It Has Not Rained. What is the Fate of my Preemergent Sugarbeet Herbicides?
Sugarbeet growers use soil residual herbicides at planting to control kochia or waterhemp. Ethofumesate applied at 2 to up to 5 pt/A controls early season waterhemp and ethofumesate at greater than 5 to 7.5 pt/A controls kochia in sugarbeet. Growers often mix Dual Magnum at 8 fl oz/A with ethofumesate at 2 pt/A in sugarbeet fields planted after April 20 since Dual Magnum takes less precipitation for activation and is readily available for control of early emerging waterhemp. The question on producers’ minds is what is the fate of either ethofumesate or Dual Magnum in fields that have receive no or trace amounts of rainfall?
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 are attached to soil colloids and generally will not volatilize during warm and dry conditions. However, herbicides may move with blowing soil and impact efficacy. Adsorption is the attachment of herbicides to soils. Herbicides must be bound to soils or they are easily leach out of the seedling zone. 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.
Ethofumesate or Dual Magnum have not degraded and are likely bound to surface soil. Where do they need to get and how are they taken up by waterhemp and how much time is left before waterhemp emergence? Dual Magnum and ethofumesate are herbicides inhibiting very long chain fatty acids (VLCFA) and are used preemergence or with shallow soil incorporation to control waterhemp. The VLCFAs affect waterhemp before emergence, but do not inhibit waterhemp seed germination. Likewise, VLCFA due not control emerged plant although we occasionally seed evidence of ‘reach-back’ with ethofumesate. The primary site of absorption and action of Dual Magnum (called a chloroacetamide herbicide) on waterhemp is the roots. Ethofumesate (called a benzofuran herbicide) are absorbed from the soil solution through both waterhemp roots and emerging shoots. The VLCFAs are not readily translocated in the plant, so herbicide placement and availability are important.
While both herbicides need to be in the seedling zone, there is more flexibility with ethofumesate since it is absorbed through both roots and shoots. However, Dual Magnum is more water soluble than ethofumesate meaning it takes less rainfall to incorporate Dual Magnum into the seedling layer. Since ethofumesate is more water insoluble, it lasts longer in the soil than Dual Magnum. We think Dual Magnum will be active in the soil for 2-3 weeks while ethofumesate will be active in the soil for 4-5 weeks.
We are unaware of any waterhemp germination and emergence in Minnesota or North Dakota at writing. Waterhemp emergence has been linked to growing degree days and soil temperature. We have not accumulated many growing degrees days and the soil remains cool. Matter of fact, agriculturalists and consultants report very little weed emergence at writing.
Extension Sugarbeet Agronomist
NDSU & U of MN