Two questions are on farmers minds. First, how long will soil-applied herbicides ‘last’ in the soil if it doesn’t rain and second, should a farmer consider using a rotary hoe or drag harrow to incorporate herbicides?
Volatility (evaporation), adsorption, and soil moisture effect soil-applied herbicides. Volatility is the change in herbicide physical state, from a liquid to a gas. Most soil-applied herbicides used by farmers have a medium or low vapor pressure meaning they generally will not volatilize during warm and dry conditions. However, understand that herbicides sprayed on soils will move with blowing soil and these effects may impact efficacy. Adsorption is the attachment of herbicides to soils. Herbicides must be bound to soils or they would easily leach away. Most herbicides are moderately or strongly bound to soils colloids and should not be impacted by our dry conditions.
Soil moisture (and rainfall) affects soil-applied herbicides in two ways. First, rainfall moves the herbicide from the soil surface and into soil. Second, rainfall contributes to the amount of herbicide available for absorption by weeds. While ‘half an inch’ is a good rule of thumb to activate herbicides, soil moisture conditions at or after the time of soil-applied herbicide application will influence herbicide activation. Rainfall must first wet the soil surface before water and the herbicide can move into the soil profile under dry conditions. Additionally, herbicides bind more tightly to soils and are less active for weed control in dry conditions. Thus, under our dry conditions, it might take more than 0.5 inch of rainfall for satisfactory levels of activation and resultant weed control. But on the other hand, your herbicide should be ‘there’ and available for activation once we get rain...provided the soil does not blow.
Our soils are extremely mellow this year due to the dry conditions during tillage. Running a tractor and pulling an implement across fields may disturb too much soil and may break the ‘herbicide barrier’ and thus do more harm than good.
We recommend farmers continue to use soil-applied herbicides for grass and small seeded broadleaf weed control in row crops in freshly planted row crops even though it has been dry. Remember, farmers are using soil-applied herbicides for control of shallow seeded broadleaf weeds such as common ragweed, lambsquarters and waterhemp. These weeds germinate at or near the soil surface and are in dry soils. We also know waterhemp will not germinate and emerge until mid to late May in our climate and we eventually will get much needed precipitation to activate herbicides.
There might be weed escapes from deep germinating weeds like giant ragweed or wild oat that germinate and emerge from moisture and will need to be controlled with postemergence herbicides.