How Long Will Residual Herbicides Remain on the Soil Surface Without an Activating Rainfall?

Spring weather has proven challenging once again in 2016.  Cool, wet conditions persisted into early May with planting not really getting started on a broad scale until late in the first week of May or the beginning of the second week of May.  In many areas of the state, particularly those south of Highway 6, rainfall has been in short supply in the past four weeks.  Some have gotten rainfall but many have been dry until this week (some still are).  A number of folks have inquired about how long a residual herbicide will remain on the soil surface without an activating rainfall.  In general, if you experience two or more weeks of dry weather, you have likely lost the vast amount of that residual herbicide even if you were to get a rainfall event.

Given that many residual herbicides did not get incorporated by rainfall, a timely application of a POST applied residual herbicide should be strongly considered.  Most POST applied residual programs will include either S-metolachlor (Dual Magnum, Medal, etc.) or acetochlor (Warrant).  Each of these products are labeled for PRE or POST application in cotton and the labels essentially state that for POST application, the crop should be fully emerged and applications made prior to weed emergence (given that neither of these products has activity on emerged weeds).  A POST applied residual will also require an incorporating rainfall (or irrigation) to be activated.  Mother nature dealt a tough hand with the lack of rainfall after application of PRE herbicides; however, do not let that discourage you from applying residual herbicides POST.  Everyone knows the level of pigweed pressure in many areas of the state and residual herbicides are one of the best tools that we have to battle them with.