Most soil-applied herbicides are stable under longer than desired periods of weather with no adequate moisture and not broken down by UV sunlight. Those herbicides in the site of action group (SOA) 8 (butylate, cycloate, triallate) and SOA 3 (trifluralin, ethalfuralin, pendimethalin) are volatile and susceptible to degradation by UV light. In no-till, these herbicides are not widely used because of the need for mechanical pre-plant incorporation. S-metolachlor has been found to be broken down by sunlight but other herbicides in the same family are not (dimethenamid-P, acetochlor).
Activation & Longevity
If we look at the most-common soil applied herbicides used in no-till they may remain on the soil surface for an extended period of time. These herbicides start working at the moment of activation with water (rain or irrigation). The herbicides then move into the weed seed germinating section of the soil profile, acting on the weed seeds germinating at that time. A half an inch to over an inch of water is good to have for activation depending on the herbicide. If weeds germinate before herbicide activation occurs, then the weeds may not be terminated by the herbicide. A few herbicides containing pyroxasulfone, flumioxazin, sulfentrazone or saflufenacil could terminate small, emerged weeds after activation. If weed germination continues after herbicide activation, those weeds should be controlled for a certain period depending upon the herbicide.
Other factors effecting soil applied herbicide longevity and activity in the soil include: microbial degradation, leaching, run off, volatilization, organic matter, soil texture, soil cation exchange capacity (CEC) and soil pH.
For any questions contact Gared Shaffer.