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Changing Corn Cuticles1
by Bob Hartzler

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Originally appeared March 26, 2001 -   Plant tolerance to herbicides is generally directly related to plant size - i.e. large plants are harder to kill than small ones. However, corn tolerance to many herbicides is known to decrease with increasing plant size.  I have often attributed this increased potential for crop injury with late applications to increased herbicide interception by the larger leaf area.  In addition, the reproductive structures are initiated at about the V4 stage and may be sensitive to herbicide applications.  A recent research paper provides further insights into why delayed herbicide applications may increase the potential for corn injury (Hennig-Gizewski, S. and W. Wirth.   2000.  Changes in the biosynthesis of epicuticular waxes in maize and their influence on wetting properties.  Pflanzenschutz Nachrichten Bayer 53:105-125.).

The researchers evaluated the leaf surfaces of corn and several other plant species, looking at both composition and structure of epicuticular waxes.  They noted that crystalline wax structures on leaf surfaces reduced both the retention of spray droplets hitting the leaves and the wettability of leaves.  A reduction in droplet retention would reduce herbicide absorption, therefore reducing herbicide effectiveness.  Corn was found to differ from the other plants in that the characteristics of the cuticle surface underwent rapid changes early in the growing season.  Leaves of corn plants from VE to the V4 stage (V4 = four leaves with visible collars) had crystalline deposits of wax on the surface of the cuticle.  These crystals reduced spray retention and leaf wettability by trapping air under the spray droplets.   The reductions in retention and wettability would enhance crop safety by reducing herbicide absorption.  There was a rapid change in cuticle properties from the V5 to V8 stage.  Whereas the leaves of small corn plants had crystalline wax deposits, the later stages had a smooth wax film on the leaves.   Spray retention increased from about 30% at the V4 stage to about 80% at the V6 stage.  Corn was the only plant species studied that had these rapid changes in cuticle characteristics, and the response was consistent with several hybrids and with plants grown in the field or in greenhouses.

The change in leaf surface characteristics correlates very well with changes in corn tolerance to postemergence herbicides.  Labels for most systemic herbicides specifically target the growth stages when the cuticle is changing.   For example, the labels of DuPont's grass herbicides (Accent Gold, Basis Gold,  etc.) prohibit applications after the V5 stage of development due to increased injury potential.  The Steadfast label was recently modified to allow applications up to the V7 state.  Rates for dicamba products (Banvel, Clarity, Distinct) are reduced at this same stage of development.  This research helps explain field observations of changes in corn tolerance and reinforces the need to follow labels to minimize injury potential. 

1 The author has long been a fan of alliteration.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.