Ohio State University Extension
Department of Horticulture and Crop Science
2021 Coffey Road, Columbus, Ohio 43210-1044

Guidelines For Corn Replant Decisions

AGF-124-95

Farmers usually take into account a 5 to 10% stand reduction when selecting their seeding rates. However, they are periodically confronted by additional stand losses due to unexpected factors, such as a late frost, hailstorm, insect attack, herbicide/ fertilizer injury, flooded fields or unfavorable seedbeds. The threat of such stand reductions often forces the unwanted and difficult decision of whether or not to replant. This decision should be based on strong evidence that the returns to replanting will not only cover replant costs but also net enough to make it worth the effort. Presented here are some guidelines to follow when making a replant decision.

When to Assess Crop Damage

The first guideline is not to make a final assessment on the extent of damage and stand loss too quickly. A corn plant's growing point remains protected below the soil surface until six to seven leaves have emerged. Thus, early damage to above- ground foliage does not necessarily kill the plant. Generally, 2 to 4 days of 70 degree F or warmer temperatures are sufficient to stimulate new leaf growth on an affected plant. If these new leaves seem to be unfolding naturally, the plant should survive and resume normal development.

Facts Needed for a Replant Decision

If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including:

• Original target plant population/Intended plant stand
• Plant stand after damage
• Uniformity of plant stand after damage
• Original planting date
• Possible replanting date
• Likely replanting pest control and seed costs

To estimate after-damage plant population per acre, count the number of viable plants in a length of row that equals 1/1000 of an acre and multiply by 1000. (Table 1 shows row length needed for various row widths.) Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient.

Table 1. Row length required to equal 1/1000 acre when corn is planted at various row widths.

When making stand counts, also note plant distribution within the row. Yield loss due to stand reduction results not only from the outright loss of plants but also from an uneven distribution of the remaining ones. Studies have indicated that gaps 14 to 37 inches long reduced yields by about 2% when compared to a uniform stand, whereas 4 to 6 foot gaps reduced yields by about 5%. Therefore, the more numerous and longer the gaps between plants within the row, the greater the yield reduction.

Guidelines for Making the "Right" Decision

A major determinant in making a replant decision is the potential yield at the new planting date and possibly different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Table 2 shows effects of planting date and plant population on final grain yield for the central Corn Belt. Grain yields for varying dates and populations are expressed as a percentage of the yield obtained at the optimum planting date and population.

Table 2. Central Corn Belt grain yields for corn planted at various dates and population rates, expressed as a percent of optimum planting date and population yield (uniformly spaced within row).

Here's how Table 2 might be used to arrive at a replant decision. Let's assume that a farmer planted on April 25 at a seeding rate sufficient to attain a harvest population of 25,000 plants per acre. As a result of poor seedbed conditions and an unfavorable environment during emergence, the farmer determined on May 15 that his stand was reduced to 16,000 plants per acre. Table 2 shows that this would likely result in a 12 percent final grain yield loss (i.e., 98 percent expected yield for a 25,000 plant per acre final population planted April 25 minus 86 percent expected yield when stand is reduced to 16,000 plants per acre).

If he could replant the next day (May 16) at his original seeding rate, the farmer's final yield (according to Table 2) should be 98 percent of optimum, which is the same as that expected from the April 25 planting. Being 12 percent better than the present reduced stand projected yield, replanting appears profitable.

But what if stand reduction occurred later or conditions delayed replanting until May 31? Table 2 shows that replanting at the initial seeding rate would yield 87% of optimum, which is only 1 percent better than the expected yield from the reduced population. Thus, replanting definitely could not be justified.

Remember that table values are based on a uniform distribution of plants within the row! Add a 5% yield loss penalty if the field assessment reveals several gaps of 4-6 feet within rows and a 2% penalty for gaps of 1-3 feet.

Determining Seed and Pest Control Costs

When making the replant decision, these factors must not be overlooked. Depending on the seed company and the cause of stand loss, expense for seed can range from none to full cost.

You also need to review herbicide and insecticide programs under late-planting conditions. For instance, it may be necessary to reapply herbicides, especially if deep tillage is used. However, try to avoid such tillage depending instead on postemergence chemicals or cultivation for weed control.

Concerning insect control, if insecticides were applied in the row at initial planting, consider reapplication if tillage is used before replanting. Also remember that later planting dates generally increase the possibility of damage from insects such as European corn borer, corn rootworm beetle and black cutworm. Therefore, understand that replanting itself does not guarantee the expected harvest population.

Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.

The cost of replanting will differ depending on the need for tillage and chemical application. The cost and availability of acceptable seed will also be considerations. These factors must be weighed against expected replanting yield gains. If after considering all the factors there is still doubt as to whether or not a field should be replanted, you will perhaps be correct more often if the field is left as is.

Adapted from: Guidelines for making corn replant decisions. 1990. National Corn Handbook. NCH-30.

Prepared by:
Peter R. Thomison
Extension Agronomist