by Jourdan Bell, Texas A&M AgriLife Assistant Professor and Agronomist – Amarillo

Post emergent herbicide applications are necessary for season long weed control. Post emergent corn herbicide labels define the latest growth stage for which herbicide applications can be made without causing crop injury. Herbicide applications past the recommended application stages can result in significant crop injury. For some herbicides, later applications can be made according to the label, but drop nozzles are recommended to prevent herbicides being sprayed in the whorl. Some herbicide labels provide a recommended height for the last herbicide application as plant height often corresponds with a particular vegetative stage. However, in high input environments with abundant irrigation and/or precipitation and fertility, internode distances can expand at a greater rate than new leaves. Consequently, plant height does not always correspond to the correct vegetative stage so, it is important to accurately stage the corn crop prior to making post emergent herbicide applications. In stressful production environments with limited water or cool temperatures, corn growth is slowed, which results in magnified crop injury because the plant does not metabolize the herbicide quickly enough to avoid injury. For labels that provide both plant height and growth stage, the applicator should follow the more conservative recommendation. Non-labeled spray adjuvants can enhance herbicide injury from a post emergent application. In addition to the crop stage and condition, knowledge of the weed species and weed size are also important for effective post emergent herbicide applications.  The vegetative stage are described using the leaf collar method (Table 1). Leaves are counted from the lowermost first (rounded-tip) leaf to the uppermost leaf with a leaf collar, which is the connection between the leaf blade and the leaf sheath. The leaves in the whorl that are not fully expanded are not counted.

Figure 1. Image identifying the leaf collar. (Image from Kay Ledbetter)

Figure 2. Image reflecting first leaf with round tip. (image from Kay Ledbetter)

Table 1. Corn vegetative growth stages noted as “V” stages important for herbicide application timing.

Table 2.  Postemergence herbicide labeled growth stages for common corn herbicides.

Jourdan Bell
Assistant Professor and Extension Agronomist
Dept. of Soil&Crop Sciences
jourdan.bell@ag.tamu.edu