Recent precipitation has alleviated the extremely dry soils in parts of Illinois; other areas of the state continue to endure excessively dry conditions. University of Illinois associate professor of weed science Aaron Hager expects that weed growth will accelerate in areas that received rain, and the effectiveness of postemergence herbicides could be challenged in dry areas.
 
Emerged weeds have continued to grow, albeit at a reduced rate, where dry conditions have persisted. In the areas that have recently received one-half an inch or more of rain, summer annual weeds could have a growth spurt within a few days of the precipitation event.
 
“It would be advisable to re-scout fields prior to applying a postemergence herbicide to determine how much weed growth has occurred and if the herbicide application rate should be adjusted to account for the now-larger weeds,” said Hager.
 
The recent precipitation might also promote additional germination and emergence of summer annual weeds such as waterhemp and morningglory. Hager recommends applying a postemergence herbicide to control existing weeds when field conditions are favorable rather than delaying the application to allow additional weed emergence to occur.
 
“Delaying the application of a postemergence herbicide will allow the existing and most competitive weeds to continue to deprive the crop of the resources needed to express its full genetic-yield potential,” he explained.
 
Weed control with postemergence herbicides can become more difficult during dry weather. Water loss by the plant is an essential process for its survival. However, as soils dry out and less water is available, the plant must reduce the amount of water it loses. A protective covering (known as the cuticle) on above-ground plant surfaces reduces the amount of water lost through the plant leaves. The thickness of the cuticle increases during dry periods, slowing the water-loss rate.
 
Unfortunately, because the cuticle is composed of a wax-like material, a thick cuticle also slows the rate at which a herbicide penetrates the leaf. “Some amount of herbicide will always penetrate into the plant, but it might not be enough to provide complete weed control,” Hager said. “Herbicide that is not absorbed will not provide any weed control.”
 
What can be done to alleviate the adverse effects of dry weather on the activity of postemergence herbicides?
 
Hager said there is no single solution, but several practices can be modified to try to optimize herbicide application and performance. Increasing the 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. Reducing the speed of the application equipment helps to reduce the amount of dust.
 
Small weeds are almost always easier to control than larger weeds, and size becomes even more important during dry periods. Moreover, the ones competing with the crop now are more competitive than those that will emerge days or weeks from now.
 
The selection of appropriate herbicide application rates is always important but can be even more important when applications are made under adverse environmental conditions. Some postemergence herbicides can be used at a reduced rate if conditions are favorable for their activity but the labels suggest a full rate when conditions are less favorable.
 
Current conditions dictate that rates of foliar-applied herbicides should not be reduced but should be as close as possible to the fully recommended rate. Weed control is often more consistent and complete when foliar-applied herbicides are applied at fully recommended rates, which are based on weed size and prevailing environmental conditions.
 
Weeds not controlled by the initial postemergence application may prove to be 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 lbs. ae/acre, respectively, and the total of all in-crop applications from emergence through 48-in. corn or R2 soybean is 2.25 lbs. 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.
 
“Under dry conditions, COC generally enhances herbicide penetration more than NIS, but if more herbicide is penetrating into the weeds, more herbicide is also penetrating into the crop plants,” Hager said. Always consult the herbicide label when selecting additives because the choice can change depending on the tankmix partners and the prevailing environmental conditions.