With the weird weather we have been experiencing, there are areas of the state that have already planted and sprayed pre herbicides, while others are still waiting. In some cases, due to the initial lack of rainfall, soil-applied herbicides may not have been properly activated for weed control in certain earlier planted fields. And with the recent deluge this past weekend, there is concern about how these various factors affect herbicide performance.
Heavy rainfall and herbicides
In those areas with regionalized heavy rainfall and flooded fields, if residual herbicides were already applied and water (and soil) is moving off of the field, some of the herbicide will likely be taken with it or it will simply degrade and lose activity. However, each herbicide will be affected differently depending on its water solubility and soil adsorption characteristics.
Sandy or coarse soils with low organic matter have low cation exchange capacities (CEC) and thus will not adsorb as much herbicide as those soils with more clay/silt and higher organic matter content with much higher CECs. Furthermore, the less soluble the herbicide, the more moisture (i.e., rainfall) will be required to activate the product and move it into the root available zone. If the herbicide is highly water soluble it can be mobilized with much less rainfall. These same principles apply to excessive rainfall situations.
Under heavy rainfall events (>4 inches) herbicide can be leached out of the zone and down into the soil profile which will negatively impact herbicide efficacy. Low solubility herbicides include atrazine, Balance Flexx, simazine, Prowl, Valor, and Zidua. While medium and high solubility products include Callisto, s-metolachlor, acetochlor, metribuzin, Outlook, Pursuit, Reflex, Spartan/Authority, and Stinger. (For additional information on water solubility and herbicides, refer to Tables 2.2-6 and 2.4-6 in the 2021-22 Penn State Agronomy Guide.)
Scouting fields is important
Since it can be difficult to determine if herbicides have leached or moved off sight, it is best to monitor those fields for lack of weed control or weed escapes over the next few weeks. Post herbicide applications may be necessary for appropriate weed control. Also, if a field needs to be replanted to corn or another crop, remember to adhere to the crop rotation guidelines of the most restrictive herbicide that was applied. Refer to Table 2.1-11 in the Penn State Agronomy Guide or the product label.
All soil-applied herbicides require rainfall to mobilize them for effective weed control. In general, rainfall should occur within 7 to 10 days after application or before weed emergence. As a general rule of thumb, a ½ inch of rain is considered the minimum depending on current soil moisture levels and the herbicide used; ¾ to 1 inch is ideal. The less mobile materials (Prowl, Atrazine) and deeper germinating weeds (e.g. yellow nutsedge, cocklebur, velvetleaf, ragweed, etc.) will require more rainfall for effective mobilization and activation into the seed germination zone.
Keep in mind that many small seeded annual weeds can germinate with minimal moisture. Thus, sometimes you may observe weed emergence before enough rainfall has activated the herbicide for effective kill. If 10-14 days have passed without rainfall following a pre treatment and weeds are starting to break, start planning for a post herbicide rescue application (or where possible, consider using a rotary hoe or that old cultivator that you have been thinking about trying again).
Some “reach back” or “recharge” can be expected on small annual weeds (esp. broadleaves) with some herbicides when rainfall occurs, although depending on this may be a little like gambling. In particular, the HPPD (Group 27) herbicides (Acuron, Balance, Corvus, Lexar, Lumax, etc.) tend to have better “reach back” potential then some other herbicides and escaped grass control is probably of greater concern.
The Group 5 herbicides (Photosystem II inhibitors) like atrazine, simazine, and metribuzin will also control small emerged susceptible broadleaves via root uptake.