Corn Herbicide Application Timings

Knowing the terminology is Important in corn herbicide application timings.
Corn Herbicide Application Timings - Articles
Corn Herbicide Application Timings

In the past there were generally two or three times when herbicides were applied in corn - preplant, pre or 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 timingNumber of corn leavesaNumber of corn collarsbAverage corn heightcWeed heightWAPdComments
Preplant000Varies0Typical of standard preplant or preemergence applications such as Bicep, Lexar, Harness Xtra, Zidua/Anthem, Corvus, SureStart, etc. Include burndown herbicides if necessary.
Pre0000 to 20 to 1Typical of standard preplant or preemergence applications such as Bicep, Lexar, Harness Xtra, Zidua/Anthem, Corvus, SureStart, etc. May need to include burndown herbicide if weeds are present.
Delayed pre00<1 inch0 to 2 inches1 to 1.5Ideal application timing for Prowl. Other products can vary, see label.
Spike1 (tightly wrapped)0<2 inches0 to 2 inches1 to 3Typical application timing for Prowl
Early post1 to 3£2£6 inches£3 inches2 to 4Ideal application timing for Clarity/Banvel and Basis Blend. Some chloroacetamide ± atrazine products and Balance Flexx can still be applied at this stage.
Mid post3 to 62 to 4Up to 12 inches2 to 6 inches3 to 5Typical of many post applications (Glyphosate, Status, Steadfast Q, Resolve Q, Callisto, Capreno, Cadet, etc.)
PostVariesVariesVariesVariesVariesGeneral term that may encompass early, mid, and late post applications.
Late post (not split)7 to 115 to 912 to 20 inches8 to 12 inches7 to 8Typical of some applications. Later timings generally not recommended except as a rescue.
Late post (as split treatment)7 to 115 to 912 to 20 inches2 to 6 inches7 to 8May be the second application in a planned pre/early post followed by a late post herbicide program.
Layby (use drop nozzles)11 to 139 to 1020 to 36 inchesVaries8 to 9Layby refers to an application at the last cultivation time or the equivalent.

Just as a reminder, with the exception of Princep (simazine), Prequel, 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 or The Penn State Agronomy Guide (Table 2.2-12) for application timing restrictions. Also, once the corn has emerged, do not apply these products in a liquid fertilizer carrier.