Determining the Yield Potential after a Storm

In assessing damage after a storm or other disaster it is important to evaluate the yield potential of the crop. To do this the grower should determine the stage of growth, number of living plants, and the amount of leaf damage. The following steps can be used in this process.

Step 1: Determine the stage of growth

Before tassel emergence corn growth is determined by the number of leaves on the plant. The first leaf is the coleoptile leaf. The coleoptile leaf is distinguished by its rounded tip as compared to the pointed leaf tip of the other leaves. The coleoptile leaf usually dies shortly after the plant emerges, so care should be taken to account for it. Leaf stages are only counted once the leaf collar is visible. This means that you must be able to distinguish the collar before counting that leaf. Once the tassel emerges from the topmost leaf whorl (the VT stage) corn enters the reproductive stages. Stage R1 refers to the emergence of silks from the ear. Table 1 shows the common stages of corn development. If you are examining a corn plant during the vegetative growth that has had significant leaf loss, the best way to determine the stage of development is to split the lower stalk lengthwise and inspect for internode elongation. At the very bottom of the stalk are four compressed nodes that are very hard to distinguish form one another. The first node above the first elongated internode generally is the fifth leaf node. The vegetative stage can be determined by using this node as a reference point and counting the number of elongated internodes above this fifth leaf. This will tell you the plant growth stage.

Table 1. Growth stages for corn in both vegetative and reproductive periods.

Step 2: Determine which plants are damaged

Following hail, herbicide, or other types of damage it is important to determine which plants will recover and which ones will not. The first thing to look for is damage to the growing point. This can be done by splitting the corn plant down the center with a sharp knife. Prior to V4, the growing point is below the surface of the soil and appears as a mass of dense tissue at the base of the plant. By V6 the growing point can be found about 1 to 1.5” above the soil surface and appears to be a dome shaped area of dense tissue. From V8 to tassel (VT), the growing point can be found above the elongating internodes. By VT, the growing point has differentiated into the tassel structures. Regardless of the growth stage, a healthy growing point should appear light green to whitish. When a growing point dies it appears watery, mushy, with a brownish color. Any new leaves above the dead growing point will first appear wilted and limp. Eventually, these leaves will turn brown and die. Stalk damage below the growing point is serious and should be carefully examined. If the stalk is showing damage at the internodes or is split at a node there is a good chance that the plant will eventually lodge due to stalk rot or other secondary invaders. Any plants that have dead growing points or stalk damage should be counted as non-productive or dead plants. Damage above the growing point is usually less serious. The corn plant can tolerate a great deal of leaf loss without severe yield reductions. However, following a hail storm or other type of leaf damage there can be a situation where the dead leaves prevent the new leaves from emerging from the whorl. To determine if this is the case, examine the whorl of the plant. If the leaves are being restricted, the outer leaf tissue will be very tight with a characteristic wrinkling known as a “buggy whip” (resembles the outer appearance of an old-fashion buggy whip). Since it is impossible to predict if these leaves will unfurl and resume normal development, these plants should be counted as dead plants.

Step 3: Determine the remaining plant population

Mark off a section of row representing 1/1000th of an acre. Table 2 shows the row length required for several different row spacings. Within this section, remove the dead plants determined in Step 2 and count those that remain. Multiply by 1000 and you have the remaining plant population.

Step 4: Determine crop yield potential

If the remaining plant population is greater than 20,000 to 22,000 plants per acre, there is still an opportunity for a profitable yield in North Carolina. In most cases, fields with this range in plant population can produce corn yields that meet or exceed the realistic yield expectations (RYE) for most soil types in the state.

If the remaining plant population is between 18,000 and 20,000 plants per acre, the grower must assess the current replanting situation carefully. If the damage has occurred early enough in the season to allow for replanting of corn (before May 10th) or the situation allows for little additional cost in replanting another crop with good yield potential and the RYE for corn on that soil type is greater than 130 bushels per acre, then replanting could be done. For corn, expect yield reductions of about 10% when plant population is between 18,000 and 20,000 plants per acre on soils with RYE’s over 130 bushels per acre. If the RYE for that soil type is below 130 bushels per acre, then there is still an opportunity for corn yields to reach that level.

If the remaining plant population is below 18,000 plants per acre and there is still time to plant an alternative crop with a good chance of normal yields, then the grower should seriously consider replanting. A corn crop with less than 18,000 plants per acre will not yield over 100 bushels per acre under normal conditions. Expect 25 to 40% yield reductions when plant population is less than 18,000 plants per acre.

Table 2. Row length needed to measure 1/1000th of an acre for determining plant populations at different row spacings.

Step 5: Determine yield loss due to leaf damage

In addition to the loss of plants in the field, most storms lead to a loss of leaf area. This leaf area loss and its effect on yield must be taken into account. Table 3 shows the effect of various levels of leaf loss on yield at different growth stages. Note that leaf loss has less effect on yield when it occurs either early or late in the growth of the corn plant. The amount of leaf loss should be determined in several representative areas of the field by examining the plant and estimating the percentage of leaf area that has been destroyed. A good estimate is adequate for determining potential yield loss from Table 3.

Table 3. Effect of various amounts of leaf area loss in grain yield at several stages of development. Adapted from Crop Loss Instructions NCIS Pub.#6102, Rev. 1984.

Step 6: Determine if replanting should be done by determining crop yield potential

The best way to determine the yield potential of the damaged crop is to start with the RYE estimate for that field. If you have yield records, use the average corn yields for the best three out of five years. If no yield records are available, use the RYE for that soil type. Multiply the RYE by the percent yield reduction expected from the loss of plant populations (see Step 4). Subtract that number from the original RYE to get the population-adjusted yield. Now multiply the population-adjusted yield by the percent yield reduction expected from the loss of leaf area (see Step 5). Subtract that number from the population-adjusted yield to get the potential yield of the damaged crop. The example below shows how this can be done:

Growth Stage is V10
Original RYE = 145 bushels per acre
Remaining Plant Population is 19,000 plants with a yield reduction of 10%
Damaged leaf area is 60% with a yield reduction of 8%

RYE X Population yield reduction: 145 X .10 = 14.5
Subtract 14.5 from 145 to get: 130.5 bushels per acre population-adjusted yield
Multiply by leaf loss yield reduction: 130.5 X .08 = 10.4
Subtract 10.4 from population-adjusted yield to get: 120.1 bushels per acre potential yield

Once the remaining potential yield of the corn crop has been determined, the grower should pencil out his expenses for replanting and the return expected from doing so, and then compare that with the potential yield and return for the damaged crop. The best way to do this is to develop a balance sheet with the costs of replanting and expected returns from the replanted crop on one side and the potential corn yield and return from the damaged crop on the other.

In the example shown above it is unlikely that this grower would gain anything by replanting if the damage has occurred after May 10 (most likely if the corn is at V10). In fact, in most cases the grower will find that it will take severe reductions in plant population and leaf area to justify replanting. In this example, plant populations below 18,000 plants per acre and leaf area loss of greater than 80% would have been required to make replanting an economic alternative.