By Dwight Lingenfelter and Bill Curran
Originally printed in PSU Field and Forage Crop News
In the past there were generally two or three times when herbicides were applied in corn: preplant, PRE (preemergence) or POST postemergence. Now with more and newer products, POST application timings in particular are being subdivided into different timings (i.e., delayed PRE, early POST, mid POST, POST, late POST, etc.). Unfortunately there are no exact definitions for these various timings. In general, the standard preplant, PRE, and POST definitions are adequate for most products, but if you would like a more refined breakdown of herbicide application timings, please read on.
Below are some general guidelines to follow, based on our opinion. Keep in mind, these definitions may vary between parties, so always refer to the product label for the proper application timing with respect to corn and weed growth stages. (For a more in-depth discussion and pictures of corn growth stages, refer to the Iowa State website entitled “How a Corn Plant Develops.”
| Application timing | Number of corn leaves | Number of corn collars | Average corn height | Weed height | Weeksafterplanting | Comments |
| Preplant | 0 | 0 | 0 | Varies | 1 to 3 weeks before planting | Typical of standard preplant or PRE applications such as Bicep, Lexar, Harness Xtra, Zidua/Anthem, Corvus, SureStart, etc. Include burndown herbicides if necessary. |
| PRE | 0 | 0 | 0 | 0 to 2 | 0 to 1 | Typical of standard preplant or PRE applications such as Bicep, Lexar, Harness Xtra, Zidua/Anthem, Corvus, SureStart, etc. May need to include burndown herbicide if weeds are present. |
| Delayed PRE | 0 | 0 | <1 inch | 0 to 2 inches | 1 to 1.5 | Ideal application timing for Prowl. Other products can vary, see label. |
| Spike | 1 (tightly wrapped) | 0 | <2 inches | 0 to 2 inches | 1 to 3 | Typical application timing for Prowl |
| Early POST | 1 to 3 | ~2 | ~6 inches | ~3 inches | 2 to 4 | Ideal application timing for Clarity/Banvel and some chloroacetamide ± atrazine (ie Bicep) products and Balance Flexx can still be applied at this stage. |
| Mid POST | 3 to 6 | 2 to 4 | Up to 12 inches | 2 to 6 inches | 3 to 5 | Typical of many POST applications (Glyphosate, Status, Steadfast Q, Callisto, Capreno, etc.) |
| POST | Varies | Varies | Varies | Varies | Varies | General term that may encompass early, mid, and late POST applications. |
| Late POST (not split) | 7 to 11 | 5 to 9 | 12 to 20 inches | 8 to 12 inches | 7 to 8 | Typical of some applications. Later timings generally not recommended except as a rescue. |
| Late POST (as split treatment) | 7 to 11 | 5 to 9 | 12 to 20 inches | 2 to 6 inches | 7 to 8 | May be the second application in a planned PRE/early POST followed by a late POST herbicide program. |
| Layby (use drop nozzles) | 11 to 13 | 9 to 10 | 20 to 36 inches | Varies | 8 to 9 | Layby refers to an application at the last cultivation time or the equivalent. |
Just as a reminder, with the exception of Princep (simazine), Verdict, and a few others, most of the corn herbicides that are typically applied PRE, also can be applied POST. However, their restrictions differ depending on the product and tank-mix partners.
Certain products are limited to corn no larger than 2-leaf stage (e.g., Instigate, Zidua), others can be applied to corn up to 11 or 12 inches tall (e.g., atrazine, Harness Xtra, Keystone NXT, Lumax EZ, Lexar EZ, Outlook, SureStart II, others) and still others can be applied up to 30-inch tall (V8 stage) corn (e.g., Prowl H2O, Zemax, Halex GT). Refer to the herbicide label for application timing restrictions. Also, once the corn has emerged, do not apply these products in a liquid fertilizer carrier.