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Issue No. 9, Article 6/May 26, 2006

Oversized Vegetation in Soybean Fields

Several areas of Illinois have experienced delays in soybean planting due to cool and wet field conditions. These planting delays often are accompanied by delays in controlling existing vegetation by either preplant tillage operations or burndown herbicide applications. Many winter annual weed species, such as butterweed, downy brome, and various mustard species, are close to completing their life cycles, but many summer annual weed species are beginning to thrive. Weeds such as common lambsquarters, giant ragweed, and smartweeds have made tremendous growth over the past few weeks, thanks in part to adequate soil moisture. Control of these larger weeds will most likely prove to be more challenging this year than during recent seasons.

While many Illinois soybean acres are planted without tillage, a significant number are tilled at least once in the spring prior to planting. Large weeds often can reduce the effectiveness of preplant tillage as a weed control tool, a problem often enhanced when tillage is done when soils are still on the moist side. Large soil clods and "transplanted" weeds frequently result when tillage occurs on fields that are still wet. Shallow tillage with a field cultivator may not always take out the larger weeds or all the weeds growing in dense patches. Weeds that survive a preplant tillage operation can be very difficult to control with herbicides once the crop has emerged.

When attempting to control large weeds prior to planting, use reduced rates of burndown herbicides with caution. Attempting to reduce herbicide costs by using low application rates of burndown products may actually increase weed control costs if poor control results. The calendar suggests that soybean yield potential will start to decline with continued delays in planting, so planting operations may very well take precedence over weed control (at least for the present time). If possible, burndown herbicides should be applied prior to planting; however, it is not likely that this sequence will always be followed. A challenging question some farmers may face is whether to spray first and then plant, or plant as soon as soil conditions are conducive and then worry about controlling existing vegetation later. The best solution may vary by field. Some points to consider when making this decision follow.

The rate of 2,4-D ester used in burndown applications generally ranges from 0.25 to 0.5 lb acid equivalent (0.5 to 1 pint) per acre. Keep in mind that 2,4-D is not effective on smartweeds. According to label guidelines, there must be an interval of seven days between the application of up to 1 pint per acre of 2,4-D and soybean planting, or 30 days for the application of 2 pints per acre. These time intervals are established to allow for dissipation of the 2,4-D, thus reducing the potential for crop injury. Soybean injury may result if seed comes in contact with the 2,4-D. Plant soybean seeds 1 to 1.5 inches deep, and be certain to obtain good closure of the seed slot.

Translocated herbicides commonly used in burndown applications (such as glyphosate) must move within the plant after absorption to provide good control. Plants under stress conditions tend to reduce their rate of growth and thus may not translocate herbicides as quickly or extensively as when growing conditions are closer to optimal. Tillage, including planting operations, stresses plants by injuring both aboveground and belowground portions. Once this "injury" has occurred, a few days must elapse before the plant resumes active growth. Application of a burndown herbicide during the time the plant is under stress may reduce the amount of herbicide translocation, which may also reduce the level of weed control. This becomes even more important when attempting to control perennial weed species with translocated herbicides. During the spring, food reserves are moving upward from the root system to support new vegetative growth. This upward movement of food reserves is not conducive for downward translocation of the herbicide. Disrupting perennial plants too soon after herbicide application can result in much-reduced control, as the herbicide lacks ample opportunity to be distributed throughout the plant.

Contact herbicides (such as Gramoxone Inteon) do not translocate extensively within the plant following absorption. Because of this, thorough spray coverage of the vegetation is more critical with contact herbicides than with translocated herbicides. Addition of spray additives (such as crop oil concentrate) generally increases the activity of contact herbicides on existing vegetation. The activity of contact herbicides on plants under stress conditions may be preserved more so than the activity of translocated herbicides, but control of perennial weed species usually will be less with contact herbicides.

In most instances, several days must elapse following application of translocated herbicides before injury symptoms become apparent. In contrast, injury symptoms from contact herbicides may be observed within one to three days, depending on the herbicide. Many of the contact herbicides used for burndown interfere with the process of photosynthesis, which requires sunlight. Therefore, injury symptoms with these herbicides will develop faster under conditions of bright sunshine. Often, combining two or more of these herbicides enhances the weed control spectrum. If weeds are over six inches high, consider increasing the spray volume for improved coverage.

In summary, it is advisable to control existing vegetation before planting. If planting is done first, you may realize reduced weed control when spraying follows planting, especially for perennial weed species. If growth regulator herbicides come in contact with the soybean seed, injury may occur. Application of 2,4-D should not occur after soybean seeds are planted. Translocated herbicides require more time to be distributed throughout the plant than contact herbicides, but contact herbicides require more thorough coverage, which usually means higher application volumes.

How much time should elapse between application of a burndown herbicide and soybean planting? As a general rule, "a few days" is better than "not many days," although exceptions exist to any general rule. Large weeds remaining in fields after soybean emergence will be very competitive with soybean, so it's better to control these weeds sooner rather than later.--Aaron Hager

Aaron Hager

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