2,4-D (2,4-Dichlorophenoxyactetic acid) is one of the oldest synthetic herbicides on the market first described in 1941, yet we still receive a number of questions about its use.
With the increased adoption of no-till, it remains a very important tool for managing problem weeds.
There are a number 2,4-D products in the marketplace with several different formulations. Most commonly, we talk about amine (mostly dimethylamine salt) solution and low volatile ester (mostly isooctyl or 2-ethylhexyl ester) emulsifiable concentrate (EC).
More recently within the ester group, solvent-less formulations are now available that have less phenoxy-associated odor. The ester formulations are commonly used in burndown applications because of their lower water solubility and greater activity under cooler air temperatures (and greater volatility), while the amine formulation is generally used postemergence during the summer months (with lower volatility).
In corn and soybeans, the recommendation for 2,4-D ester is to delay planting a minimum of 7 days after application at rates up to 1 pint per acre.
With the increasing concern for glyphosate resistant weeds, using 2,4-D ester in the burndown application can be an important resistance management tactic. However, the legitimate use of 2,4-D for burndown in crops other than corn and soybean is less certain.
Most 2,4-D labels state not to replant treated fields in the same growing season with crops other than those labeled for 2,4-D use. A number of labels state in the Fallow or Crop Stubble section “only labeled crops can be planted within 29 (or 30) days of application and all other crops, 30 or more days after application.”
However, if you read the fine print, “there may be a risk of injury to susceptible crops and under normal conditions, any crop may be planted without risk of injury if at least 90 days of soil temperatures above freezing have elapsed since application”.
Finally, some labels have a Fallow Land statement that may say, “do not plant any crop for 3 months after treatment or until 2,4-D has disappeared from the soil”.
What does this all mean? In practical terms, the half-life of 2,4-D (time for 50% of the herbicide to degrade) is 7 to 10 days in warm, moist soil.
So if you apply 1 pint/acre, ½ pint remains after 7 days, ¼ pint remains after 14 days, and by 21 days after application, only 1/8 pint could be remaining. Obviously, 30 days out, 1/16th of a pint or 0.031 lb. could remain in the soil which is generally not enough to harm even sensitive crops like alfalfa or clover.
Rates greater than1 pint per acre or planting sensitive crops not on the label prior to 30 days after application obviously carry greater risk for injury (or illegal herbicide residues).