Farmers commonly add 2,4-D to glyphosate for use as a burndown herbicide treatment on existing weeds in no-till fields. However, this spring as wet weather keeps delaying the time available between spraying and planting, the wisdom of including 2,4-D in the burndown treatment is being questioned.
There are several advantages of including 2,4-D in with the glyphosate, says Bob Hartzler, Iowa State University Extension weed management specialist. It provides better activity on dandelion, horseweed and many winter annual broadleaf weeds than glyphosate alone. It provides more consistent control than glyphosate used along during cool conditions. It reduces the selection pressure for glyphosate resistant horseweed.
"The risk associated with 2,4-D use is injury to emerging corn or soybean plants," notes Hartzler. For soybeans, a seven-day interval between application of 0.5 pounds 2,4-D/A (2/3 part of a 6 pounds LVE formulation) and planting is required. For corn, most labels recommend applying up to 1 pounds acid equivalent 2,4-D per acre (1.3 pints low volatile ester 6) from 7 to 14 days prior to planting or 3 to 5 days after planting. Following these label restrictions minimizes, but does not eliminate, the threat of crop injury.
Avoid risk of injury to corn, bean seedlings
The risk of injury to both crops is determined by how much herbicide reaches the depth of the germinating seed and developing seedling. Hartzler says this is determined by several factors, including--depth of planting, 2,4-D rate and formulation, soil type and rainfall.
"Planting seed at proper depth reduces the risk of injury by providing a more favorable environment for germination and minimizing the amount of herbicide reaching the seed," says Hartzler. "Shallow planting or failing to close the seed furrow increases the risk of injury. Ester formulations are recommended for burndown applications because they are less mobile in the soil than amines, thus they are less likely to reach the seed. Adsorption of 2,4-D to soil colloids minimizes movement through the profile, thus injury is most likely to occur on coarse textured soils or soils with little organic matter."
"Finally, rainfall is required to move the herbicide through the profile to the depth of the emerging seedlings," he explains. "Since 2,4-D breaks down relatively quickly in the soil (approx. 10 day half-life), it is the rainfall that occurs within the first two weeks after application that determines the threat of injury. After this period the 2,4-D should have degraded to levels unlikely to injure the crop."
Corn is most sensitive to 2,4-D when the herbicide is present in the water that is initially imbibed by the seed, this is why 2,4-D can be applied shortly after planting, says Hartzler.
"2,4-D is a valuable tool in no-till systems, but it must be used properly to manage the risk of crop injury. In situations where the planting interval restrictions cannot be followed, alternative herbicide products are available," he adds. "The alternatives may not be as broad-spectrum as 2,4-D, thus their selection must be based on specific weeds present in individual fields."