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Weather Concerns That Could Affect Weed Management Decisions in 2003

April 3, 2003

The 2002 growing season marked a year in which growers were faced with a variety of weed management challenges because of weather. Although we can't predict what the weather will be this growing season, reflecting back on last year's challenges can give us some insight on what potential problems may arise in 2003. The 2002 growing season began with cold, wet weather conditions that soon turned hot and dry. Excessive rains early in the season reduced the activity of some soil-applied grass herbicides that had been applied in March and early April. Research on herbicide movement has shown that with 2.5 inches of rainfall, as much as 50% of a soil-applied herbicide can leach out of the top 2.5 inches of the soil, depending on the herbicide (Bunting and Simmons, UI). Movement of these herbicides below the weed seed germination zone and breakdown of these products under excessive moisture conditions can lead to reduced control and late-season weed escapes.

Cool-to-cold temperatures, particularly in mid-May, also had an effect on postemergence herbicide performance. Some postemergence herbicide applications were made to frost-damaged corn when average daily temperatures were below 50°F. Herbicide applications that occurred under these stressful growing conditions often resulted in reduced weed control and increased crop sensitivity. For example, post-emergence applications of Callisto (mesotrione) under cold temperatures increased corn sensitivity and resulted in bleached corn leaves. Corn plants were able to recover when temperatures increased and plants were able to metabolize the herbicide. Warnings about postemergence herbicide applications under cool temperatures are on a number of labels. It is important to follow these warnings and allow crops to recover from these stressful conditions before making postemergence herbicide applications.

The early-season cool conditions also left the corn crop at several different developmental stages when post-emergence herbicide applications were made. This phenomenon became extremely critical when these applications were approaching the maximum corn size window for certain herbicides. Most herbicide labels often refer to plant height, growth stage, or both when discussing timing of postemergence applications. During the cool conditions experienced early in the 2002 season, the corn stayed relatively small in regard to plant height. However, corn continued to advance developmentally. For example, the Accent label indicates that applications can be made to corn up to 20 inches in height or that has six or fewer collars (V6), whichever is more restrictive. If the herbicide application was made by only looking at corn height, the possibility exists that corn injury could occur because the application was made to corn beyond the labeled growth stage. Following the more restrictive labeling is extremely critical, especially under cool growing conditions.

Hot and dry. In contrast to the cold and wet conditions growers faced early in the season, hot and dry conditions were more prevalent throughout many areas of Illinois in June, July, and August. Weeds, such as common lambsquarters and common water-hemp, were tougher to control under these conditions with many of the postemergence herbicides. Decreased control of these species is often related to decreased herbicide absorption into the plant. Under hot and dry conditions, many weed species form thicker leaf cuticles that act as barriers to herbicide absorption. Herbicide additive selection can sometimes enhance weed control under these conditions. For many products, a nonionic surfactant (NIS) may be the additive of choice; however, many labels allow the use of a crop oil concentrate (COC) under very dry conditions to enhance weed control.

Dry conditions last year not only affected the 2002 season but also may impact rotational crops in 2003 because of persistence of herbicide residues in the soil. There are several corn and soybean herbicides that have the potential to carry over after a dry season. Rotational restrictions for many of these herbicides can be found in the Illinois Agronomy Handbook or Illinois Agricultural Pest Management Handbook or on the herbicide label. Tables 2a and 2b contain rotational restrictions for most corn and soybean herbicides, respectively. Some of the factors that need to be considered to determine whether carryover may be a problem in 2003 are (1) the herbicide's ability to persist in the soil, (2) the amount of rainfall or soil moisture available for degradation, (3) soil temperature, and (4) soil pH.

Soil moisture was the most limiting factor for the degradation of herbicides last season. Dry soil conditions generally reduce the rate of herbicide degradation. Soil moisture is extremely important, especially in the first 2 to 4 weeks after application. If rainfall and soil moisture were not sufficient during this time, dissipation of the herbicide was likely reduced, increasing the potential for carryover. Additionally, lack of soil moisture can result in increased herbicide adsorption to soil particles and organic matter, reducing herbicide availability for degradation.

Because of the planting delays last season, time of herbicide application also may influence rotational crop injury concerns. Several late-season "rescue" applications were made last year, so be sure to observe the rotational crop interval on the respective herbicide labels before planting rotational crops.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
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