Herbicide application
- Herbicides vary in the amount of precipitation needed for activation.
- Activation depends on a number of factors including the solubility of the herbicide, sensitivity of the weed to the herbicide, and other factors.
- 0.25 to 0.75 inches of precipitation within the first week after application of a residual herbicide is the minimum amount needed to activate soil applied herbicides.
As we approach warmer weather, preplant and preemergence spray applications that contain residual herbicides will commence as soil and weather conditions allow. Herbicides that have soil residual activity require moisture for activation. What this really means is that the herbicide has to be dissolved in the soil water (aka “soil solution”) and moved down into the upper inch or two of the soil profile, so it can be taken up by the germinating weed seedling roots or shoots. When soil conditions are dry, herbicide molecules will remain closely associated with soil particles and are not able to move into weed seedlings via mass flow processes. As a result, weed control with soil-applied herbicides under dry conditions can be less than desirable as we saw in much of the corn-belt in 2023.
We are asked quite often how much rainfall it takes to activate a soil-applied, residual herbicide. The answer depends on many factors, which include:
- How water-soluble the herbicide is?
- How sensitive the weed species is to the specific active ingredient?
- What stage is the weed seedling at when exposed to the herbicide?
- Did the weed seedling receive a high enough dose to overcome any natural herbicide tolerance or metabolism mechanisms?
- How moist was the soil when the herbicide was applied?
As you can see, the answer to the question “how much moisture is required to activate my herbicide” requires consideration of several factors.
A quick review of several herbicide labels (but not all) shows the following edited comments with regards to precipitation and herbicide activation:
| Single-Active Ingredient Products |
Precipitation Required for Activation |
| Metolachlor |
0.5 inches on coarse soils, 1 inch on fine-textured within 2 days after application |
| Dimethenamid-P |
Nothing about precipitation amounts mentioned |
| Acetochlor |
0.25 to 0.75 inches within 7 days after application |
| Pyroxasulfone |
When adequate moisture is not received after application, weed control may be improved by irrigation with at least 0.25-acre inch of water. |
| Flumioxazin |
0.5 inches before weed emergence |
| Sulfentrazone |
If adequate moisture (0.5 to 1 inch) from rainfall or irrigation is not received within 7 to 10 days after application, a shallow incorporation may be needed to obtain desired weed control. |
| Atrazine |
Nothing about precipitation amounts mentioned |
| Metribuzin |
In areas of low rainfall, preemergence applications to dry soils should be followed with light irrigation of 0.25-acre inch of water. |
| Isoxaflutole |
Most effective in controlling weeds when adequate rainfall is received within 14 days after application |
As you can see, the answer varies by herbicide. As a rule of thumb for most soil-applied herbicides, we would like to see about 0.75 to 1 inch of precipitation within the first week. Also, we would like to see approximately 2 inches of precipitation spread out over the first two weeks after the herbicide was applied for optimal herbicide performance. Some herbicides can “recharge” or “reach back” on small, emerged weeds when rainfall occurs. This can be a roll of the dice though. The HPPD inhibiting (Grp. 27) herbicides, specifically those containing isoxaflutole and mesotrione, tend to have better “recharge” potential than some others. Grass escapes are most likely a greater concern. Photosystem II inhibitors (Grp. 5) like atrazine and metribuzin will also control small, emerged broadleaf weeds via root uptake if a precipitation event is delayed. In the event precipitation is not received and herbicides are not activated, and weeds start to break, consider a rotary hoe pass to dislodge small seedlings or move your postemergence application up to timely control the weeds. For questions regarding herbicides for operation, reach out to your local FS Crop Specialist.