Wet weather throughout Kansas in May created some difficult conditions for properly applying herbicides for adequate weed control in corn. At this point, numerous fields in southern Kansas are at the tasseling to blister stages in development and early planted fields are just beginning to tassel north of Interstate 70 in the eastern half of the state. However, numerous fields are still in the vegetative stage throughout the state. To complicate spray decisions even further, saturated soil conditions have led to various growth stages of corn being present in the same field (non-uniform fields), which makes applications of herbicides in accordance with the label a difficult task.
In 2008 and 2009, Kansas experienced similar wet weather conditions that delayed many corn herbicide applications. On occasion, fields with late applications of pesticides experienced poor pollination or had abnormally developed ears. It was speculated that the off-label applications might have been involved. In 2009, a demonstration of off-labeled applications of glyphosate at high rates was applied to Roundup Ready corn just prior to the tassel stage. Another demonstration area received a similar application at about the V12-V14 stage.
Figure 1. Ears from an untreated area. The tipping back is what we’d normally expect with high plant populations in a dryland setting with drought and temperature stress (in the year this photo was taken). Photos by Stu Duncan, K-State Research and Extension.
Figure 2. Ears from the pretassel treatment with glyphosate showing a few blank kernels, with sections of rows missing.
Figure 3. Ears from plants treated over the top with an off-label application of a high rate of glyphosate at about V12-V14. The corn shows much more severe problems with ear development.
Remember, this was simply a demonstration of off-label applications. These types of symptoms should not be evident if glyphosate is applied according to label restrictions.
Reviewing glyphosate labels: Glyphosate applied with drop nozzles can be applied on corn up to 48 inches tall. Increased risk of injury and arrested ear occur when application are made after this stage. Tank-mix partners at the late stages are minimal in number. Impact herbicide may be applied to corn up to 45 days before silage or grain harvest on corn. Again, drop nozzles are essential to attempt to get weeds controlled at that stage. All other herbicides are likely off label at these late stages. Control of large weeds at this stage often is marginal.
Agronomists at Purdue University have seen similar problems with off-label, late applications of glyphosate in Indiana, but primarily with something they have termed “arrested ear development.” They did a more detailed study looking at different combinations of fungicides, additives, and glyphosate applications at V14. Many of the symptoms they observed are similar to what we have seen in the field and in our demonstration. A full description of the Purdue study can be found at: http://www.agry.purdue.edu/ext/corn/news/articles.08/ArrestedEars-1209.html
A K-State publication on abnormal corn ears was recently released as a web-based document. In that document, we have other examples of damage from off-label herbicide applications. This publication can be found at: https://slate.adobe.com/a/Z0xaA
Stu Duncan, Northeast Area Crops and Soils Specialist
Doug Shoup, Southeast Area Crops and Soils Specialist
Curtis Thompson, Extension Agronomy State Leader and Weed Management Specialist
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist