9/16/2020

Why Dry Conditions Increase Risk of Herbicide Carryover

Written by Stephen Strachan, Ph.D. and Kevin Hahn, Ph.D.

Key Points

  • Herbicides break down through microbial and/or chemical degradation in the presence of soil moisture.
  • When soils are very dry, herbicide breakdown via microbiological activity is diminished.
  • Chemical degradation continues to occur in dry soils and may increase due to higher soil temperature.
  • If there is a concern about planting a sensitive crop into soil that was treated with a herbicide that degrades via only microbial activity, carefully check the rotational crop portion of the label and plant the crop according to these guidelines.

Watch Out for Herbicide Carryover

In the growing season following a drought, growers should be wary of potential herbicide carryover. Herbicides break down through microbial and/or chemical degradation in the presence of soil moisture. When soils are very dry, herbicide breakdown via microbiological activity is diminished. Growers who suspect and need to diagnose herbicide carryover issues arising from severe drought conditions can use this information to identify how the applied herbicide degrades and how its degradation rate may be affected.

Herbicide Degradation in Dry Soils

To understand herbicide degradation in dry soils, it is important to understand how drought affects soil water and microbiological activity, herbicide degradation pathways, and the interaction between microorganisms and herbicides.

Characteristics of Water in Soil

A saturated soil contains about 50% solids, 25% plant available water in the micropores, and 25% air space in the macropores. As the soil dries, plant-available water from the micropores is consumed. There is also a third type of water in soil called adhesion water. This is the water that surrounds the soil colloids and is held in the soil by strong chemical and hydrogen bonds. This water is not plant available. In addition, this adhesion water does not evaporate under dry conditions and comprises about two to five percent of the weight (~20 to 50 tons) of an acre furrow slice of air-dry soil. One acre furrow slice comprises one acre of soil to a depth of approximately six inches and weighs approximately 1,000 tons. (Foth, H.D. and L. M. Turk, Fundamentals of Soil Science Fifth ed., Wiley and Sons, New York. Pp. 63-96.)

Illustration - Characteristics of water in soil.

How Microorganisms Function in Soil

Microorganisms (bacteria, fungi, etc.) require water for life and must live in a “sea of water” for survival. As the soil dries, the “seas” in the micropores diminish, thus reducing microbiological populations. As soils become very dry, fewer microorganisms are present for herbicide degradation, so the rate of microbial herbicide degradation decreases.

Illustration - How microorganisms function in soil.

Herbicides in Soil

Herbicides tend to exist as single molecules in the soil profile. These molecules tend to bind or associate with soil colloids, soil clay or soil organic matter. A large portion of the herbicide molecules is, therefore, associated with the adhesion water. Another portion of herbicides is dissolved in the water contained in the soil micropores. Herbicide molecules associated with the soil adhesion water and with the micropore water eventually reach equilibrium concentrations and move between the two types of water. As the soil dries, the relative amount of herbicide molecules associated with the adhesion water increases.

Illustration - herbicides in soil

How Microorganisms Degrade Herbicides in Moist Soil

Microorganisms must either ingest or be very closely associated with herbicide molecules in order to degrade these molecules. Most microbiological degradation, therefore, occurs in soil micropores. When a microorganism degrades a herbicide molecule in the micropore, a new equilibrium is established between the herbicide in the micropore water solution and herbicide associated with the adhesion water.

Illustration -How Microorganisms Degrade Herbicides in Moist Soil

As the herbicide molecules are removed from the micropore water by microbiological degradation, the amount of herbicide molecules associated with the adhesion water subsequently decreases until, eventually, all herbicide molecules are consumed.

Drier Soil Slows Microbial Degradation

The rate of microbiological degradation of herbicides decreases as soils become drier for two reasons. First, microorganisms require water to live. If there is less available water, there are fewer microorganisms. If there are fewer microorganisms, there are fewer “factories” to degrade the herbicide molecules. Second, the very small size of the herbicide molecule allows these molecules to penetrate very tiny pore openings. Herbicide molecules are a few angstroms in size (1 angstrom = 10-10 m), while microorganisms are a few microns in size (1 micron = 10-6 m). Microorganisms are about 10,000 times larger than herbicide molecules. These small molecules remain “hidden” or “protected” from microbiological attack because the relatively larger microorganisms cannot penetrate these openings.

Illustration - Drier Soil Slows Microbial Degradation

Chemical Degradation of Herbicides in Soil

Chemical degradation occurs wherever water is present. This includes water associated with soil micropores and water closely associated with the soil colloids (adhesion water).

Illustration - Chemical Degradation of Herbicides in Soil

Even the driest soil, in its natural state, contains about two to five percent of water by weight. As long as water is present, chemical degradation can occur.

Chemical Degradation of Herbicides Continues in Drier Soils

Soil temperature also plays a critical role in herbicide degradation. Chemical reactions typically occur faster as temperatures increase. Under drought conditions, soils are drier, and soil temperatures also tend to be hotter. Therefore, chemical degradation of herbicides tends to increase. It is not known how much the rate of chemical degradation increases relative to the rate of decrease in microbiological degradation as soils become drier under drought.

Illustration - Chemical Degradation of Herbicides Continues in Drier Soils

How Common Herbicides Degrade

The following table contains a few examples of how some of the more common herbicides degrade in soil. Note that the chemical class is more important than the mode of action in determining primary pathways for herbicide degradation. As an example, imidazolinone and sulfonylurea herbicides both affect the ALS binding site. However, imidazolinone herbicides degrade primarily via microbiological degradation, whereas sulfonylurea herbicides (e.g., chlorimuron ethyl, rimsulfuron, and tribenuron) degrade via both microbiological and chemical pathways.

Table 1. Degradation pathways of herbicides (typically based on chemical class, not mode of action).

Table - Degradation pathways of herbicides.

Table - Degradation pathways of herbicides.

1References: Herbicide Handbook, Weed Science Society of America, 9th edition, 2007, and EPA-published documents.
2Greater rotational crop concern if followed by metribuzin ahead of soybeans.
3High pH: microbial only. Low pH: chemical and microbial.

Herbicide Degradation During a Drought Year – Follow the Label

Many of the active ingredients listed in Table 1 have been used for many years. These herbicides have been applied during drought years (e.g., 1988) and in very wet years (e.g., 1993). Product labels commonly have a “safety buffer” built into the label guidelines. If there is a concern about planting a sensitive crop into soil that was treated with a herbicide that degrades via only microbial activity, carefully check the “following crop” or “rotational crop” portion of the label, and plant the crop according to these guidelines.


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The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary. Pioneer® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents.