Corn Response to Post-Emergence Herbicide Applications

Post-emergence herbicide tank mixtures are an important element of integrated weed management of tough-to-control broadleaf weeds in corn. Environmental conditions, such as those that have favored development of thin cuticles on the leaf surfaces of corn this spring, influence the absorption of post emergence herbicides and potential crop tolerance. Warm, humid conditions promote rapid absorption while cool conditions may slow crop development, herbicide uptake and crop selectivity. Crops under stress may not metabolize herbicides quickly enough to avoid crop injury. Therefore, it is important to take appropriate steps to minimize the risk for injury.

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Basics of Leaf Cuticles

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The leaf cuticle changes during early corn development. From emergence to V4, under normal conditions, corn leaves have crystalline deposits of wax on the cuticle which reduce herbicide spray retention and leaf wet-ability by trapping air under the spray droplets. There is rapid change in the cuticle from V5 to V8 as the wax becomes a smooth film on the leaves1. Spray retention increases from approximately 30% at V4 to about 80% at V6 as a result of wax changes on the leaf surface. Also, certain environmental conditions, including cloudy days, can result in thin cuticles. Changes in leaf surface characteristics have been shown to correlate well with corn tolerance of post-emergence herbicides1.
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Thin Cuticles and Herbicides

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Thin cuticles can cause corn to be more susceptible to leaf burn from contact herbicides, formulation emulsifiers, additives and nitrogen based carriers (Figure 1). Enhanced absorption of systemic herbicides can also occur with thin cuticles. Additives can also enhance plant uptake and increase the risk for crop injury under some conditions. It is critical to consult individual product labels to determine which products and additives to use during different environmental conditions.
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Increased absorption combined with conditions for slow growth, can slow the recovery rate from typically safe herbicide applications. This response is commonly associated with herbicides in the ALS family, although it can also occur with other types of herbicides (Figure 2).

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The potential for a crop response to herbicide applications can also increase with enhanced absorption. This can be especially evident with plant growth regulator (PGR) herbicides. At V6 the majority of rapid stalk elongation begins2. Warm, humid growing conditions can enhance the growth rate, increasing susceptibility to brittleness. Application of PGRs during these conditions can increase the risk of stalk breakage. PGRs and other herbicides can have multiple formulations. Ester formulations tend to be absorbed more quickly than amine formulations.

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Thin Cuticles and HerbicidesPossible Effects on Yield Potential

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There are various ways herbicide injury can potentially affect the corn plant. Some are more damaging than others. Understanding what the corn plant is doing at the time of application can help determine the potential ramifications. Corn initiates ear formation around V5. The girth of the ear is determined around V7. The kernels per row is determined from V6 through one week prior to silk emergence2. While it is not pleasant to view burned corn leaves, the potential for recovery from this crop response is highly likely. Emerged corn with these injury symptoms will often recover with favorable weather and loss of yield potential is not common. This is because new whorl leaves without visual symptoms will emerge as the plant continues to grow. If a herbicide is applied during ear initiation, or girth or length determination, enhanced absorption can increase crop sensitivity and affect ear development. If the crop cannot tolerate a herbicide application at V7, while girth is being determined, a reduction in kernel rows can occur (Figure 3).
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Reduction in kernel  rows

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Management Options

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Post-emergence herbicide applications should continue. Be cautious when adding additional herbicides and additives to the spray solution. While each may be needed in certain situations, evaluate rate options, formulations, and additives to reduce the risk of symptomology. Herbicide sensitivity ratings in product characteristic charts should be reviewed. If residual herbicides are applied POST, consider the formulation. Oil-based formulations are more likely to cause leaf burn than water-based formulations.