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Corn replanting considerations

Agriculture.com Staff 07/06/2010 @ 5:21pm

Don't make a final assessment on the extent of damage and stand loss too quickly. Replant decisions 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.

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:

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.

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. Tables 2 and 3 show effects of planting date and plant population on final grain yield for the central Corn Belt.

Table 2 is a newer chart developed by Dr. Emerson Nafziger at the University of Illinois that includes earlier planting dates and higher optimum plant populations. Table 3 is based on older data from the 1970s, but it still provides a reasonable assessment of potential yield losses, especially for planting dates in June.

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. University of Illinois replant chart developed under high yielding conditions (adapted from Nafziger, 1995-96)

*Adapted from NCH-3-"Guidelines for Making Corn Replant Decisions"

Here's how these tables might be used to arrive at a replant decision. Table 2 will be used in this example:

Let's assume that a farmer planted on May 9 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 2, the expected yield for the existing stand would be 79% 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 is 81 minus 79, or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to three bushels per acre which would probably not justify replanting costs.

It's also important to note plant distribution within the row. Remember that the values in Tables 2 and 3 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. 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.

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 planting dates generally increase the possibility of damage from European corn borer (ECB) and may warrant selection of ECB Bt hybrids (if suitable maturities are available).

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.

The following are some additional on-line sources of information on making replant decisions:

Nielsen, Bob. 2002 (rev). Estimating Yield and Dollar Returns From Corn Replanting. Purdue Univ. Cooperative Extension Service publication AY-264-W. [URL verified 5/17/04].

Nielsen, Bob and Peter Thomison. 2002. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Service publication AY-312-W. [URL verified 5/17/04].

Don't make a final assessment on the extent of damage and stand loss too quickly. Replant decisions 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.

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