No. 11 Article 8/June 3, 2005

Dry Weather Concerns with Postemergence Herbicides

While some areas of Illinois received near-average precipitation for May, many other areas fell well below average. Farmers should be aware that if dry soil conditions persist, it may very well become more difficult to control weeds with postemergence corn and soybean herbicides. The reason for the difficulty is the protective covering, or cuticle, on aboveground plant surfaces. The cuticle functions to reduce the amount of water lost through plant leaves. Why do plants lose water in the first place? Water moves into the plant from the soil in response to differences in water concentration between the soil and the atmosphere. After being absorbed into the plant, water moves upward in the water-conducting tissue (xylem) to the leaves. Water is then lost to the atmosphere through the leaves by transpiration, a cooling process the plant uses to maintain a relatively constant temperature. Water evaporating from the plant removes with it a large amount of heat, providing a cooling effect. This process in plants is analogous to perspiration in humans, which helps cool our bodies on hot days.

Water loss by the plant is an essential process for its survival. However, as less water is available as soils become dry, the plant must reduce the amount of water it loses. The plant accomplishes this reduction through changes in the structure and composition of the cuticle. The thickness of the cuticle can increase during dry periods, which slows the rate of water loss. Unfortunately, because the cuticle is composed primarily of waxlike material, a thick cuticle also reduces the amount of herbicide that is able to penetrate the leaf. Herbicide that remains on the leaf surface and is not absorbed will not provide any amount of weed control. Even in the driest times, some herbicide will penetrate into the plant, but the amount may not be enough to provide for complete weed control.

What can be done to offset the adverse effects of dry weather on the activity of postemergence herbicides? Small weeds are almost always easier to control than large weeds, and this becomes even more important during dry periods. Don't delay making a postemergence application in order to allow more weeds to emerge. The ones competing with the crop now are more competitive than those that will emerge days or weeks from now.

Some postemergence herbicides allow for use of a reduced rate if conditions are favorable for activity but suggest a full rate when conditions are less favorable. With the paltry amount of precipitation received in many areas of Illinois over the past few weeks, conditions are far from "ideal" for activity from postemergence herbicides. Reduced rates of postemergence herbicides are not advisable because of the potential for poor weed control. Weeds not controlled by the initial postemergence application may prove even more challenging to control should a second application be made.

Many postemergence herbicides require the addition of a nonionic surfactant (NIS), crop oil concentrate (COC), or ammonium nitrogen fertilizer (28% UAN or AMS) for optimal activity. Some products, such as certain glyphosate formulations, may not require additional spray additives for optimal activity. Under dry conditions, COC generally enhances herbicide penetration more than NIS; but remember, if more herbicide is penetrating into the weeds, more herbicide is also penetrating into the crop plants. Always consult the respective herbicide label for additive selection, as choice of additive(s) can change depending on prevailing environmental conditions.

Herbicide antagonism, resulting from tank mixes of two or more products, is often more frequent during periods of dry soils. Antagonism with herbicide tank mixes occurs when the control of a certain type (grass or broadleaf) or species of weed is reduced by the presence or action of one component of the tank mix. Usually, antagonism occurs more often with grass species than with broadleaf species. For example, control of annual grass species with ACCase-inhibiting herbicides (sethoxydim, quizalofop, clethodim, etc.) is sometimes reduced when these herbicides are tank-mixed with certain broadleaf herbicides.

Glyphosate has become the most widely used herbicide for postemergence weed control in glyphosate-resistant soybean, and tank-mixing another postemergence soybean herbicide with glyphosate is rare. There does appear, however, to be some interest in adding a tank-mix partner that has soil residual activity with glyphosate for postemergence applications in glyphosate-resistant corn. Atrazine is frequently mentioned as a partner of choice, as it can provide some cost-
effective residual weed control. In areas where soils are dry and weeds (especially grasses) have been moisture stressed for several days, antagonism may be more likely to occur with this type of tank mix.

Additionally, pay close attention to the number of leaves on broadleaf weed species during dry periods. Most postemergence herbicide labels indicate application rates based on weed height and/or leaf number. A velvetleaf plant growing under dry conditions may not exceed the maximum height on the herbicide label, but it may actually exceed the maximum number of leaves for a given application rate. The same general considerations apply to the crop; a corn plant may be "older" than its height would suggest, so be sure to count leaf collars when staging a cornfield for a postemergence herbicide application.--Aaron Hager and Dawn Nordby

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