This guide helps Minnesota corn growers evaluate hail damage and other crop injuries, determine regrowth potential, and make informed decisions about whether to replant.
We use hail injury as the model, but the principles of evaluating crop damage and regrowth potential are similar to other crop injury situations.
Causes of crop injury
Corn crop injury occurs somewhere in the state every year. The major cause of crop injury is hail, which causes many millions of corn crop losses.
Insect feeding, flooding, low air temperature, nutrient deficiencies, crusted soils, soil borne fungi and bacteria, cold seedbeds or chemical injury (fertilizers, insecticides or herbicides) may also cause crop injury and reduced stands.
Replant factors to evaluate
Deciding whether to replant after crop injury is one of the most stressful and important decisions a farmer has to make.
Extreme injury to agricultural crops is far too common and can be an emotional event. But growers should make replant decisions based on facts from research rather than their “gut feelings” of anger and despair at the loss.
Here are the seven factors for evaluating whether to replant:
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The existing plant stand.
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Distribution of the plant stand.
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Calendar date.
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Weed situation.
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Seed availability of earlier maturing hybrids.
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Cost to replant.
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Yield potential of the existing crop.
When considering whether to replant the crop, the first two factors to evaluate are the plant stand and the distribution of the remaining plants.
Evaluating the plant stand
Establishing and maintaining an optimum plant stand is important for profitable crop yields.
Poor stands may occur for several reasons, such as poor germination, crusted surface (or other poor conditions for emergence), a cold seedbed, excess moisture, insects, chemical injury (fertilizers, insecticides or herbicides) or hail.
If hail caused the stand reduction, wait until three to five days after the storm so plants can begin to regrow. This gives time for some regrowth and, ultimately, a better evaluation of whether the plants will survive.
During this waiting time, growers can make all the necessary plans for replanting such as financial considerations; preparing the equipment; determining the availability of early-maturing, good-yielding hybrids and others.
If insects caused the stand reduction, growers can make the replant decision as soon as you find and evaluate the damage.
Growing point
When hail damages young corn plants, they usually regrow if the growing point remains healthy.
In corn, the growing point remains protected below the soil surface until the V5 stage (five collared leaves). Locate the growing point by splitting a stalk down the center (Figure 1).
If the growing point has been damaged, bacteria will often invade the plant and the growing point becomes brown and soft. These plants will not recover, so count them dead.
Leaves
Some plants severely damaged by hail may have difficulty regrowing.
Plants with leaves loosely bound in the whorl usually grow or blow out, and continue with normal development.
But plants with very tightly bound leaves in the whorl usually don’t. These plants are often referred to as buggy whips or ties (Figure 1). The leaves remain so tightly wrapped that some of the uppermost leaves and the tassel cannot emerge from the whorl.
It’s impossible to determine if these plants will recover, or the degree to which they will recover. Some of these tied plants might shoot an ear and produce some grain (tassel emergence is not necessary on each plant to allow pollination).
But do not count them as living plants when making the population count, as they are unlikely to significantly contribute to grain yield.
A healthy growing point will be white to light green in color, and firm in texture.
Calculating the plant population
Measure the distance for one-thousandth of an acre for your row spacing, and count the number of live plants in that row section. Table 1 gives the length of row equivalent to one-thousandth of an acre for various row spacings. Then multiply by 1000 to determine the number of healthy plants per acre.
Make several checks throughout the field, because you may identify areas that do not need replanting.