How Long Will Cotton Residual Herbicides Lay on the Soil Surface Awaiting an Activating Rainfall?

Several questions have come in over the past week or so regarding how long residual herbicides will lay on the soil surface without rainfall and still have activity when rainfall does arrive.  This is a very difficult question to answer and the answer depends on many factors.  Some herbicides are broken down by sunlight, some by water, and some by microbial activity, as well as by other processes.  In addition, herbicide formulation, application rate, soil texture, and residue on the soil surface can play a part in how much herbicidal activity will be achieved once rainfall is received.

Generally speaking, if rainfall is received in 7 to 10 days after herbicide application, a good deal of residual activity will occur.  If rainfall is not received within 10 days of application, the amount and length of residual activity becomes very cloudy.  Having said this, keep the following in mind.  During the period of time between when a herbicide is applied and rainfall is received, weeds may still germinate and create issues if moisture is present in the soil.  The residual herbicide did not fail in this instance, but an activating rainfall was not received to ensure herbicidal activity.  This phenomenon may make it appear that no residual activity was achieved; however, you may still get activity which may prevent some level of weed seed germination after rainfall is received.

If you have applied a residual herbicide and not received rainfall in 10 days or so, a second application of a residual herbicide should be considered.  Make sure that label directions are followed with respect to number of applications and total application rate allowed per year for a given herbicide.