University of Illinois

No. 13/June 20, 1997

Preharvest Weed Control Treatments for Wheat

With the wheat crop nearing maturity in southern portions of Illinois, some producers may consider applying a preharvest herbicide treatment to control weeds that could make harvesting operations difficult. There are, however, only two herbicides that are labeled for this type of application.

Roundup Ultra may be applied using either ground or aerial application equipment. Applications must be made after the harddough stage of grain (30% or less grain moisture) and at least 7 days before harvest. Do not apply more than 1 quart per acre for preharvest applications, and do not treat wheat being grown for seed as a reduction in germination or vigor may occur. Additionally, do not apply to wheat with a legume underseeding.

Some formulations of 2,4-D ester are labeled for preharvest applications in wheat. Use rates vary somewhat by product but usually fall in the range of 1 to 2 pints per acre. Applications should be made after the wheat is in the hard-dough stage, but not to wheat with a legume underseeding.

If wheat harvest is timely and moisture conditions remain favorable, double-crop soybeans could likely be planted in many wheat-stubble fields. If a preharvest application of 2,4-D ester was made in the preceding wheat crop, planting soybeans should follow the time intervals set for preplant applications of 2,4-D in no-till systems: 7 days for 1 pint of 2,4-D ester, 30 days for 2 pints. There are no soybean planting restrictions if Roundup Ultra was used as a preharvest treatment.

What if the wheat was previously treated with some other herbicide? Soybeans may be planted 60 days after an application of Harmony Extra, 10 months after an application of Peak, and 10.5 months after an application of Stinger.

Aaron Hager and Marshal McGlamery, Department of Crop Sciences, (217)333-4424