No. 12 Article 2/June 22, 2012

Considerations With Postemergence Herbicides Applied During Wet or Dry Conditions

Recent precipitation has alleviated the extremely dry soils in parts of Illinois, but other areas remain excessively dry. Weed growth is likely to accelerate where rain has fallen, while effectiveness of postemergence herbicides could be challenged where dry conditions persist.

Growth of emerged weeds has continued, albeit more slowly, where dry conditions have persisted. Where there has been recent precipitation of about ½ inch or more, many summer annual weeds could demonstrate a "growth spurt" within a few days of the rainfall. Before applying a postemergence herbicide, rescout fields to determine how much weed growth has occurred and if the application rate should be adjusted accordingly.

Recent precipitation might also promote more germination and emergence of summer annual weeds, such as waterhemp and morningglory. Applying a postemergence herbicide is recommended to control existing summer annual weeds when field conditions are conducive, not delaying to allow additional weed emergence to occur. A delay will allow the existing--and most competitive--weeds to continue depriving the crop of the resources needed to express its full genetic yield potential.

Weed control with postemergence herbicides can become more difficult during dry soil conditions because of the protective covering (known as the cuticle) on aboveground plant surfaces. The cuticle reduces the amount of water lost through the plant leaves. Water loss is essential for a plant's survival, but as less water becomes available as soils dry, the plant must reduce the amount of water it loses. This is accomplished through changes in the structure and composition of the cuticle. Cuticle thickness can increase during dry periods, which slows water loss, but because the cuticle is composed primarily of waxlike material, a thick cuticle also reduces the amount of herbicide that can penetrate the leaf. Herbicide that remains on the leaf surface and is not absorbed provides no weed control. Some herbicide will penetrate into the plant even in the driest times, but the amount might not be enough to provide complete weed control.

What can be done to alleviate the adverse effects of dry weather on the activity of postemergence herbicides? There is no one solution, but several practices can be modified to try to optimize herbicide application and performance. The influence of dust on herbicide performance was described in issue 8 (May 25). Increasing application volume might help alleviate the deleterious effects of dust on herbicide performance, but reducing the amount of dust generated during the application could be even more beneficial. Simply reducing the speed of the application equipment is one technique that can also reduce dust.

Small weeds are almost always easier to control than larger ones, and this becomes even more important in dry periods. To reemphasize: don't delay applying a postemergence herbicide to allow more weeds to emerge. The ones currently competing with the crop are more competitive than the ones that will emerge days or weeks from now.

Selecting appropriate application rates is always important, but it can be even more so when environmental conditions are adverse. Labels for some postemergence herbicides allow for a reduced rate if conditions are favorable for activity but suggest a full rate when conditions are less favorable. Current prevailing conditions dictate that rates of foliar-applied herbicides not be reduced but rather remain as close as possible to the full recommendation. Weed control with foliar-applied herbicides is often more consistent and complete when they are applied at full recommended rates, which are based on weed size and prevailing environmental conditions. Weeds not controlled by the initial postemergence application may prove even more challenging to control should a second application be made. As a reminder, the maximum allowable rates per application for glyphosate in glyphosate-resistant corn and soybean are 1.125 and 1.54 lb ae/acre, respectively, and the total of all in-crop applications from emergence through 48-inch corn or R2 soybean is 2.25 lb ae/acre.

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 that if more herbicide is penetrating into weeds, more is also penetrating into crop plants. Always consult the herbicide label for additive selection, as choices can change depending on tank-mix partners and prevailing environmental conditions.--Aaron Hager

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