Corn emergence across the Wheat Growers territory this spring has been uneven due to variable environmental conditions. Pre-emergence herbicides have generally been performing extremely well, but where a second pass is needed, or if a pre-emergence pass was not possible for whatever reason, there are certainly excellent post-emergence corn herbicide options for growers.

 Effective weed management in corn may take multiple herbicide applications.

 Variable emergence due to environment can affect herbicide applications. 

A product like Halex GT provides excellent post-emergence weed control, and combines solid residual herbicides- mesotrione (Callisto) plus encapsulated S-Metolachlor (active in Dual II Magnum), good burndown and glyphosate in a single formulation. The HPPD class of herbicides are good residual products and have some solid burndown on small weeds: Laudis, Armezon, Callisto, for example. Many of the pre-emergence herbicides- like Harness Xtra, Acuron or Resicore can also can be used early post-emergence. Of course, the dicamba-containing herbicides like Status, Diflexx or Diflexx Duo (Diflexx plus Laudis) provide excellent, consistent broadleaf weed control in an early post-emergence timing.

The uneven stands can create potential problems for the proper timing of post-emergence herbicide applications. The first step in making a proper herbicide application post-emergence is to scout fields prior to applications and determine the growth stage of the crop. This is also a great opportunity to truly evaluate crop stands and understand field variability in emergence. Differing growth stages for emerged corn are generally not a huge issue for corn herbicides post-emergence, but herbicide labels do define the maximum growth stage (and occasionally a minimum), to maximize efficacy and minimize crop injury. Weed stages and sizes are also often specified on herbicide labels.

Growth staging corn early in the season is relatively simple. The corn growth stage is identified by the leaf stage or plant height. Leaf staging is accomplished by counting all corn leaves that have a distinct “collar” at the base of the leaf. This method is the most scientific, and most commonly used, but some labels (like atrazine) still define maximum growth in terms of plant height.
To stage corn by the leaf collar method, look for the collar- a visible, light colored band at the base of the leaf. Count all leaves with a visible collar, including the first round -tipped leaf (V1). Every leaf collar after that adds a “V” (or vegetative) stage- so V2, V4, etc. Older plants will lose that first round-tipped leaf. On these plants, split the stalk. At the base, there will be a hard, triangular base section. Immediately above that section will be a visible, horizontal node slightly above that triangular base section. This node is the attachment point of leaf V5. Variable plant stages make making a single “call” more difficult, but if over 50% of the plants are at a certain developmental stage, that is the growth stage to use. Plant height is less consistently defined, but is usually accepted to mean the height to the arch of the uppermost leaf that is at least 50% emerged from the whorl.

Proper application timing is critical to remove weeds before they interfere with corn development at some critical growth stages. At around the V5 stage of growth the tassel and ear are just being initiated. This growth stage is sensitive to stresses, and a late herbicide application can be one of the stressors.

Be cautious with additives in a post-emergence herbicide program. Always consult the herbicide label for additives approved post-emergence in corn. For example, 28% UAN is generally not recommended post-emergence in corn because of increase crop injury. Under warm, humid conditions, growth regulator type herbicides can rapidly be absorbed into plants, and can increase susceptibility to brittleness. Incorrect additives, such as strong crop oils, can also enhance plant uptake and risk for crop injury. Following label guidelines will ensure a successful application to increase weed control and reduce potential crop issues.
POST herbicide tank mixtures are an important element of integrated weed management of tough-to-control broadleaf weeds in corn. Talk to your local Wheat Growers Agronomist for post-emergence weed control options that will work on your farm.