Herbicides: Don't Let the Planter Outpace the Sprayer

With the recent warm spell, many are planting corn and soybeans to take advantage of the nice weather conditions. Some have sprayed burndowns and residual already while others are planting prior to spraying preplant burndown herbicides. As usual, there can be issues with both scenarios that need to be considered. One thing to be aware of is not to have the planter outpace the spraying operation so the crop and weeds get too far ahead. Here are some considerations for both corn and soybean fields.

Don't apply these herbicides to emerged soybeans.

Keep in mind that once soybeans start to emerge (soil cracking and beyond), do not apply herbicides to soybeans that contain:

• metribuzin (Tricor, Canopy, Tendovo, etc.)
• flumioxazin (Valor, Trivence, etc.)
• sulfentrazone (Authority, Spartan, etc.)
• saflufenacil (Sharpen, Verdict, OpTill, Zidua Pro, etc.)
• pendimethalin (Prowl, etc.)
• linuron (Lorox)
• flumetsulam (Python)

There are many post herbicide options for soybean however depending on the type of trait platform, some options cannot be used for effective control of various weeds. For example, if using a non-GM, Plenish, Roundup Ready only trait, marestail/horseweed will be difficult to control, especially in-crop, if marestail is not completely controlled during the burndown stage and an effective residual is not applied.

Considerations for corn herbicides under different scenarios.

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 broadspectrum 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.)

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). As previously mentioned, 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 (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 or Agrian ).

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 temperatures are rather warm and rainfall is scarce in many parts of the state, how should herbicide programs be adjusted? With many 10-day forecasts predicting limited rainfall in certain parts of the state, 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.  The Group 5 herbicides like atrazine, simazine, and metribuzin will also control small emerged susceptible broadleaves via root uptake. Since several herbicides, including residual products, can be applied after planting 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.