By Tom Peters
Two questions are on farmers minds. First, how long will soil-applied herbicides ‘last’ on the soil surface 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. 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. Herbicides can lay on the soil surface for 7 to 10 days, perhaps even 2-weeks without loss of efficacy.
A greater concern in 2016 is blowing soil, especially soil-applied herbicides attached to blowing soils. It is very possible that herbicide will move from target field to off-target fields with blowing soils. Damage once activated might be similar to spray drift; damage greatest near the source and diminishing 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 ‘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.
Can I assist Mother Nature by using light tillage such as a dray harrow or rotary hoe? It depends on your purpose. I would consider light tillage if the concern is small seeded broadleaves starting to emerge. I would not consider light tillage to incorporate the herbicides as good intentions may lead to steaks from inadequate or uneven incorporation?