Preharvest Weed Control in Wheat

June 9, 2000
The early maturing of the wheat crop may increase the interest in planting soybeans after wheat harvest (double cropping), especially in fields not underseeded with a legume. While the weed control in many wheat fields appears fairly good, a weedy wheat field can slow or delay harvest. Three herbicides are cleared for preharvest applications in wheat. Clarity, Roundup Ultra, and some formulations of 2,4-D are labeled for preharvest applications in wheat, but if you intend to plant soybeans 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 plantings (crops, ornamentals, gardens, etc.).

Some 2,4-D labels allow preharvest treatment in wheat after the wheat is in the hard dough stage. No 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 more than 85°F. Use of amine formulations increases the preplant interval for soybeans. The soybean preplant 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.

Roundup Ultra (glyphosate) can be used at 1 to 2 pints per acre after the hard dough stage (30% moisture or less) and at least 7 days prior to harvest. Do not apply to wheat grown for seed; a reduction in germination or vigor may occur. There is no preplant delay before planting soybeans if Roundup Ultra was used as a preharvest treatment in wheat.

Clarity (dicamba) can be used at 8 fluid ounces per acre preharvest 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. 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 and Christy Sprague

Author: Aaron Hager Christy Sprague