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Drought and Herbicides: Controlling Escaped Weeds in Corn

By Dwight Lingenfelter

With the prolonged dry weather pattern the past few weeks, many areas across the state have not received at least 0.5 inch of rainfall within a week or so after application to properly incorporate herbicides for effective weed control. However, there is usually enough moisture in the soil to allow weeds to germinate and grow despite herbicide being present. 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 if rainfall occurs soon, although control is not always consistent and effective. In particular, the HPPD (Group 27) herbicides (Acuron, Corvus, Lexar, TriVolt, etc.) tend to have better “reach back” potential then some other herbicides but escaped grass control is of greater concern.  The Group 5 herbicides like atrazine and simazine might control small emerged susceptible broadleaves via root uptake if rainfall occurs soon.

Nonetheless, to control escaped weeds, postemergence herbicide will likely be necessary. Make sure to scout fields to know how effective pre herbicides have worked before spraying post herbicide programs. In most cases, weedy grasses and larger seeded broadleaf weeds (velvetleaf, ragweed, cocklebur, etc.) seem to be the mostly likely to break through in these situations. Herbicides that are effective for grass control include products such as, glyphosate, Resolve Q, Steadfast Q, Impact/Armezon, Capreno, and a few others. Typical products for broadleaf control include, glyphosate, Liberty, dicamba (Clarity/Status/DiFlexx, etc.), Callisto, and others. There are numerous premixed and/or generic products that contain various combinations of herbicides that can be used. However, keep in mind there are crop height restrictions on many of the post herbicides. Most products can be applied to corn that is up to 20 or 30 inches tall. Also, for best performance, spray weeds when they are not beyond their maximum height guidelines.

Most annual grasses need to be sprayed before they reach 3 inches tall, but this can vary depending on what herbicide is used. The typical height for broadleaf weeds is 4 to 6 inches tall but again it can vary by product. For a listing of additional herbicides and maximum corn heights and information on maximum weeds sizes for these products please refer to Tables 2.2-14 and 2.2-15 in the 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— or

Crop injury and post herbicides

Even though weeds may be more “tolerant” to herbicides during times of drought, crop injury can still be a concern. Since crops are stressed during hot weather, it is more difficult for them to detoxify and metabolize the herbicides, in addition, leaf burn can occur if too many different pesticide formulations and adjuvants are added to the tank. So at this point, don’t try to combine too many operations into one.


On a final note, most post herbicides require adjuvants to be included in the spray mixture to optimize performance. Make sure to check the herbicide label for details on the use of surfactants (NIS), oils (COC, MSO), and/or nitrogen fertilizer additives (AMS, UAN). You may need to use a higher rate or switch to MSO (methylated seed oil) or COC (crop oil concentrate) if they are allowed which can increase herbicide uptake and improve control.  However, remember oil-based adjuvants can also increase the potential for crop injury.  Sometimes there is a fine line between controlling the weeds and injuring the crop.

Here are some general adjuvant guidelines to consider:

Use adjuvants with most post herbicides to improve weed control

  • Oils with dicamba or 2,4-D, may cause crop injury; however, products like Status and Diflexx allow the use of oils under certain conditions
  • Always use adjuvants in droughty conditions and for stressed weeds

If both COC/MSO and NIS are labeled:

  • Use NIS if “normal weather” and small weeds
  • Use COC/MSO if drought and more mature weeds

If labeled, include COC/MSO for control of grasses

  • Include N-Fertilizer (AMS or UAN) only if on herbicide label
  • Include AMS in hard water to improve glyphosate activity

If potential for crop injury is great (high humidity, succulent crops), use NIS instead of COC

Use the “150 rule” to reduce crop injury

  • Temperature + relative humidity is >150, then use NIS instead of COC
  • For example: 85 F with 50% relative humidity = 135…use oil adjuvant
  • If it’s 85 F with 70% humidity = 155…NIS may be substituted for oil adjuvant

Spray tank should be half full of water before adding any herbicides and adjuvants

  • Make sure dry products are fully mixed before adding other ingredients

Other comments:

  • Glyphosate is enhanced by NIS and AMS or UAN but can be antagonized by oil adjuvants
  • HSOC adjuvants were developed to enhance lipophilic herbicides (clethodim, dicamba, HPPDs, others) without antagonizing glyphosate when these types of products are tankmixed

Table 2.2-16 in the Penn State Agronomy Guide contains basic information about what type of adjuvants are commonly used with specific herbicides.

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