Early Season Corn Residual Herbicide Issues
Drought Effects on Residual Herbicides.
Drought in late April/early May is not usually a problem for us in this part of the country. However, now that in many parts of the state the temperature is rather warm and there is a lack of rainfall, how should herbicide programs be altered? With many 10-day forecasts predicting limited rainfall, some are wondering if residual herbicides included with burndown programs will be effective. Therefore, should they even be included in the burndown programs if droughty conditions persist? Dry weather can affect both soil-applied and postemergence herbicide performance. All soil-applied herbicides require rainfall to mobilize them for effective weed control. In general, rainfall should occur within 7 to 10 days after application or before weed emergence. As a general rule of thumb, a ½ inch of rain is considered the minimum, depending on current soil moisture levels and the herbicide used; ¾ to 1 inch is ideal. More water soluble products such as Callisto, and metribuzin require less rainfall (1/4 to 1/3 inch) to activate them while the less mobile materials (Balance Flexx, Prowl, atrazine, simazine, Valor, Zidua) and deeper germinating weeds (e.g. yellow nutsedge, cocklebur, velvetleaf, ragweed, etc.) will require more rainfall for effective mobilization and activation into the seed germination zone. Keep in mind that many small-seeded annual weeds can germinate with minimal moisture. Thus, sometimes you may observe weed emergence before enough rainfall has incorporated the herbicide for effective kill. If 10-14 days have passed without rainfall following a pre-treatment and weeds are starting to break, start planning for a post herbicide rescue application. Some "reach back" or "recharge" can be expected on small annual weeds (esp. broadleaves) with some herbicides when rainfall occurs, although, depending on this, it may be a little like gambling. In particular, the HPPD (Group 27) herbicides (Acuron, Balance, Corvus/TriVolt, Lexar, Lumax, etc.) tend to have better "reach back" potential than some other herbicides, and escaped grass control is probably of greater concern. Group 5 herbicides like atrazine, simazine, and metribuzin will also control small emerging susceptible broadleaves via root uptake. Since 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, it might be best to wait until the crop is planted and consider modifying your residual herbicide program at that point, if possible.
Residual Herbicides Applied after Planting.
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 that 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 the 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 (old formulation), 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 Rev, Kyro
- 30-inches (V8): Acuron Flexi, Armezon Pro, Callisto, Enversa, 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 that 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 the 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 weed sizes for these products, please refer to Tables 2.2-10 and 2.2-15 in the 2025 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 or Greenbook - labels or Agrian - labels).
