How Rainfall Influences Residual Herbicides

Heavy rains the past month have caused concern that residual preplant incorporated and preemergence herbicide applications may and not provide the length of weed control expected and increase the risk of crop injury compared to when rainfall is closer to average.  The affect of the additional rainfall on herbicide activity and crop safety is complex, and is influenced the soil and the chemical properties of each herbicide.

Soil characteristics that influence herbicide effectiveness include texture, percent organic matter and pH.  Sand particles are the largest, silt is medium in size, and clay particles are the smallest.  Soils with a large percentage of large sand particles are considered to be coarse in texture and are called sand, loamy sand, or sandy loam.  Soils with a moderate amount of each size soil particle are considered to be medium in texture, and are called loam, or silt loam. Soils with a large percentage of small clay particles are considered to be fine in texture and are called silty clay loam, clay loam, or clay.

Soil particles are negatively (-) charged. The negative charge attracts positively (+) charged fertilizer nutrients such as H2PO4⁺, K⁺, Ca⁺⁺, Mg⁺⁺ and many herbicides.  The attraction of a positively charged herbicide to the negative charge of the soil particles slows leaching. Other fertilizer nutrients, such as NO3^– and a few herbicides, have a negative charge.  Negatively (-) charged molecules are not bound to the soil and are more subject to leaching, especially if they are highly soluble in water.

Since substances that are positively (+) charged are called cations, the measure of a soil’s ability to hold onto cations is called the Cation Exchange Capacity or CEC.  Sand is the largest particle in size and has the lowest CEC value, less than one.  Silt is intermediate in size and has an intermediate CEC value, near five.  Clays are the smallest soil particles and have the highest CEC value of the mineral component of soil, near thirty-five, depending on the type of clay.  The CEC of your soil should be listed on your soil test results.

Organic matter makes up only a small part of most soils, usually between 0.5 and 5.0 percent in soils across the northeastern United States, but it has the highest CEC value, near two hundred.  Even small changes in the percent organic matter in soils, especially sandy soils, can have a strong influence on herbicide performance.  That is the reason small changes in percent organic matter may require herbicide rate changes.  Rate tables may have several columns with different herbicide rates for different levels of organic matter in each soil type.

Soil pH also affects the performance of some herbicides by influencing the degree of attraction to soil particles.  Low pH, below 6.0, or high pH, above 7.0, may affect the availability of certain herbicides by changing the positive (+) charge of the molecule.  Weed control may be reduced and/or herbicide carryover may be increased if the herbicide is more tightly bound to the soil.  The risk of crop injury may increase if an herbicide is less tightly bound to the soil, and more available.  Herbicides that are affected by pH may have DO NOT USE warnings on the label if the soil pH is above or below a value that increases the risk of crop injury, herbicide carryover, or poor weed control.

Herbicides leach more quickly through coarse textured (sandy) soils that are low in organic matter and more slowly through fine textured (clay) soils and soils high in organic matter, but the water solubility and adsorption characteristics of the herbicide also influence leaching (see table 1).  Herbicides leach more quickly as solubility in water increases and adsorption decreases.

Herbicides, that have “Very Low” solubility in water and are “Strongly” adsorbed to soil,  including Dacthal, Curbit, Goal, Prefar, Princep, Prowl, and Treflan,  and are not leached by heavy rainfall.  Since herbicide breakdown by soil microorganisms occurs when the soil is warm and moist, loss of these herbicides occurs due to increased speed of breakdown by microorganisms when the soil is continuously moist and warm.  Erosion of treated soil can also affect herbicide performance.  Some herbicides, such as Devrinol and Solicam, that are “Moderately” soluble in water are very “Strongly” adsorbed onto the soil and also do not leach.  In general, herbicides that have long carryover restrictions, two years or more, are less likely to leach quickly.

Herbicides that have “Moderate to High” solubility in water and are “Weakly or Moderately” adsorbed to soil, including Dual Magnum, Intrro, Metribuzin, Sinbar, and Stinger, are more likely to leach too deeply into the soil to provide weed control during extended periods of heavy rainfall.  Reduced length of weed control is more apparent when lower rates are used and when the soil is coarse (sandy) in texture and organic matter is low.  Split the herbicide treatments into more than one application to compensate for herbicide loss due to heavy rainfall.

Example 1:  Tomato growers use metribuzin to control annual broadleaf weeds.  Metribuzin can be applied preplant incorporated, postemergence, and late postemergence in tomatoes.  Crop safety and weed control are both improved by splitting the total rate applied into three applications, compared to a single application at a higher herbicide rate.

Example 2:  Transplanted cabbage growers have several herbicide options to consider.  Treflan, Dacthal and Prefar have “Very Low” solubility in water and are “Strongly” adsorbed to the soil. Dual Magnum has “Moderate” solubility in water and is “Moderately” adsorbed to the soil.  You should expect Treflan, Dacthal and Prefar to leach more slowly than Dual Magnum, and resist leaching in wet years.  You should also expect Dacthal and Prefar to require more rainfall or overhead irrigation for activation than Dual Magnum in dry years.  Goal has “Very Low” solubility in water and is “Very Strongly” adsorbed to the soil, therefore Goal does NOT Leach away from the soil surface.  Goal is so “Very Strongly” adsorbed to the soil that any mechanical incorporation or cultivation deactivates the herbicide, resulting in poor weed control!  Stinger has “High” solubility in water and is “Weakly” adsorbed by soil.  Stinger is most likely to be leached by extended periods of high rainfall resulting in reduced length of weed control.  This is one reason Stinger is applied as a postemergence herbicide for annual broadleaf weeds in cabbage.

Example 3:  Sweet corn growers often apply atrazine preemergence for annual broadleaf weed control. Atrazine’s solubility in water is “Low to Moderate” and adsorption onto soil is “Moderate” when the soil pH is between 6.0 and 7.0.  Princep’s (simazine) solubility in water is “Very Low” and adsorption onto soil is “Moderate to Strong”.  Atrazine is more easily activated, requiring less rainfall than Princep, but can be leached below the zone of weed seed germination by extended periods of heavy rain, especially in coarse textured (sandy) soils.  Princep requires significantly more rainfall for activation, but is unlikely to leach below the zone of weed seed germination.  Tank-mixing atrazine and Princep provides more consistent weed control in dry and wet years.

What YOU Should Do:  All herbicides will be affected to some degree by extended periods of excessive rainfall. Expect herbicides with “Very Low” or “Low” solubility in water and/or “High” soil adsorption to be least affected.  Soil adsorption is more important in fine (clay) textured soils than in coarse (sandy) soils.  Herbicides that have “Moderate” solubility in water and “Moderate” adsorption will be more affected, and herbicides that have “High” solubility in water and “Weak” soil adsorption will be most affected by heavy rainfall.

1.  If you have overhead irrigation for herbicide activation, choose herbicides that have low water solubility and high soil adsorption to control the weed species in your field, when you have a choice.

2.  Tank-mix two herbicides with different solubility and adsorption characteristics, such as Atrazine and Princep for sweet corn in Example 3.

3.  Split the herbicide treatment into two or more smaller applications when the label allows, such as the Metribuzin for tomatoes in Example 1.

4.  Be prepared to apply a postemergence treatment such as Stinger in cabbage in Example 2, when available, when preemergence herbicides do not provide full season control.