Pennsylvania Corn: Herbicide Application Timings and Restrictions

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.

Also, refer to last week’s article ” Delayed Pre/Early Post Residual Corn Herbicides and Weather Effects ” on crop size and application timing of various herbicides.

Herbicide application timings with respect to corn and weed growth stages and weeks after planting:

Application timing Number of corn leavesa Number of corn collarsb Average corn heightc Weed height WAPd Comments
Preplant 0 0 0 Varies 0 Typical of standard preplant or preemergence applications such as Acuron, Bicep, Corvus, Harness Xtra, Keystone, Resicore, Verdict, etc. Include burndown herbicides if necessary.
Pre 0 0 0 0 to 2 0 to 1 Typical of standard preplant or preemergence applications such as Acuron, Bicep, Corvus, Harness Xtra, Keystone, Resicore, Verdict, 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 Some herbicides can be applied at this stage. Refer to label.
Spike 1 (tightly wrapped) 0 <2 inches 0 to 2 inches 1 to 3 Some herbicides can be applied at this stage. Refer to label.
Early post 1 to 3 £2 ≤6 inches ≤3 inches 2 to 4 Ideal application timing for dicamba products. Some chloroacetamide + atrazine products 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 (Callisto, Capreno, Halex GT, Impact/Armezon, glyphosate, Liberty, Resolve Q, Status, Steadfast Q, 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 (e.g., glyphosate). 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 Usually 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.

a number of leaves with greater than 50% of leaf exposed
b number of leaf collars fully exposed
c Average corn height measured to the arch of the highest leaf that is >50% emerged
d WAP= weeks after planting

Source URL: https://extension.psu.edu/corn-herbicide-application-timings-and-restrictions