Corn Herbicides: Burndown Issues, Liquid Fertilizer Carriers, and Other Problems

Over the past week, I have heard about a couple scenarios regarding corn planting and the weed control process, so let's review some considerations.

Scenario 1: Corn was planted and has emerged but residual herbicides still need to be applied as well as fertilizers; thus, can herbicides be applied in liquid fertilizer carriers?

The short answer is no. Commonly used herbicides such as Acuron, atrazine, Harness Xtra, Lexar EZ, Resicore Rev, Storen, SureStart II, TriVolt and many others should not be applied postemergence in liquid fertilizer carriers, even if it is diluted with water. There is too much risk for severe crop injury. The labels stipulate water as the carrier when applying these herbicides postemergence.

Scenario 2: Corn has been planted (or will be planted soon) but no burndown or residuals have been applied. What herbicides can be used to burndown large weeds and get residual activity on germinating summer annuals?

If corn has not yet emerged, the use of products like Sharpen or Gramoxone can still be used. The same is true if using glyphosate or Liberty in conventional corn. However, keep in mind that if weeds are taller than 6 inches, poor burndown can occur. Also, 2,4-D and dicamba could be used but it is best to wait 3–5 days after planting for better crop safety. On a side note, for those thinking about using Surtain as a burndown as well as getting some residual, this product does not provide burndown/foliar control of weeds due to its special formulation to protect the crop.

There are some situations in which 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 just make sure to adhere to the 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 broad spectrum burndown are very limited. We do not recommend application of Gramoxone/paraquat 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/Anthem Maxx, 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 TriVolt contain a safener and can be applied up to early POST (V2 growth stage) to corn, while Anthem Maxx can be applied up to the V4 growth stage.
• Acetochlor-containing products such as Degree (Xtra), Harness (Xtra and Max), FulTime NXT, Keystone NXT, 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, Lumax EZ, Lexar EZ, and Outlook.
• Resolve Q and Python can be sprayed on 20-inch-tall corn.
• Resicore Rev and Kyro up to 24-inch corn.
• And finally, Acuron Flexi, Armazon Pro, Halex GT, Prowl H2O, Warrant/Enversa, and Zidua SC can be applied up to 30-inch-tall corn.
• Dual II Magnum can be applied to corn that is 40 inches tall.

Keep in mind, 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 weed 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.