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Issue No. 8, Article 2/May 28, 2010

Postemergence Corn Herbicide Reminders

The ample soil moisture combined with very warm air temperatures has contributed to rapid corn growth and development over the past several days. As you can imagine, these conditions also promote rapid growth and development of weeds. Postemergence herbicide applications in corn are progressing as conditions allow, and we would like to offer a few reminders:

  • As mentioned in issue 7 last week ("Corn Growth Stages and Postemergence Herbicides"), it's always advisable to scout corn fields before applying postemergence herbicides to accurately determine the crop's growth stage. Adverse environmental conditions sometimes result in corn plants that are physiologically older than their height suggests, so be sure to accurately assess plant developmental stage (i.e., leaf/collar number) in addition to plant height. Pay close attention to the maximum corn stage listed on the herbicide label, and do not apply the product if corn exceeds the labeled stage. If tank-mixing two or more products, follow the most restrictive growth stage listed on any of the tank-mix component labels. For example, glyphosate can be applied broadcast to glyphosate-resistant corn up to 30 inches tall (V8), but the height limit shrinks to 11 inches if glyphosate is tank-mixed with Harness or Harness Xtra.
  • Tank-mixing two or more postemergence herbicides can provide several advantages over single-product applications. Perhaps one of the most obvious is that the spectrum of weeds controlled can be broadened over that controlled by a single product. Tank-mixes generally have been less common during the past decade as glyphosate alone has been effective controlling many broadleaf and grass weed species. However, with the occurrence of glyphosate-resistant weed populations and weed species inherently less sensitive to glyphosate, tank-mixing products with glyphosate to control these challenging species will increase. In glyphosate-resistant corn, tank-mixing growth regulators (such as dicamba and 2,4-D) or HPPD inhibitors (such as mesotrione, topramezone, and tembotrione) with glyphosate can improve control of glyphosate-resistant waterhemp and other tough-to-control broadleaf weed species, such as annual morningglory and giant ragweed. Be aware that some labels restrict tank-mixes based on the formulation of the tank-mix partner; for example, the Callisto label restricts tank-mixes with certain herbicides formulated as emulsifiable concentrates. Be sure to follow all label restrictions and additive recommendations when tank-mixing postemergence herbicides.
  • High air temperatures and relative humidity favor enhanced absorption of postemergence herbicides. Soil moisture in many areas of Illinois has been ample (though excessive in other areas), and the corn's leaf cuticle is likely to be somewhat "thinner" than it would be under moisture-limiting conditions. This condition may contribute to rapid uptake of foliar-applied herbicides. Be sure to consult the product label when selecting spray additives to include with postemergence herbicides. Many labels suggest changing from one type of additive to another when the corn crop is under stressful growing conditions. Attempting to save a trip across the field by applying a postemergence corn herbicide with a liquid nitrogen fertilizer solution (such as 28% UAN) as the carrier is not advisable. While applying high rates of UAN can itself cause corn injury, adding a postemergence herbicide can greatly increase corn injury.
  • Corn stalks can become "brittle" during periods of rapid growth and stem elongation. Be cautious with applying growth regulator herbicides to corn at this time, as these products may enhance stalk brittleness.
  • Effectiveness of postemergence herbicides can be reduced if weeds are stressed before or after the application. Labels of postemergence labels often recommend avoiding inter-row cultivation within a certain number of days before or after application to avoid reducing herbicide effectiveness. While side-dressing corn generally does not disturb as much soil as inter-row cultivation, growth of weeds near where the applicator knives passed could be slowed for a few days following this type of nitrogen application.
  • Air temperature at the time of application can influence herbicide selection. Several postemergence corn herbicide labels have application restrictions based on air temperature. High air temperatures enhance the possibility of volatilization of certain herbicide formulations. Vapors are easily moved by air currents and could potentially move out of the treated area and cause injury to nearby sensitive vegetation.

--Aaron Hager

Aaron Hager

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