Dr. Chad Lee

Extension Grain Crops

859.257.3203

cdlee2@uky.edu

Dr. Jim Herbek

Extension Grain Crops 270.365.7541 ext. 205

jherbek@uky.edu

Assessing Damaged Corn Stands and Management Options

Chad Lee and James Herbek, Extension Grain Crops Specialists, University of Kentucky

May 2005

Assessing damaged corn stands and determining the proper management options is often a difficult task. Survival, health and expected yield of the current corn crop must be weighed against the expected management and yield of a replanted crop. The decisions are rarely clear-cut as damaged corn is rarely uniform throughout the field. Herbicides already applied must be factored to determine if any prevent corn from being replanted. The replanting decision must be based on economics and whether or not the replanted corn will result a financial advantage.

Damaged corn plants are never fun to examine and replanting decisions are often very difficult to make. These guidelines can help to separate some of the emotion of seeing the damaged field with the business of whether or not replanting is justified.

Determine if damaged plants will recover.

Check for new growth, root condition and growing point. New growth on damaged corn will usually occur within five to seven days after the damage first occurred. In addition to checking for new growth, dig up a few corn plants to check on the roots. If the roots appear healthy and new growth has occurred, then the corn plant will likely recover from the damage. If new growth has not occurred, but the roots appear healthy, then cut open the growing point and determine if it is turgid and white to light green in color. If new growth has not occurred, but the roots and growing point appear healthy then wait another day or two to see if new growth occurs. If new growth has not occurred after a couple more days, then check the roots and growing point again.

Corn that will not recover from damage will have a black growing point and roots will start to turn black as well. In addition, both the growing point and the roots will begin to lose turgidity and start to collapse.

Determine the remaining corn stand in damaged area(s) of field.

Injured, but surviving corn plants should be counted along with healthy plants to determine the population. Multiple stand counts should be made in both injured and non-injured areas of the field. Use Table 1 to determine how long of a row to count to estimate plant stand.

Table 1. Estimating Corn Stand. Determine the length of row to count. Count the plants within that row. Multiply that number by 1,000. The product is the estimated number of plants per acre. This process should be repeated throughout the field in injured areas and non-injured areas.

Table 2. Grain yields for various planting dates and population rates, expressed as a percent of optimum planting date and population rate (uniformly spaced within row).

Determine yield potential of reduced stand

Compare the population number obtained in Table 1 to the population numbers in Table 2 to help determine maximum yield. The information in Table 2 was obtained and adapted from the National Corn Handbook, NCH-30, "Guidelines for Making Corn Replanting Decisions” and is Table 5 in ID-139, “A Comprehensive Guide to Corn Management in Kentucky”. Table 2 should be viewed as a general guide. Most of the data in the table is averaged across the Midwest and may need adjustment for your particular area. For example, populations above 25,000 plants per acre should provide yields comparable to stands at 25,000 plants per acre based on Kentucky research. Planting corn after June 1 may reduce yields more than estimated in the table.

Once the expected yield from the surviving stand has been determined, you can use Table 2 to estimate expected yields from replanting. If the expected yields of the replanted corn are lower than the expected yields of the surviving population, then there is no advantage to replanting.  If the expected yields from replanting are higher than the expected yields from the surviving population, then you can proceed further with the replanting decision process.

Determine percent of field that is damaged.

In addition to knowing how much yield will be lost from the reduced stand, knowing how much the damaged area(s) will affect the entire field can help you make a decision on replanting. Estimate how much of the entire field has a reduced stand. This estimation can be done several ways, from using a measuring wheel to using GPS devices. By knowing the expected yield loss in the damaged area(s) of the field and knowing how much the damaged area comprises of the entire field, you can make a good estimate of yield loss for the entire field.

For example, if a reduced stand results in a 20% yield loss, but yield losses for the entire field are less than 2%, then you may decide not replant the damaged area. On the other hand, if the entire field has an expected yield loss of 20%, then you may want to look further into the options for replanting.

Determine if Replanting is Allowed

Some preplant and several postemergent herbicides have restrictions on replanting corn. Check the labels of herbicides already applied to the field to determine if and when corn can be replanted.

Comparing Returns of the Field As-Is to Replanting

Once you have determined that a yield gain could occur from replanting and that the herbicides applied will allow corn to be replanted, then you must consider the economics of replanting. Replanting does bring some additional costs. Seed costs, fuel costs, hourly wages for replanting (if hired labor is used), increased nitrogen costs (if some has been used by the first planting), and increased pesticide costs (if additional pesticides are needed) all must be factored into the replanting equation. The expected returns of the replanting minus the increased costs from replanting must be compared to the expected returns from not replanting. The final decision to replant must be made on economics for most all situations. The only time replanting may be justified regardless of economics is when the damaged area of the field is right next to a major road or if it is close to the homestead. In these cases, replanting is probably justified every time!!

While general guidelines have been mentioned above, there may be additional considerations for replanting in your situation. For more information on replanting decisions, and for a second opinion on whether or not you should replant a field, contact your county extension office.