With the onset of weed resistance, especially giant ragweed and increased broadleaf weed pressure in corn in the northern Burrus territory, many have turned to post applications of growth regulator herbicides that include 2,4-D and dicamba. Some examples of PGR herbicides include products premixes that contain the following Trade Names®: Banvel, Clarity, and Status. For more information you can refer to the Take Action Herbicide Classification Chart. This chemical family combats weeds by disrupting their growth processes, such as protein synthesis, cell division, and cell enlargement. Because of this, the potential for the PGR herbicides to injure corn increases when application occurs during rapid growth, later in the growing season, or during adverse weather conditions. Most PGR injury to corn has been attributed to weather, variable corn growth, and non-label applications.
All corn hybrids can be susceptible to PGR injury during post applications and it occurs more frequently when high rates or concentrated amounts are placed in the whorl of the corn plant. Some hybrid germplasm may be more sensitive to PGR herbicides, so many seed companies, like Burrus, have charts that help you consult with your account manager on the appropriate chemical use with particular corn hybrids. You can find the Are There Chemicals to Avoid chart in the 2016 Burrus Product Selection Guide on page 39 and the 2016 Growing Guide (Pocket Handbook) on pages 58-59.
When looking through the Burrus Are There Chemicals to Avoid chart, keep in mind that this information has been summarized and is based on replicated, trial results from the last two years. PGR herbicide trial results can vary within different environmental conditions. Also, in order for our research department to visually evaluate a hybrid’s response to herbicide, an increased amount of chemical must be applied between V2 and V10 or a corn height of 4 to 36 inches. This is an important reminder for our growers to always follow herbicide label guidelines to avoid herbicide injury.
PGR injury symptoms to corn can appear as malformation of the brace roots, temporary leaning, or as twisting shortly after herbicide application, because the PGR herbicides affect the meristematic (actively growing) areas of the plant.
|Suspect "onion leaf" appearance due to PGR herbicide|
Symptoms of PGR injury can include an “onion leaf” appearance during the early growth stages or “goose necking” (a kink in the stalk) and “s” shaped stalks later in the growing season. If PGR herbicides are applied during periods of rapid growth or elongation of corn, plants can become brittle and vulnerable to snapping. In some cases, injury symptoms do not occur on every plant, but show up on isolated plants or in an irregular pattern, which could be traced back to application error. Do not confuse with other herbicide injury or twisted whorl syndrome.
To avoid injury, it is best to apply PGR herbicide during early growth stages (before V5), but always refer to PGR herbicide labels for proper application timing, as emergence may be too early. If applying PGR herbicides, do not spray when daytime temperatures are high and plants are growing rapidly. Dry growing conditions or heavy winds are examples of stressful times after application that could increase PGR injury potential. Corn injury has also been observed when PGR applications were made just before cool weather or with hot conditions right after application. Some PGR herbicide labels may recommend reduced rates if corn is over 8 inches tall. Additives to the PGR spray mixture can influence the uptake of PGR herbicides, so always refer to herbicide label for more information.
For additional choices of herbicides for broadleaves, refer to the Corn Herbicide Effectiveness ratings on page 8 of the 2016 Herbicide Guide for Iowa Corn and Soybean Production.