We have exited the Roundup Ready^® era when all we had to do was apply a shot or two of a glyphosate herbicide and were rewarded with clean fields. Weed control was never so easy. Today we have entered the era of near-mandatory use of preemergence residual herbicides, along with different post combinations.

Residual herbicides have been on the market for a long time now and savvy growers made routine use of them in their weed control program. They controlled weeds as seeds germinated and provided some protection when post applications were delayed due to rainfall or inclement weather. Today, however, residual herbicides should be a common ingredient for every weed control program.

Residual, or soil-applied herbicides, are no longer applied preemerge alone. They can be applied in the fall to control winter annuals, preplant in the early spring, preemerge with the planted crop or as an overlapping residual with a post program. There are a lot of opportunities to apply them today and we need to take advantage of these products.

Frequent rainfall events during May can delay planting and preemerge applications. And cool and rainy weather, wet soil conditions or cloudy weather may disrupt herbicide effectiveness and cause crop damage if the soybean seedlings can’t metabolize the herbicide.

Residual herbicides are sprayed on the soil, creating a zone of protection near the soil surface where the herbicide’s active ingredients lie in wait to intercept the germinating weed seed. Remember yellow trifluralin? It needed to be physically incorporated in the soil. And some herbicides need to be activated by rainfall. Or, in the case of some residuals, reactivated to maintain effectiveness over a longer period of time. Today’s better residuals do not need to be incorporated by any means.

Application of residual herbicides is important because they deliver several weeks of weed control, cover a broad spectrum of weeds, and aid in weed resistance management by incorporating additional site(s) of action. Most residual products do not have foliar activity, will not control emerged weeds present at application and need to be tank-mixed with foliar herbicides labeled for soybeans.

Several important factors must be considered when using residual herbicides applied after emergence:

• Crop stage when applied
• Weed height (<4”)
• Tank mix partner (Compatible or antagonistic? See label.)

Residual herbicides are an effective tool for controlling herbicide-resistant weeds. Length and effectiveness of residual activity from in-crop application will vary depending on:

• Weed species (will the a.i. show have efficacy against target weeds)
• Application rate and volume (see label)
• Rainfall following application (minimum of 0.5 inches of rainfall within a week of application)
• Density of the weed and crop canopy at the time of application
• Length of subsequent weed germination events (amaranth species can germinate from May to August)

Aaron Hager, weed scientist with the University of Illinois at Urbana-Champaign (UIUC) offers these tips for improving the effectiveness of soil-residual herbicides taken from “Revisiting the Realm of Residuals.”

1. Select a soil-residual product that offers the best solution for the problem weed species encountered in each field.
2. Pay careful attention not only to what products are contained in a premix, but how much of each active ingredient will be applied at the intended use rate.
3. Herbicides applied close (within 14 days) to crop planting generally control weeds longer into the growing season compared with applications made several weeks before planting.
4. Some residual herbicides commonly applied to the soil also can be applied after the crop has emerged. Applying these products following crop emergence may extend residual weed control for a few additional weeks.
5. Higher application rates generally provide a higher level of weed control longer into the growing season. However, don’t assume that a higher application rate will provide season-long weed control.
6. For a soil-applied herbicide to be effective, the herbicide needs to be available for uptake by the weed seedling. If no precipitation is received between application and planting, mechanical incorporation can help move the herbicide into the soil solution.
7. Herbicide selectivity arises from the crop’s ability to metabolize (break down) the herbicide to a nonphytotoxic form before it causes much injury. When the crop is growing under favorable conditions, it rapidly metabolizes the herbicide before excessive injury occurs.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.