Soil-applied, pre-emergence herbicides applied at the beginning of the growing season require rainfall or irrigation for activation. This water turns the herbicide into a solution that can be taken up by weeds. About 0.5 to 1.0 inch of rain/irrigation is needed to activate most soil-applied herbicides. Too much rain, however, will cause the herbicide to become diluted and leach or wash away. Heavy precipitation in southern and eastern Nebraska the last two weeks has created areas of standing water in corn and soybean fields. Most corn planting was underway or completed and soybean planting had been started before the rains started. Pre-emergence, residual herbicides had been applied in some of these corn and soybean fields. Given the current conditions and continued forecast for rain, it is important to understand the effect of excessive rainfall on the efficacy of residual herbicides. This effect is complex and influenced by the soil and the chemical properties of the herbicide applied. The solubility of a herbicide in water helps determine its leaching potential. Herbicides that readily leach due to heavy rainfall may be carried away from weed germination zones, reducing their efficacy.

Soil Characteristics Major soil characteristics such as soil texture, organic matter content, and soil pH can affect residual herbicide efficacy. Sandy soil is coarse in texture and can be classified into sand, loamy sand, or sandy loam soil. Soils with a moderate amount of each size of soil particle are considered medium in texture and are classified as a loam or silt loam. Soils with a large percentage of clay particles are considered fine in texture and are called silty clay loam, clay loam, or clay. Soil particles are negatively charged and attract positively charged herbicides. The attraction of a positively charged herbicide to the negative charge of the soil particles slows leaching. A few herbicides have a negative charge and do not bind to the soil, making them more subject to leaching, especially if they are highly soluble in water. Residual herbicides must be applied based on organic matter content of the soil and soil type (texture). The herbicide label has information about residual herbicide rates for different levels of organic matter in each soil type. Figure 2. Heavy early season rains may have hampered and allowed weed escapes. In many areas farmer efforts to make timely post-emergence herbicide treatments also may be hampered by standing water in some fields.

Soil pH also affects the efficacy of some herbicides by influencing the degree of attraction to soil particles. Low pH (below 6.0) or high pH (above 7.0) soils may affect the availability of certain herbicides by changing the positive (+) charge of the molecule. If the herbicide is more tightly bound to the soil, weed control may be reduced and/or herbicide carryover may be increased. If an herbicide is less tightly bound to the soil and more available, crop injury may increase. Herbicides leach more quickly through coarse-textured (such as sandy) soils that are low in organic matter and will leach more slowly through fine textured (clay) soils and soils with high organic matter.

Herbicide Characteristics

Major chemical properties of herbicides that affect their persistence in the soil include

water solubility,

soil adsorption, and

microbial degradation.

The solubility of herbicide in water helps determine its leaching potential. Leaching occurs when herbicide is dissolved in water and moves down through the soil profile. Herbicides that readily leach due to heavy rainfall may be carried away from weed germination zones, reducing their efficacy.

Soil adsorption (Koc) is the ability of herbicide molecules to bind to soil particle surfaces. The higher the Koc value, the greater the adsorption of herbicide to soil colloids.

Other factors also affect herbicide leaching. These include herbicide-soil binding properties, soil physical characteristics, rainfall frequency and intensity, and herbicide concentration. In general, herbicides that are less soluble in water and strongly attracted to soil particles (adsorption) are less likely to leach. For example, simazine (Princep), pendimethalin (Prowl H2O), and trifluralin (Treflan) are less likely to be leached by rainfall compared with some other herbicides (Table 1).

Herbicides leach more quickly as solubility in water increases and adsorption decreases. For example, dicamba (Banvel, XtendiMax), metribuzin (Sencor/Tricor), and clopyralid (Stinger) are more likely to leach during extended periods of heavy rainfall due to their high water solubility and reduced adsorption. A reduced period 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.

Several residual herbicides applied pre-emergence in corn and soybean are pre-mixes of herbicides. For example, Acuron is a premix of four herbicide active ingredients, including atrazine, bicyclopyrone, mesotrione, and S-metolachlor. To consider the leaching potential of this herbicide mixture, consider the leaching potential of each of its component herbicides. The advantage here is some active ingredients in the premix will have more water solubility than others. For example, atrazine has low water solubility (33 ppm); Dual II Magnum has medium water solubility (488 ppm), and Callisto has high water solubility (2200 ppm).

The Bottom Line

The efficacy of herbicides will be affected to some degree by extended periods of excessive rainfall.

If irrigation is available in the field for herbicide activation, choose herbicides that have low water solubility and high soil adsorption, when you have a choice. Tank-mix two herbicides with different solubility and adsorption characteristics, such as atrazine and Callisto or consider using premix herbicides such as Acuron (Aatrex + bicyclopyrone + Dual II Magnum + Callisto) or Resicore (Stinger + Surpass NXT + Callisto). This will also provide multiple effective modes of action for effective management of herbicide-resistant weeds. Split the herbicide treatment into two applications when the label allows. Scout the field two to three weeks after applying the pre-emergence herbicide(s) to determine weed pressure. This will help you think about additional weed control options. Remember that the period of weed control will be reduced in soil that is coarse (sandy) in texture and has low organic matter. Be prepared to apply a post-emergence herbicide when weeds are less than 4 inches tall.