Is Herbicide Carryover a Concern for 2000?

March 17, 2000
The dry weather conditions that have occurred over the last 9 months have raised the question of the potential for herbicide carryover in the 2000 growing season from applications made in 1999. Certainly, due to the drier conditions in certain parts of the state, a number of products may have an increased potential to carry over into the 2000 season. Areas in Illinois that have been hit the hardest with droughty conditions are the western and southern regions of the state. When making planting decisions for the 2000 growing season, especially in these droughty areas, make sure to check the rotational restrictions from the labels of herbicides used during 1999. The following information describes some factors to consider when assessing the potential for herbicide carryover in the 2000 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 of these factors are more important for specific herbicide chemistries.

Herbicide Persistence

First of all, it is important to ask what products were used in the 1999 growing season, how long they persist, and what crops are sensitive to soil residues from these herbicides. 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 enough time for atrazine to degrade between application and soybean planting the following year.

Soil Moisture and Temperature

Other questions to ask are how much rainfall there has been since the herbicide application and what the soil temperatures have been. Dry weather conditions reduce the rate of herbicide degradation. Soil moisture is extremely important, especially in the first 4 to 6 weeks after application. If rainfall and soil moisture were not sufficient during this time, dissipation of the herbicide by weed uptake is 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 the herbicide's availability for degradation. Herbicide labels that have minimum rainfall precaution statements include imazaquin-containing products (Scepter, Squadron, and Steel), prosulfuron-containing products (Exceed, Spirit, and Peak), and clopyralid-containing products (Stinger, Hornet, Accent Gold, Scorpion III). Other herbicides that are affected by low rainfall 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, soil pH above 7.0 affects the persistence of atrazine and simazine. Also, degradation 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 into the next season. Degradation of prosulfuron-containing products (Exceed, Spirit, and Peak) is also affected by high soil pH. In addition, Command's degradation is affected 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 7 and Table 8, a field or greenhouse bioassay may be conducted to predict potential carryover problems. These bioassays are generally inexpensive and provide an estimate of the potential for rotational crop injury from herbicide residues in the soil. For more information on testing for herbicide residues in soil, refer to Chapter 21 of the Illinois Agricultural Pest Management Handbook. Another factor that may predict herbicide residues in the soil is differences in winter annual weed growth in a particular field. This could determine herbicide persistence due to differences in soil pH or moisture throughout a field.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague