Considerations for Postemergence Herbicide Applications

Mark VanGessel, Extension Weed Specialist;

The past week or two has resulted in a lot of weed emergence and its time to consider postemergence sprays. Here are some items to consider:

When to Spray
In many situations “when” is more important than “what” to spray. For most weeds we don’t have good options for rescuing fields when weeds get too large. My rule of thumb is spray when the first flush of weeds reach 3-inches tall. The approach many folks have voiced is they want to wait for all the weeds to emerge – this is a recipe for disaster.

  • Weeds like Palmer amaranth, nightshades, and crabgrass emerge over a long period of time and by waiting those weeds that emerged first are getting big. The first flush of weeds are the ones that are going to cause the most yield reduction, get the largest and interfere with combining, and produce the greatest amount of seeds. Later emerging weeds are much less competitive and produce a lot fewer seeds.
  • If you expect a prolonged germination period, there are a lot of options that provide residual control for weeds. Glyphosate, Liberty, dicamba, 2,4-D do not provide residual control but tankmixtures with Dual, Warrant, Zidua/Anthem, Callisto+atrazine, Capreno+atrazine, Reflex, and others can be used to provide residual control until the crop canopy produces enough shade to suppress weed growth and prevent additional weed emergence.
  • As we get into the warmer part of the summer, weeds are going to start growing a lot faster. It is important to scout regularly so that weeds do not get ahead of you, particularly if you have to rely on someone else to spray your fields. It’s unlikely that you can wait for your Palmer amaranth to reach 3-inch height, and call for a spray operator to come and they will be there the next day. In the case of Palmer amaranth our research has shown that postemergence sprays should be applied within 28 days of when the residual herbicides were applied. So if you are not scouting regularly, you can use this as a guideline. Local research with soybeans has demonstrated good to excellent Palmer amaranth control at 26 to 28 days after the residual application, but control was not acceptable at 32 days after the residual application. Don’t wait.
  • Another reason for spraying earlier instead of waiting is because it gives you options to retreat if needed. There are no guarantees for 100% control and so you want to leave yourself some time in case you need to follow up with an additional treatment.

Use Adjuvants When Needed
Most of the glyphosate formulations have their own adjuvant system “built in”. Those adjuvants will optimize the performance of glyphosate. So, if you are tankmixing another herbicide with glyphosate you need to ask, “Why is this additional herbicide included?”

  1. Is it there only to provide residual control?
  2. Is it there to help improve the control on a couple of weed species (and the weeds are small)?
  3. Is it there because there are weeds in the field that glyphosate is not very effective on (for instance morningglory)?
  4. Is it there because weeds in the field are resistant to glyphosate?

In the first situation no additional adjuvant is needed. In the second situation there is a good chance there is no need for additional adjuvants. However, if you are using tankmix partners with glyphosate for the last two scenarios then you need to consider additional adjuvants. Read the label because there may be specific recommendations when tankmixing with glyphosate or Liberty (i.e. Callisto, DiFlexx, Laudis, Capreno, Realm Q), otherwise, include the adjuvants recommended for these additional herbicides. Remember you are using the additional product(s) because glyphosate will not control the weeds, so you need to maximize the effectiveness of these products.

Use Atrazine with HPPD-Herbicides
Be sure you use atrazine with any HPPD-herbicide (Group 27) such as Callisto, Impact, Armazon, or Laudis. Without atrazine these products look pretty pedestrian (trying to find a nice way to say they look very poor without atrazine).

No Silver Bullet
If there was an ideal herbicide in the market we would not have so many options. Remember when Roundup Ready came out, a lot of products were discontinued. In fact, the project for Liberty Link crops was put on moth balls because of Roundup Ready. Well, we now know glyphosate alone is not the answer anymore. Dicamba and 2,4-D are not the silver bullet either. So be sure to select your postemergence herbicides based on weeds present in the field. Refer to the Mid-Atlantic Weed Management Guides, specifically Tables 2.11 and 2.12 for postemergence in corn and Tables 4.11 and 4.12 for postemergence in soybeans (order online at

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