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Is Herbicide Carryover a Concern for 2001?

April 6, 2001
The number of carryover problems that occurred during the 2000 growing season has raised the question regarding what the potential is for herbicide carryover during the 2001 growing season. Looking back to last season, there were numerous reports of fomesafen (Flexstar or Reflex) carryover injury to corn from applications made during 1999. Most of these carryover problems were from late-season Flexstar applications for waterhemp control, followed by early corn planting that resulted in a less than 10-month rotational interval. However, there were a few instances where the recropping interval was met, but there were still problems. In many locations where this occurred, there was generally a lack of sufficient precipitation following herbicide application to allow for herbicide dissipation. However, over the last 9 months throughout most of Illinois, there was generally more precipitation than during the same period in 1999. So does more precipitation during the fall of 2000 mean that carryover will not happen in 2001? The following information describes some factors to consider when assessing the potential for herbicide carryover in the 2001 growing season.

Herbicide carryover is a function of four properties: (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. Although these four factors all influence herbicide persistence, certain factors are more important than others for specific herbicide chemistries.

Herbicide Persistence

First of all, it is important to know what products were applied in 2000, how long these products persist, and what rotational crops are sensitive to soil residues. Different herbicides persist in the soil different lengths of time. It is important to know when a product was applied to determine the time the product has been available for degradation. For example, the atrazine label indicates that soybeans can be planted the following year if applications were made before June 10 of the previous year. This rotational restriction was made to ensure that there was sufficient time for atrazine to degrade between application and soybean planting the following year.

Soil Moisture and Temperature

Other factors to consider include the amount of precipitation received following application and soil temperatures. Dry soil conditions generally reduce the rate of herbicide degradation (DNA herbicides, however, degrade more rapidly under saturated soil conditions). Soil moisture is extremely important, especially during 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. Herbicide labels that include minimum precipitation requirements include imazaquin-containing products (Scepter, Squadron, and Steel), prosulfuron-containing products (Exceed, Spirit, and Peak), and clopyralid-containing products (Stinger, Hornet, Accent Gold). Other herbicides that are affected by low soil moisture include Command, Pursuit, and Lightning. Soil temperature also plays an important role in herbicide degradation. Colder soil temperatures decrease herbicide degradation. That is why certain herbicides such as Scepter and Exceed are not used in the northern regions of Illinois.

Soil pH

Soil pH affects the persistence and degradation of many herbicides. In particular, high soil pH (above 7.0) reduces the dissipation of atrazine and simazine. Also, dissipation of chlorimuron-containing products (Classic, Canopy, Canopy XL, and Synchrony STS) is reduced if the soil pH is greater than 6.8, increasing the likelihood of persistence. Dissipation of prosulfuron-containing products (Exceed, Spirit, and Peak) is also reduced by high soil pH. Clomazone (Command) dissipation is reduced by soil pH levels less than or equal to 5.9, increasing the chances of carryover to corn the following year.

Concerned About Carryover?

If you are still concerned about the potential for carryover this spring after considering all of these factors and referring to the rotational restrictions for the corn and soybean herbicides listed in Table 4 and Table 5, a field or greenhouse bioassay can be conducted to help determine potential carryover problems. These bioassays are generally inexpensive and will help estimate the potential for rotational crop injury from herbicide residues. For more information on testing for herbicide residues in soil refer to Chapter 16 of the Illinois Agricultural Pest Management Handbook. Another diagnostic tool to help predict herbicide residues in the soil is winter annual weed growth. This could help determine herbicide persistence from differences in soil pH or moisture throughout a field.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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