Issue No. 13, Article 5/June 20, 2008
Preharvest Herbicides for Wheat
As the wheat crop progresses toward maturity, some farmers may consider applying a preharvest herbicide treatment to control weeds that could make harvesting operations difficult. Few herbicides are cleared for preharvest applications in wheat. Clarity, glyphosate, and some formulations of 2,4-D are labeled for preharvest applications in wheat. If you intend to plant soybean after using one of these herbicides as a preharvest treatment, be sure to follow herbicide label precautions. Preharvest treatments in maturing wheat fields often require aerial application, and drift of any of these products out of the target area can seriously injure susceptible plants (crops, ornamentals, gardens, etc.).
Some 2,4-D labels allow preharvest treatment 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 prior to planting the seed from fields where 2,4-D was used as a preharvest treatment. Ester formulations are generally more active on weeds than amines, but esters can volatilize more easily than amines, especially when temperatures are over 85 degrees. Amine formulations increase the preplant interval for soybean. The soybean planting 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.
Several formulations of glyphosate can be used after the hard dough stage (30% moisture or less) and at least 7 days prior to harvest. Application rates depend on the formulation used. Do not apply to wheat grown for seed, as a reduction in germination or vigor may occur. There is no preplant delay before planting soybean 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 when 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 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 prior to planting soybean.--Aaron Hager