Corn planted in late April and last week has already emerged. But unusual weather patterns interspersed with recent rainy and windy periods have made it difficult for timely herbicide applications, acknowledge Penn State Extension Weed Specialists Bill Curran and Dwight Lingenfelter.
In some cases, burndown herbicides and residuals were applied a few weeks ago and are now running out of steam. Or the corn is already coming up, and weather didn't allow getting a herbicide, even a burndown, applied. Here are some of their recommendations from Curran and Lingenfelter for such situations:
Unless it's Roundup Ready or Liberty Link corn, your options for broad-spectrum burndown are very limited. They don't recommend applying Gramoxone even if the corn is in spike stage. Instead, try other herbicide tank mixes or using two-pass herbicide programs."
A number of herbicides, including residuals, can be applied after planting until corn and weeds reach a certain size or growth stage. The greatest risk of failure comes with trying to control annual grasses such as foxtail and panicum as they're emerging without including a foliar applied herbicide.
With the increasing acres of Roundup Ready (glyphosate) and Liberty Link (Liberty 280) corn, you have more flexibility in managing weeds after emergence. And, a number of "conventional corn" products are available to control emerged grasses -- Accent Q, Basis Blend, Capreno, Equip, Impact/Armezon, Laudis, Option, Realm Q, Resolve, Resolve Q, Steadfast (ATZ) and Steadfast Q. Even more products are available for broadleaf weed control.
In most cases, these postemergence or foliar-applied herbicides can be tank-mixed with residual products to provide several weeks of control. For most products, do not apply in a liquid fertilizer carrier if corn has emerged or injury may occur.
Sizing up corn and weeds critical to control
Maximum corn and weed sizes vary for early-post corn applications depend on the product. "Pushing it" may compromise control. Stick with these labeled guidelines:
- Balance Pro, Radius, and Princep must be applied before corn emergence.
- Balance Flexx and Corvus contain a safener and can be applied up to early-post (V2 growth stage) to corn.
- Bicep II Magnum, Bullet, Dual II Magnum, etc. can be applied to corn up to five inches tall.
- Acetochlor-containing products such as Degree (Xtra), Harness (Xtra), FulTime, Keystone (LA), and SureStart can be applied to corn up to 11 inches tall.
- Herbicides including Atrazine, Lumax, Lexar, Guardsman Max, and Realm Q can be applied to corn up to 12 inches tall (20 inches tall for Resolve Q).
- Prowl H2O, Halex GT, and Warrant can be applied to 30-inch tall corn or less.
When tank-mixing with other pesticides, follow the most restrictive product label. For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products, click on corn_weed_max.