Planting occurred at a near record pace in much of Iowa due to lack of rain in late April. This string of dry weather also allowed most herbicide applications to be made in a timely fashion, but now there is concern about herbicide performance after prolonged time on the soil surface.
Role of rain Preemergence herbicides (PRE) must be present within the soil profile when weed seeds begin to germinate to have maximum effectiveness, they are inactive while on the soil surface. It typically takes a half inch of rain to move herbicides into the soil profile where weeds germinate, there are relatively small differences among herbicides in the amount of rain required to activate. Most consistent performance occurs when rain occurs with 5 to 7 days of application, especially if soils are warm enough for weed germination.
The second role of rain is to keep a portion of the herbicide dissolved in the soil solution so the chemical can be absorbed by germinating seeds. Residual herbicides are less active in dry soil due to less herbicide being in solution. The terms reactivation and recharge are used to describe the activity of herbicides on weeds that escaped control due to dry conditions during germination. Group 27 herbicides display this property more frequently than other herbicide groups, but these products still perform best when available while seeds germinate.
Fate of herbicides on soil surface Herbicides typically are more persistent in dry soil due to lower degradation rates. However, in some cases herbicide may be lost from the soil surface. Volatilization is a concern since the soil surface reaches temperatures much higher than air temperatures. Several older herbicides (e.g. trifluralin, EPTC, propachlor) had relatively high vapor pressures, and if left on the soil surface enough herbicide could be lost to compromise performance. Photodegradation can also be a source of loss for certain herbicides from the soil surface. For most products I don’t think sufficient herbicide will be lost from photodegradation or volatilization to compromise activity (Appendix).
Another concern is wind erosion. Much of the state experienced very strong winds earlier this week that undoubtedly moved herbicide on the soil surface. I suspect there are areas within many fields where enough herbicide was moved to limit activity once rain occurs to activate the herbicide. This risk will be influenced by field topography and tillage practices, crop residue on the soil surface should reduce movement. I suspect wind movement will be more of a concern than losses due to volatility or photodegradation in most fields.
Herbicide performance Herbicides on the soil surface are not absorbed by germinating seeds, thus any weeds that germinate will be unaffected by this herbicide. Fields at greatest risk for weed escapes are those planted as soon as they were fit to work. In this situation there probably was enough soil moisture in the upper soil surface for weeds to germinate. Fortunately, soil temperatures during early planting were cool enough that many of our important weeds (foxtails, waterhemp) hadn’t begun to emerge. However, early-season weeds such as giant ragweed and common lambsquarters could easily escape due to lack of activation.
In some areas surface soils might have dried sufficiently at the time of planting/PRE application that few if any weeds will germinate until more rain is received. In these situations, the main concern is the loss/movement of herbicide on the surface.
When rainfall occurs the herbicide on the soil surface will be moved into the profile where it will be active. It takes slightly more rain to activate herbicides with dry soil since the initial rain wets the soil rather than percolates through the soil, carrying the herbicide into the profile.
What actions can be taken to minimize performance failures? Weed scientists of my age like to promote the use of rotary hoes or harrows when rain fails to provide timely herbicide application. These tools move some of the herbicide off the surface, but more importantly control weeds that have germinated and escaped the PRE. For maximum effectiveness they should be used while weeds are in the white root stage – germinated but not yet emerged. While they are operated at high speeds and thus cover acres quickly, they are not as effective in fields with high amounts of crop residue on the soil surface.
Probably the most important thing to do is to systematically scout fields beginning about 14 days after planting. PRE performance across fields likely will be more variable than in other years, so be sure to cover the entire field.
Some fields will require an earlier POST application than normal to control weeds that escaped the PRE treatment due to lack of activation. Consider including a layered residual with these POST applications since there will be a longer interval between the POST application and canopy closure.
Since it is impossible to know whether significant herbicide was lost from the surface, there is no need to ‘blindly’ try and supplement the herbicide with additional residual herbicide. Use scouting to determine when to make POST applications at the optimum time, and knowledge of weed history to guide the need for a layered residual with the POST treatment.