Farm Progress

Apply preemergence herbicide on emerged beans?

Check your preemergence herbicide options for bean fields that have already been planted.

May 31, 2017

2 Min Read
WHAT TO DO: Your soybeans are emerging or are already up. With wet weather, you haven’t been able to get into the field to apply the preemergence herbicide. So now what are your weed control options?

The spread of multiple herbicide-resistant weeds has brought an end to the era of using total postemergence programs for soybean weed management in Iowa. Unfortunately, a prolonged rainy period this spring has prevented applications of preemergence treatments on many planted soybean fields in certain areas of the state.

By the time fields have dried out enough and are fit for field operations such as spraying, the soybeans likely will be emerging and limit the herbicide options that can be used in those fields. “Without careful management and a bit of good fortune, this situation will make it difficult to stay ahead of the weeds for the remainder of the growing season,” says Bob Hartzler, Iowa State University Extension weed scientist. “Controlling emerged weeds and applying an effective residual herbicide in these fields should be a top priority as we move forward.”

Most can’t be used on emerged beans
If soybeans are already in the ground, there is a good likelihood the planned preemergence program cannot be used once fields dry out, says Hartzler. The HG 14 (herbicide group 14) products (such as flumioxazin, sulftentrazone) are the backbone of many preemergence programs, and labels of most products containing these active ingredients restrict applications to within three days of planting. Delayed applications greatly increase the risk of soybean injury due to the likelihood of the hypocotyl being exposed to high concentrations of the herbicide on the soil surface (Figure 1). In severe cases, this results in widespread seedling death from girdled hypocotyls (Figure 2).

The table below  lists herbicides that can be applied following soybean emergence for residual weed control. Herbicide common names are used due to the abundance of products on the market. “Read the herbicide label prior to use as restrictions may vary on products,” says Hartzler. “Although the products available for application after crop emergence are not as broad-spectrum, as are the herbicides used before emergence, they will increase the odds of staying ahead of weed problems throughout the season.”


Post-application may be needed
Even with the recent cool temperatures, weeds are still emerging in fields, notes Hartzler. Scout fields prior to application of the preemergence product to determine if a postemergence product is necessary. Control these weeds as soon as possible with an appropriate herbicide (such as glyphosate if you have glyphosate-resistant beans), and include the preemergence herbicide with this application. Many winter annual weeds (such as marestail) may be too large for consistent control in no-till fields. Adding multiple products to try to improve control is usually unproductive.

Corn does not pose as significant a challenge due to the wider availability of herbicide products and generally wider windows for application. However, treating these fields with a combination of products to both control emerged weeds while they are small and to provide residual control is also critical, says Hartzler.

Source: Iowa State University


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