Residual herbicides are a key component of effective weed management programs. Applying multiple residual herbicides is a recommended practice, with combinations generally designed for effectiveness on the range of weed species present in the field. In addition, selecting multiple effective herbicides is increasingly important to mitigate resistance to residual herbicides, such as the Group 14 herbicides [flumioxazin (Valor, others) and sulfentrazone (Spartan, others)] and the Group 15 herbicides [acetochlor (Harness, Warrant, others), dimethenamid (Outlook), S-metolachlor (Dual, others), and pyroxasulfone (Zidua, others)]. One factor that may be overlooked when selecting herbicide combinations is the water solubility of the herbicides.
Water solubility is a numeric value, usually reported in the metric unit mg/L. It indicates how much of the herbicide's active ingredient can be dissolved in water at room temperature. The solubilities of some residual herbicides are listed in Table 1.
In general, herbicides with a solubility of less than 100 mg/L have low solubility, while herbicides with a solubility greater than 1,000 mg/L are considered to have high solubility. Herbicides with relatively low water solubility will be more easily ‘activated’ or available for plant uptake via water present in the soil when compared to herbicides with relatively high water solubility. Herbicides with relatively high solubility will require more rainfall to move the herbicide to the zone where weed seeds are germinating and/or roots are growing. Conversely, the duration of residual activity provided by herbicides with high water solubility will be shortened if the herbicide is lost from the root zone by rainfall or irrigation. However, herbicides with lower water solubility will not be as readily leached. Herbicide availability for plant uptake or leaching out of the root zone is also affected by the strength with which the herbicide is bound to clay and organic matter in the soil, which is not necessarily a function of water solubility. The strength of the herbicide binding to clay and organic matter is called sorption. This interaction's effects are reflected in how some residual herbicides adjust application rates based on soil texture or organic matter content.
Table 1. Herbicide solubility and precipitation requirements for several commonly used active ingredients.
|
Herbicide active ingredient |
Example |
SOA Group |
Precipitation requirement on label |
Water solubility (mg/L at 68 F) |
Solubility category |
|
Dicamba |
Clarity |
4 |
adequate |
250,000 |
Very high |
|
Atrazine |
Aatrex 4L |
5 |
sufficient |
35 |
Moderately low |
|
Metribuzin |
Tricor |
5 |
1/4" |
10,700 |
High |
|
Diflufenican |
Contrivo |
13 |
N/A |
0.05 |
Very low |
|
Flumioxazin |
Valor |
14 |
activating |
0.8 |
Very low |
|
Sulfentrazone |
Spartan |
14 |
1/2-1” |
784 |
Moderate |
|
Acetochlor |
Harness |
15 |
1/4-3/4” |
282 |
Moderate |
|
Dimethenamid-P |
Outlook |
15 |
needed |
1,499 |
High |
|
Flufenacet |
Component of Trivolt |
15 |
N/A |
51 |
Moderately low |
|
Pyroxasulfone |
Zidua |
15 |
1/2” |
3.5 |
Low |
|
S-metolachlor |
Dual II Magnum |
15 |
1/2-1” |
480 |
Moderate |
|
Bicyclopyrone |
Component of Acuron |
27 |
N/A |
1,200 |
High |
|
Isoxaflutole |
Balance Flexx |
27 |
adequate |
6.2 |
Low |
|
Mesotrione |
Callisto |
27 |
1/4” |
1,500 |
High |
Combining herbicides with different water solubilities can reduce the risk of residual herbicide failure. Precipitation during the first two weeks after an herbicide application is the key factor influencing the effectiveness. In addition, developing a tank mix that includes herbicides with high, medium, and low solubility helps ensure at least one residual herbicide is activated across a range of precipitation conditions.
For example:
As stated above, herbicide availability is a function of both water solubility and sorption, so this is not a strict rule. Still, it can be a useful guide to ensure you have the best residual herbicide program possible.
For more detailed information, see the “2025 Chemical Weed Control for Field Crops, Pastures, and Noncropland” guide available online at https://bookstore.ksre.ksu.edu/pubs/SRP1190.pdf or check with your local K-State Research and Extension office for a paper copy.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Weed Management Specialist
slancaster@ksu.edu
Jeanne Falk Jones, Multi-County Specialist
jfalkjones@ksu.edu