Last week in this column Extension Agronomist Bill Curran discussed how dry weather will influence soil applied and post herbicide performance. We know that soil applied herbicides require rainfall to mobilize them for effective weed control and that generally it must occur within 7 to 10 days after application or before weed emergence. We have observed and are also hearing that annual weeds are breaking through some fields that did not receive an activating rainfall.
In our Palmer amaranth trial this year, we observed emerged seedlings on May 12 in eastern Centre County. We included some additional glyphosate in some PRE treatments with the hope that this will control small emerged weeds. On May 8 we prepared a final seedbed and planted corn at our Rock Springs research farm, but did not apply PRE treatments until May 11. Giant foxtail was already breaking through. The research farm received about 0.3 inch of rainfall that night, so we’ll see if this helps control these emerged annuals. As a general rule, if the seedlings have developed true leaves and are still healthy, then it is unlikely that the soil applied herbicide will still provide control.
As mentioned last week, the exception would be herbicides belonging to the HPPD inhibitors such as Acuron, Balance, Corvus, Lumax Lexar, Instigate, and Prequel (Group 27) and Group 5 or the photosynthesis inhibitors (atrazine, simazine, and metribuzin) for control of annual broadleaves. The good news is that once the soil applied herbicides receive activating rainfall that should provide the residual control you expected. The bad news is that you will almost certainly need to come back with POST control. The emerged seedlings will not compete with the cash crop for a few more weeks, but now would be a good time to scout fields and perhaps plan for POST control a bit earlier than you might have otherwise.