Slow planting progress this spring has led to all efforts being focused on getting the crop in the ground. Often the first operation thrown to the wayside is application of preemergence herbicides in soybeans. A decade ago this practice could be successful, but with widespread multiple herbicide resistance now showing up in fields, skipping the application of the residual herbicide either sets up an imminent control failure to control weeds or contributes to further evolution of herbicide resistance.
That warning comes from Bob Hartzler, Iowa State University Extension weed management specialist.
"Applying preemergence herbicides ahead of planting provides the advantage of increasing the likelihood of the herbicide being activated by rainfall prior to weed emergence," he explains. "And it reduces the risk of injury from certain preemergence herbicides, particularly the group 14 herbicides used in soybean production. However, application of residual herbicides after planting still provides major benefits in terms of protecting yield potential, controlling resistant biotypes and reducing selection pressure from postemergence herbicides."
Adjust strategy to include applying residual herbicide after planting
Hartzler says the Group 3 herbicides (Prowl, etc.) and group 14 herbicides (Authority, Sharpen, Valor, etc.) must be applied before soybean emergence. Group 15 herbicides (Dual II Magnum, Outlook, Warrant, Zidua, etc.) can be applied after soybeans have emerged, but these herbicides do not have activity on emerged weeds. If weeds are present at the time of application, you should include an appropriate postemergence product to control the emerged weeds, he advises.
"The limited time to complete spring field operations rarely allows things to fall in place exactly as one might have planned," Hartzler notes. "However, the days when weeds could be treated as an afterthought are over. If planned preplant applications are bypassed due to delays in planting, you should adjust your weed management strategies to include post-planting residual herbicides."
Planting restrictions when using 2,4-D as burndown treatment
On another weed management topic, Hartzler is reminding farmers that if 2,4-D is used as a pre-plant herbicide for corn or soybeans, there are recommended planting restrictions of 1 or 2 weeks depending on rate applied. He provides these guidelines.
Weeds present at planting time in no-till fields are commonly controlled by adding 2,4-D to glyphosate. Its use broadens the spectrum of weeds controlled, provides more consistent control during cool weather, and reduces selection pressure for glyphosate resistance compared to glyphosate only. The primary disadvantages of including 2,4-D are the added cost and the potential for crop injury.
Crop injury risk is minimized by following the planting delays stated on product labels. Ester formulations of 2,4-D are recommended over amine formulations for pre-plant applications, due to a slightly shorter half-life and less mobility in the soil profile. Planting seed at the proper depth and ensuring closure of the seed furrow also is important in managing risks.
The restrictions regarding corn planting are based solely on injury risk and vary among manufacturers. For soybeans, the restriction is based on both injury risk and residue tolerance, therefore they are uniform on all products.
Planting restrictions following pre-plant applications of 2,4-D. (Rates are based on products containing 4 lb. of active ingredient per gallon.)
Soybeans: For 2,4-D ester—wait 7 days following 1 pint; 30 days following 1 to 2 pints applied per acre. For 2,4-D amine—wait 15 days following 1pint; 30 days following 1 to 2 pints applied per acre.
Corn: For either 2,4-D ester or amine—wait 7 days following 1 pint applied per acre of 4 lb. active ingredient per gallon; 14 days following 1 to 2 pint application rate.