Weed control may be improved when more than one active herbicide ingredient is included in the burndown application. Burndown applications often include growth-regulator herbicides, such as 2,4-D. Both amine and ester formulations of 2,4-D are labeled for preplanting burndown applications, but the ester formulation is usually preferred over the amine formulation.
Hager explains, "The low water solubility of an ester makes it less likely to be moved into the soil by precipitation, where it could injure germinating crop seeds." Also, esters are better able to penetrate the waxy surfaces of weed leaves, so they provide better control of large weeds, especially when air temperatures are cool.
Some 2,4-D ester formulations can be applied without a waiting period before planting corn, while a seven-day wait is recommended for others. "When applied prior to planting soybean, wait at least seven days between application of one pint per acre and planting, or 15 to 30 days between application of up to two pints per acre and planting," said Hager. "Pay careful attention to label statements of any 2,4-D formulation used prior to crop planting."
Some retail applicators might be experiencing difficulty acquiring sufficient quantities of 2,4-D to meet the need for burndown applications. "Substituting an amine formulation of 2,4-D for an ester formulation is possible," Hager said, "but remember, there are significant differences between these formulations." Specifically:
- Higher application rates might be necessary because an amine formulation does not penetrate the weed's leaf cuticle as easily as an ester formulation does,
- Amine formulations are less volatile but more water soluble than ester formulations, meaning that they are more likely to move into the soil after precipitation, increasing the likelihood of crop injury,
- The interval between application of 2,4-D amine and soybean planting is longer (15 days for one pint or less, 30 days for greater than one pint) than the interval with ester formulations.
Dicamba is another growth-regulator herbicide used to control existing vegetation before planting. Several commercially available products contain dicamba, but not all of them are labeled for application prior to crop (especially soybean) planting.
"In comparison with 2,4-D, dicamba provides greater control of chickweed, henbit and prickly lettuce, comparable control of dandelion, and less control of horseweed and mustard species," said Hager.
Application rates used in burndown applications can impact the interval between application and planting. For example, Clarity can be applied at up to 16 fluid ounces per acre before planting corn (up to eight fluid ounces on coarse soils or medium- and fine-textured soils with less than 2.5 percent organic matter) with no interval between application and planting. However, the 16-ounce use rate requires a minimum accumulation of one inch of precipitation and a 28-day waiting interval between application and soybean planting. An eight-ounce application rate still requires a minimum accumulation of one inch of precipitation, but the waiting interval is only 14 days. These intervals must be observed prior to planting soybean or crop injury might occur.