Late-Season Herbicide Tips for Corn and Soybeans

By Blake Jackson

As harvest season approaches, many farmers ask how late is too late to apply herbicides for controlling weeds such as marestail, Palmer amaranth, waterhemp, and burcucumber.

The answer depends on several factors, including the herbicide being used, the crop, the specific weeds, and the timing relative to crop growth.

Most herbicides have label restrictions based on crop stage and pre-harvest intervals (PHI). Spraying beyond these guidelines can reduce effectiveness and may lead to illegal residues, potential load rejection, or penalties. Late-season applications can also cause crop damage and yield loss, especially from sprayer wheel tracks.

At this stage, crops are often beyond the optimal growth stage for standard herbicide applications, making harvest aids the primary option.

Harvest aids help desiccate green weeds to facilitate harvesting, but they do not speed crop maturity. Contact herbicides are generally more effective at this stage, though results can vary by weed species.

For corn, options include:

• Aim 2EC: 2 fl oz/acre, 3 days before harvest, to desiccate broadleaf weeds.
• Defol 5L: 4.8 qt/acre, 14 days before harvest, may be tank-mixed with Aim or Gramoxone.
• Glyphosate: Up to 0.75 lb ae/acre, applied when grain moisture is 35% or less, at least a week before harvest.
• Gramoxone 3SL: 0.8-1.3 pt/acre after black layer, 7 days before harvest, with nonionic surfactant.
• Sharpen 2.85SC: 1-2 fl oz/acre after physiological maturity, 3-10 days before harvest.
• 2,4-D LV4: 1-2 pt/acre after hard dough stage, do not feed treated fodder for 7 days.

For soybeans, recommended products include:

• Aim 2EC: 1.5 fl oz/acre, at least 3 days before harvest.
• Clarity 4S: 8 fl oz-2 qt/acre, after pods turn brown, 7 days before harvest.
• Defol 5L: 4.8 qt/acre, 7-10 days before harvest.
• Glyphosate: Up to 0.75 lb ae/acre for pre-harvest weed control, 7 days before harvest.
• Gramoxone 3SL: 5.4-10.7 fl oz/acre, 15 days before harvest.
• Sharpen 2.85SC: 1-2 fl oz/acre, after physiological maturity and 50% leaf drop, 3 days before harvest.

Always follow label instructions, include recommended adjuvants, and consider crop and weed growth to achieve effective control while protecting both the crop and the end product.