Q: How long will soil-applied herbicides last on the soil surface if it doesn’t rain?
A: Volatility (evaporation), adsorption and soil moisture effect soil-applied herbicides. Volatility is the change in a herbicide’s physical state from a liquid to a gas. Most soil-applied herbicides have a medium or low vapor pressure, meaning they generally will not volatilize during warm and dry conditions. 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 dry conditions. Herbicides can lie on the soil surface for seven to 10 days, perhaps even two weeks, without loss of efficacy.
A greater concern is blowing soil, especially soil-applied herbicides attached to blowing soils. It is very possible that herbicide will move from target fields to off-target fields with blowing soils. Once activated, the herbicide may cause damage similar to spray drift. Damage would be greatest near the source of the herbicide and diminish with distance.
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 a half inch of rain 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 dry conditions, it might take more than a half inch of rainfall for satisfactory levels of activation and resultant weed control. On the other hand, your herbicide should be there and available for activation once it rains, provided the soil does not blow. — Tom Peters, North Dakota State University and University of Minnesota Extension sugarbeet agronomist
Q: What’s the minimum time interval in days after spraying glyphosate to harrow the field to scatter/break up straw and roll out rocks?
A: Small annual weeds may require only three days following application for maximum glyphosate absorption and kill. However, large winter annual weeds that have bolted, and biennial and perennial weeds may require five days before tillage should be performed. Plants that have bolted or have an extensive underground root system will require at least five days for the herbicide to translocate to stems and roots for optimal control. In summary, small weeds, warm temperature, high humidity will increase adsorption and possibly allow a shorter interval between herbicide application and tillage. Bolted winter annual weeds, biennial and perennial weeds will require the full five days to get maximum kill. — Rich Zollinger, NDSU Extension weed specialist