Several weeds, such as common lambsquarters and ragweeds, can interfere with wheat harvest. With wheat nearing maturity in several parts of the state, some producers may want to consider applying a preharvest herbicide treatment to control weeds that could make harvesting operations difficult. Only a few herbicides are cleared as harvest aids for wheat, including Clarity, glyphosate, and some formulations of 2,4-D. Applying these herbicides to maturing wheat fields often requires aerial application, and drift of any of these products out of the target area can cause serious injury to susceptible plants (crops, ornamentals, gardens, etc.). If you are planning to double-crop soybeans after wheat harvest, be sure to follow herbicide label precautions.
Some 2,4-D labels allow for preharvest treatments in wheat after the wheat is in the hard dough stage. Few label specifications are given regarding using treated wheat for seed, but it's probably wise to conduct a germination test before planting seed from fields where 2,4-D was used as a harvest aid. Ester formulations are generally more active on weeds than amines, but esters can volatilize more, especially when temperatures are over 85°F. Use of amine formulations in wheat increases the preplant interval for double-cropped soybeans. The interval after 2,4-D application is 7 days for 1 pint of ester, 14 days for 1 pint of amine, and 30 days for 2 pints of amine or ester formulations.
Several formulations of glyphosate can be used after wheat is in the hard dough stage (30% moisture or less) and at least 7 days before harvest. Application rates depend on the formulation used. Do not apply to wheat grown for seed because these applications can reduce germination and/or vigor. There is no preplant delay for soybeans if glyphosate is used as a preharvest treatment in wheat.
Clarity (dicamba) can be used as a preharvest treatment at 8 fluid ounces per acre once wheat is in the hard dough stage and the green color is gone from the nodes (joints) of the stem. Apply at least 7 days prior to harvest. Do not use preharvest-treated wheat for seed unless a germination test is performed on the seed, with an acceptable result of 95% germination or better. Clarity may be tank-mixed with 2,4-D (Weedmaster) or certain glyphosate formulations. Following a Clarity application up to 8 fluid ounces, a minimum accumulation of 1 inch of precipitation and a waiting interval of 14 days must occur before planting soybean.--Christy Sprague and Aaron Hager