A recent article published by Purdue University Extension titled “Soil Applied Herbicides and Rainfall for Activation” reminds farmers about how soil-applied chemistries can either work or fail depending on the amount of rainfall fields receive after application.
Much of the Corn Belt is busy planting corn and even soybeans, so sprayers are not far behind with pre-emerge-type chemistries to control weeds during the early growing season. Residual herbicides (soil-applied or pre-type chemistries) work the same way; they require moisture for full activation and then act on emerging weed seedlings in some way. What really needs to happen for the herbicides to be successful is for the chemistry to be mixed with water in the soil and spread out to form a blanket of protection that weed seedlings must grow through. However, sometimes they fail to work.
Consider these contributing factors:
- How much water does it take for the active ingredient to mix into the soil solution and become the barrier it is intended to be? We refer to this as water solubility. Not all chemicals react the same, so you need to know and understand the solubility of your particular chemistry.
- How sensitive are weeds to the active ingredient? Again, not all chemicals are the same; some are better at controlling grasses, while others specialize in broadleaves. Knowing this simple characteristic about your chemistry is important.
- What is the lethal dosage for my particular active ingredient? While this factor is not widely known and published, weeds grow so fast in certain conditions, or soil is moved because of erosion, that weeds do not take in a lethal dose or enough of the active ingredient to kill the seedling.
Purdue University Extension created a table referencing known water required for activation of soil-applied chemistries. I recommend growers take a look at this table before getting the sprayer out this growing season. Growers can also reach out to their local Stine agronomist for additional guidance.