University of Illinois

Herbicides as Harvest Aids

August 28, 1998

As crops across Illinois reach maturity and harvest approaches, weeds remaining in fields could potentially present harvest difficulties. Only a few herbicides can be used as harvest aids, helping dry down existing weed vegetation before harvest.

We are often asked if any of these harvest-aid herbicides cause nightshade plants to drop their berries, especially in soybean fields. While both Gramoxone Extra and Roundup can cause the leaves of the plants to dry and drop, neither of these herbicides will cause the berries to drop from the plant. Preharvest applications of translocated herbicides may also help suppress weed seed production, but caution must be used because application of these herbicides too soon (with respect to crop developmental stage) may allow some herbicide to move into the developing seeds. Translocated herbicides may also provide some suppression of perennial weed species if enough leaf surface is available for herbicide absorption.

Preharvest herbicides for corn. Some formulations of 2,4-D may be applied as preharvest treatments. Applications, generally 1 to 2 pints per acre, should be made after the hard- dough stage; some formulations are labeled for aerial application. 2,4-D controls only certain broadleaf species and provides no control of grass species. Be cautious about off-target movement of 2,4-D, especially if sensitive plants are growing nearby.

Roundup Ultra (glyphosate) may be used as a preharvest treatment in corn. Application rates range from a maximum of 1 quart per acre for aerial applications to 3 quarts per acre for ground applications. Grain moisture should be at 35 percent or less, and plants should be physiologically mature (black layer fully formed). Allow at least 7 days between application and harvest. Applications to seed production fields are not recommended.

Preharvest herbicides for soybeans. Gramoxone Extra (paraquat) may be applied as a preharvest treatment at 12.5 fluid ounces per acre. Include 1 quart of NIS per 100 gallons of spray solution. Applications should be made after 65 percent of soybean pods have turned brown or when seed moisture is 30 percent or less. Allow at least 15 days between application and soybean harvest.

Roundup Ultra may be used as a preharvest treatment in Roundup Ready and conventional (nonresistant) soybeans. For Roundup Ready soybeans, apply up to 1 quart per acre after loss of green color in soybean pods until 14 days prior to harvest. The combined total of in-crop and preharvest Roundup Ultra applications cannot exceed 3 quarts per acre. Preharvest applications to nonresistant soybeans may be made after pods have set and lost all green color and at least 7 days prior to harvest. Rates range from 1 to 6 quarts per acre, but do not apply more than 1 quart per acre for aerial applications. Do not apply to soybeans grown for seed because a reduction in germination or vigor may occur.

Author: Aaron Hager Marshall McGlamery