No. 23 Article 10/October 7, 2005

What's the Potential for Herbicide Carryover in 2006?

Dry soil conditions across much of Illinois in 2005 contributed to many instances of poor weed control by reducing the performance of soil-applied and/or foliar-applied herbicides. Soil-residual herbicides applied to a dry surface often did not receive an "activating" rainfall for several days or weeks, whereas weeds growing under hot and dry conditions were frequently "hardened off' and difficult to control with postemergence herbicides. Reduced weed control was one obvious outcome of the dry growing season, but herbicide degradation and dissipation also can be reduced when soil moisture is limited. Reduced herbicide dissipation in soils may result in herbicide residues high enough to injure susceptible rotational crops.

Several factors should be considered when determining the potential for herbicide carryover, including herbicide application timing, soil pH, and soil moisture. The labels of most soil-residual herbicides indicate the time that must elapse between application and planting of a rotational crop. Late-season applications of herbicides with soil-residual activity may result in rotational crop injury if the specific rotational interval is not achieved.

Within a given chemical family, herbicides can vary in persistence due to differences in chemical structures. For example, Harmony GT XP (thifensulfuron) and Classic (chlorimuron) are both in the sulfonylurea herbicide family, but Classic is inherently more persistent in the soil. Rotational crop-planting intervals range from 0 to 45 days after application of Harmony GT XP, while the range is 0 to 30 months for Classic.

Soil pH affects the stability and persistence of some herbicide families. High soil pH (7.0 and higher) may slow the dissipation of certain herbicides by reducing the degradation process known as hydrolysis. Even under conditions of adequate soil moisture, degradation of some triazine and sulfonylurea herbicides in high pH soil can be reduced enough to result in carryover.

Soil moisture is often the most critical factor governing efficacy and persistence of soil-residual herbicides. Many herbicides are degraded in soil by the activity of soil microorganisms, and populations of these microorganisms can be greatly depressed when soil moisture is limited. Additionally, dry soils can enhance herbicide adsorption to soil colloids, rendering the herbicide unavailable for plant uptake and degradation by soil microbial populations. Some herbicide rotational intervals are increased if a specified amount of precipitation is not received by a certain date. Rotational intervals of some imazaquin-, prosulfuron-, and clopyralid-containing products can be increased under low precipitation environments.

Points to Consider

1. Dry soils can increase the risk of herbicide carryover. Herbicides that are primarily degraded by soil microbes can become more persistent under dry soil conditions. Persistence of herbicides degraded by nonmicrobial pathways (hydrolysis) also may increase during dry soil conditions. Herbicide carryover concerns are virtually eliminated if the rotational crop of choice is tolerant to the herbicide (for example, if atrazine carryover is a concern, corn and sorghum are crops tolerant to soil residues of atrazine).

2. Check herbicide rotational intervals. These intervals appear on product labels, or see Tables 5a and 5b in the 2005 Illinois Agricultural Pest Management Handbook (IAPM). Note the extended intervals for some products (a) if applied after a certain calendar date or (b) if a minimum amount of precipitation is not received between application and a certain calendar date. Be sure to check rotational intervals if your crop rotation plans include fall seeding of small grains or forages.

3. Additional information. For additional details on herbicide carryover, including suggestions on testing soils for herbicide residues, see Chapter 15 of the IAPM ("Herbicide Persistence and How to Test for Residues in the Soil.")--Aaron Hager

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