be done with tillage or a good burndown herbicide program. Waiting to control weeds after planting already places the crop at a disadvantage and there are some instances where there are few options to control specific weed problems once the crop has emerged. Below are some possible scenarios and approaches Michigan State University Extension recommends that can be done to help manage weeds under this year’s planting constraints.

Scenario 1: Corn has been planted and rain or time constraints have prevented preemergence herbicide applications prior to emergence.

Approach 1. Continue with the planned preemergence (soil-applied) herbicide program. There are several soil-applied herbicides that are also labelled for application after corn emergence. If you decide to continue with your planned preemergence herbicide program, it is important to determine if these herbicides can be used after corn has emerged. For example, Sharpen, Verdict and Princep cannot be applied after corn emergence. A complete listing of soil-applied herbicides that can be applied after corn emergence can be found in Table 1G (pg. 47) of the 2014 Weed Control Guide for Field Crops (E0434).

These herbicides should still be applied as soon after planting as possible since many of these herbicides do not provide post-emergence activity, especially for grass control. However, in glyphosate-resistant or glufosinate-resistant corn, many of these herbicides can be tank-mixed with glyphosate or glufosinate to provide control of emerged weeds. Refer to the herbicide labels for specific restrictions on tank mixtures.

Approach 2. Switch to a total post-emergence herbicide weed control strategy. There are several different total post-emergence weed control strategies available for use in corn. Many of these programs include herbicides that will provide residual control of later emerging weeds. The important thing to keep in mind is the timing of these weed control programs.

Generally, we would like to control weeds with a total post-emergence weed control program when they are 2 inches tall or less. This timing allows us to minimize the effects of early-season weed competition with corn. For more details on post-emergence herbicide programs for use in corn, consult the 2014 Weed Control Guide for Field Crops (E0434) and www.msuweeds.com.

Scenario 2: A burndown herbicide has not been applied and no-till soybeans need to be planted.

Approach 1. There is really only one viable approach that does not lead to reductions in soybean yield from early-season weed competition. That is to make sure a burndown herbicide application is made prior to soybean emergence. Generally, we like to have burndown herbicide applications made a minimum of seven days prior to planting soybeans. This strategy allows us to include 2,4-D ester at 1 pint per acre; that is one of the options essential for control of glyphosate- and multiple-resistant horseweeds, like marestail. However, with the condensed planting window, many growers will not have this seven-day waiting period.

Other possible strategies that can be used, especially if glyphosate-resistant horseweeds are a problem, is to include the use of Sharpen or premixes that contain Sharpen like Optill, Optill Pro, and Verdict in the burndown application with glyphosate. These treatments will need to include methylated seed oil (MSO) in the mix.

Another potential option for management of herbicide-resistant horseweeds is to apply tank-mixtures of Gramoxone or Liberty plus metribuzin for effective control. All of these applications must be made prior to soybean emergence. There are virtually no post-emergence herbicide options available for control of herbicide-resistant horseweeds unless Liberty Link soybeans are planted, so effective burndown applications are essential. A complete listing of strategies to control horseweeds in no-till soybeans can be found on pg. 180 of the 2014 Weed Control Guide for Field Crops.

While glyphosate-resistant horseweeds may not be a problem in all no-till soybean fields, all soybean growers should also consider the use of a residual herbicide in with their burndown treatment. Unlike corn, almost all soil-applied (preemergence) residual herbicides used in soybeans need to be made prior to soybean emergence. For more information and product restrictions for weed control in no-till soybeans, consult Table 2N (pg. 90) of the 2014 Weed Control Guide for Field Crops (E0434).

Dr. Sprague’s work is funded in part by MSU’s AgBioResearch.

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