By Peter Thomison

There's little evidence that most corn plantings in Ohio have been jeopardized by the cool temperatures and wet soils we've experienced during the past 1-2 weeks. However, there are fields that may have been adversely affected by localized ponding and protracted saturated soil conditions. Freeze injury followed by heavy rains or brief flooding may have also predisposed some corn to bacterial soft rots.

Farmers confronted with poor stands due to these conditions, as well as other problems that affect corn stands, may be considering replanting their fields.

Replant decisions in corn 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. Don't make a final assessment on the extent of damage and stand loss too quickly. For more on assessing low temperature and ponding effects on corn survival, consult recent newsletter articles (Low temperature effects on corn and soybean survivalInjury to corn from ponding and saturated soils).

The following are some guidelines to consider when making 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. Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient. Table 4-12 in the OSU Agronomy Guide (on-line at http://ohioline.osu.edu/b472/0005.html) shows row lengths required to equal 1/1000 acre when corn is planted at various row widths.

A major consideration 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 4-15 in the OSU Agronomy Guide is a chart developed by Dr. Emerson Nafziger at the University of Illinois (on-line at http://ohioline.osu.edu/b472/0005.html) that show effects of planting date and plant population on final grain yield for the central Corn Belt. Dr. Bob Nielsen has modified this table to provide estimates of potential yield losses for planting dates in early June (online).

Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population.

Here's how the table from the OSU Agronomy Guide can be used to arrive at a replant decision.

Let's assume that a farmer planted on April 30 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding. According to Table 4-15, the expected yield for the existing stand would be 82 percent of the optimum. If the corn crop was planted the next day on May 29, and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting would be negative (81 minus 82, or minus 1 percentage point) and indicate no advantage to replanting.

However, it's also important to note plant distribution within the row. Remember that values in replant charts like Table 4-15 from the OSU Agronomy Guide are based on a uniform distribution of plants within the row. Add a 5-percent yield loss penalty if the field assessment reveals several gaps of 4-6 feet within rows and a 2-percent penalty for gaps of 1-3 feet. 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. The more numerous and longer the gaps between plants within the row, the greater the yield reduction. It's also important to consider the condition of the existing corn.

When making the replant decision, seed and pest control costs 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. As for the correct hybrid maturity to use in a late planting situation, continue to use adapted hybrids switching to early/mid maturities, if necessary, depending on your location in Ohio.

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 soil insecticides were applied in the row at initial planting, check insecticide label restrictions before re-application. Also remember that later May and June planting dates generally increase the possibility of damage from European corn borer (ECB) so planting ECB Bt hybrids is often beneficial.

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.

SOURCE: Ohio State University.