One of the most often asked questions Iowa State University crop specialists get in late April and early May deals with putting a little 2,4-D in with burndowns to help on some of the winter annual weeds -waterhemp, giant ragweed, etc. Here is the short explanation from ISU weed control specialists, as to what you can do, as allowed by the herbicide label.
Since the EPA issued approval for pre-plant application of 2,4-D in soybeans, most 2,4-D labels have carried similar recommendations.
For the use of 2,4-D ester products, the standard recommendation allows application of up to 0.5 lbs. active ingredient per acre (1 pint of a 4 lb/gal formulation) at least 7 days before soybean planting. Rates higher than 0.5 lb, but not exceeding 1 lb (1 quart of a 4 lb/gal formulation) must be applied at least 30 days before soybean planting. These recommendations are designed to prevent the 2,4-D ester from injuring soybeans.
Follow label directions to avoid bean injury
Avoidance of injury to soybeans is based on the lack of water solubility of ester formulations, which limits downward movement into soil, and the relatively rapid degradation of 2,4-D in soil. These are generally considered to be conservative recommendations, especially with regard to the 30-day restriction for the 1 lb. rate. Don't accidentally use the amine formulation of 2,4-D.
With glyphosate-resistant Marestail on the way (if not here already), if you have a field that has much marestail pressure, ISU crop specialists would recommend adding 2,4-D to the mix. That is, if it can work with your bean spraying and planting program.
What about 2,4-D prior to corn planting?
Labels vary regarding use of 2,4-D relative to corn planting. Labels for a number of products, including Weedar and Weedone, recommend application either 7 to 14 days before corn planting or 3 to 5 days after planting before the corn has emerged. Labels for other products allow application anytime after planting.
Corn injury can occur when 2,4-D ester is applied at the time of corn planting, especially when combined with acetamide grass herbicides (Harness, Degree, Keystone, Dual, Cinch, Bicep, Outlook, Define, etc). Injury is most likely to occur in coarse-textured soils with low organic matter content, and when above-average rainfall and prolonged soil moisture occur within a week after planting.
Having looked at a few fields almost every season, injured from what is suspected to be pre-emergent 2,4-D related, if the field isn't a total weed disaster, ISU crop specialists recommend asking about other burndown options.
While it is true that the pre-emerge grass herbicides occasionally can cause similar symptoms, the addition of 2,4-D into the equation can increase the risk and severity of injury, says ISU extension weed specialist Bob Hartzler.
You may want to use a safer alternative
Some older, obscure research done at ISU documented that there can be yield loss associated with 2,4-D/acetamide injury. "But admittedly many dealers, including me, have used this program since the inception of no-till with great success," says ISU Extension crop specialist Clarke McGrath. "However, with the low cost of glyphosate products today, pre-emerge (after planting) options such as removing the 2,4-D from the tank mix and using safer alternatives like glyphosate don't add much cost."
If you get into an at-planting or early pre burndown situation, ask your local dealer about alternatives such as glyphosate. Or Aim, Sencor, Atrazine, Gramoxone or other suggestions. Make sure the seed trench is closed to limit movement of herbicides into the seed zone.
Try to spray when daytime temps are projected to be around 60* or above (you can still spray when it is colder, talk to your local dealer about adjusting rates).
Follow planting delays on labels if you use 2,4-D or dicamba burndowns.
Use the hottest additives available if the crop isn't up yet (your dealer can help with this). As always, be sure to follow label directions to avoid crop injury.
Here is a great quick reference burndown chart from (it hurts to say this) NU: cropwatch.unl.edu/archives/2006/crop5/weedtables.pdf.