By Dwight Lingenfelter
For various reasons, timely herbicide applications in the spring sometimes can be difficult. In some cases, the corn is already coming up and no herbicides, including a burndown have been applied. Or if soil-applied herbicides have been applied but were not properly incorporated with adequate rainfall, weed escapes can occur. Or in other situations, herbicides were applied and there was adequate rainfall but due to wet conditions the crop could not be planted. Whatever the situation, there are some herbicide options to consider. With many acres of corn that contain Roundup Ready (glyphosate) and/or Liberty Link (glufosinate) traits, there is more flexibility in how we manage weeds after emergence. However, we do not recommend application of Gramoxone/paraquat even if the corn is in spike stage and be aware that few broadspectrum control options exist in non-GM varieties.
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. The Group 15 products (like Dual, Harness, Outlook, Zidua, etc.) and Prowl do not control emerged weeds, so additional herbicides will need to be included in the mixture that control existing weeds. In addition, several “conventional” corn products are available to control emerged grasses (e.g., Accent Q, Basis Blend, Capreno, Impact/Armezon, Resolve Q, Shieldex, Steadfast Q and a few others) and even more options are available for broadleaf weed control. However, keep in mind, if certain herbicides have already been applied and additional herbicide applications need to be made to obtain effective weed control, make sure not to exceed the maximum yearly limit for that product or group of herbicides.
In most cases, these post or foliar-applied herbicides can be tank-mixed with residual products to provide several weeks of control (again, if residual herbicides were already applied before or at planting, be cautious of maximum use loads per season). For many products, do not apply in a liquid nitrogen fertilizer carrier if corn has emerged or injury may occur (but refer to product label for specific details). Maximum corn and weed sizes vary for early post herbicide applications in corn depending on the product. Below are some maximum corn growth stage/height parameters for commonly used corn herbicides that are broadcast applied:
- Before corn emergence: Axiom, Princep/simazine, Verdict
- 2-leaf (V2 corn): Basis Blend, Balance Flexx, Bicep Lite II Magnum, Corvus, TriVolt
- 4 collars (V4): Anthem Maxx
- 8 inches (V5): Clarity (1 pint/acre)
- 11 inches: acetochlor-containing products such as Degree (Xtra), Harness (Xtra and Max), FulTime NXT, Keystone NXT, Resicore, SureStart II
- 12 inches: atrazine, Acuron, Bicep II Magnum, Cinch ATZ, Lumax EZ, Lexar EZ, Outlook, Resolve
- 18 inches (V6): Maverick, Perpetuo
- 20 inches (V6): Accent Q, Capreno, Liberty, Peak, Realm Q, Resolve Q, Shieldex, Steadfast Q, Python
- 24 inches: Resicore XL, Kyro
- 30-inches (V8): Acuron Flexi, Armezon Pro, Callisto, generic glyphosates, Halex GT, Impact/Armezon, Prowl H2O, Warrant, Zidua SC
- 36 inches (V8): Clarity (0.5 pint/acre), Status, Yukon
- 40 inches: Dual II Magnum
- 48 inches: Cadet, Permit, Roundup products
Keep in mind when tank-mixing with other pesticides follow the most restrictive product label. Also, some herbicides can be applied later if applied with drop nozzles. Furthermore, be aware of maximum weed sizes for most of these products. Just because a herbicide can be applied later in the corn crop doesn't mean it will provide effective control if weeds are too large. 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 and Table 2.2-15 in the 2023-24 Penn State Agronomy Guide and check the most recent herbicide label for specific use guidelines. (Or refer to the herbicide label for additional use information from these sites — CDMS Advanced Search, AgWorld DBX or Agrian).
Stressed crops and herbicide injury
Finally, herbicide injury to corn is always a possibility especially in the early season if the crop is stressed by cloudy, cool, wet weather while it’s trying to metabolize herbicides that have been applied. Certain herbicides (Groups 2, 3, 4, 15, and 27) and EC product formulations tend to be the most common culprits, however, others can be at fault as well.
Source : psu.edu