The wet weather has made it difficult for timely herbicide applications, to say the least. In some cases, the corn is already coming up and no herbicides, including a burndown, have been applied. Some of these post-herbicide options can be used to control escaped weeds from a pre-application, but make sure to adhere to maximum herbicide load per season for some product active ingredients, namely atrazine (Group 5) and the HPPD (Group 27) herbicides.
Unless it's Roundup Ready or Liberty Link corn, the options for broadspectrum burndown are very limited. We do not recommend application of Gramoxone even if the corn is in spike stage but would suggest other herbicide tank mixes or using 2-pass herbicide programs.
There are several herbicides, including residual products, that can be applied after planting up 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 are emerging without including a foliar applied herbicide. Products like Prowl, Dual, Harness, Outlook, Zidua, etc. do not control emerged weeds, so additional herbicides will need to be included in the mixture that control existing weeds.
With many acres of Roundup Ready (glyphosate) and Liberty Link (glufosinate) corn, we have more flexibility in how we manage weeds after emergence. In addition, several "conventional corn" products are available to control emerged grasses (e.g., Accent Q, Basis Blend, Capreno, Impact/Armezon, Laudis, Resolve Q, Shieldex, Steadfast Q and a few others), and even more are available for broadleaf weed control. In most cases, these post 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. Maximum corn and weed sizes vary for early post herbicide applications in corn depending on the product. Herbicides such as Princep and Verdict 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, while Anthem Maxx and Zidua SC can be applied up to the V4 growth stage. Acetochlor-containing products such as Degree (Xtra), Harness (Xtra and Max), FulTime NXT, Keystone NXT, Resicore and SureStart II can be applied to corn up to 11 inches tall.
Those herbicides that can be applied to corn up to 12 inches tall include: atrazine, Acuron, Bicep II Magnum, Cinch ATZ, Lumax EZ, Lexar EZ and Outlook. Resolve Q and Python can be sprayed on 20-inch tall corn. And finally, Acuron Flexi, Halex GT and Prowl H2O can be applied up to 30-inch-tall corn, while Dual II Magnum can be applied to corn that is 40 inches tall. 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 please refer to Table 2.2-10 in the Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines.