Corn & Soybean News

April, 2007, Volume 7, Issue 5

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Cooperating Departments: Agricultural Economics, Biosystems and Agricultural Engineering, Entomology, Plant and Soil Sciences, Plant Pathology

Editor: Chad Lee

In this Issue:

1.    Corn replanting options

2.  Wireworms

3.  Herbicides and replant options

4.  Diseases on replanted corn

1.

Corn replanting options

Chad Lee and James Herbek, Plant and Soil Sciences

The warm weather in mid-March prompted many producers to plant corn early, putting those fields at greater risk for injury and stand losses. Every year farmers in Kentucky are faced with the question of letting a poor stand of corn remain or removing that corn and planting another field of corn.

Temperatures below 28°F for more than two hours will kill plant growth above the soil. The growing point of corn will remain below ground until corn has about six fully emerged leaves (V6 growth stage). Even though the soil surface will help to protect the growing point from cold temperatures, repeated nights of temperatures below 28°F will likely cause damage to some growing points.  

If corn stands are damaged, then there are several factors that go into deciding between keeping the existing stand and replanting. Some of these factors include:

1.) current stand of corn and expected yield

2.) replanting date, replanting cost, and expected yield

3.) hybrid corn seed availability

4.) herbicide(s) that have a replant restriction

5.) methods for destroying the existing crop

6.) insect management changes, if any

7.) adjustments in soil fertility programs, if any

Expected corn yields based on planting date and plant population were estimated for Illinois in 1994 (Table 1). The information in Table 1 serves as a rough estimate for well-drained soils in Kentucky. In general, final stands between 27,000 to 33,000 plants per acre provide maximum yield on well-drained soils. Lower populations would be needed on poorer soils.

Table 1 indicates that maximum yield is achieved when corn is planted near the end of April. Ideal planting dates for western and central Kentucky are closer to April 17 to April 27 (Table 2). Plantings in mid to late May in western and central Kentucky will result in close to a 20% yield loss while plantings in early June will result in nearly 40% yield losses.

The dates in Table 1 and Table 2 can be shifted back seven days for eastern Kentucky. For example, the ideal planting dates in Table 2 are from April 17 to April 27, while in eastern Kentucky ideal planting dates are closer May 1 to May 7.

·    Temps below 28°F can damage young corn plants.

·    Replant decisions usually are not a simple “yes” or “no”.

Reprinted from: Nielsen, R.L. and P. Thomlinson. 2002. Late-planted corn & seeding rates.

URL: http://www.kingcorn.org/news/articles.02/Delayed_Planting_Populations-0510.html

If corn is to be planted past May 1 in western and central Kentucky, then cornborer Bt hybrids are recommended. These cornborer Bt hybrids provide a yield advantage over non-Bt hybrids at later plantings.

Determine plant populations in fields by taking several stand counts in both injured and non-injured areas of the field, about 5 days after the injury event occurred. Tables 3 and 4 can be used to help make stand counts. Table 3 identifies the length of row needed to equal 0.001 acre. Counted plants can be multiplied by 1,000 to estimate plants per acre. If stands are erratic, then a longer row length may need to be counted. Table 4 provides plant populations for corn plants counted in 50 feet of row in 30-inch row widths. If 60 corn plants were counted, then final population would be about 21,000 plants per acre. Expected yield of the surviving stand can be determined from Table 1.

Once the surviving stand has been estimated, then other factors can be considered, such as hybrid seed availability. The demands for seed corn are strong and the availability of seed, especially seed from desirable hybrids is a real question mark for the 2007 growing season. If seed from good hybrids is not available, then farmers may have to stay with the existing corn stand or consider switching to grain sorghum or soybeans. Hybrids with relative maturities of 113 days or earlier should be considered for late corn plantings.

No changes in nitrogen fertilizer should be needed for replanted corn. Any nitrogen applied to the original planting of corn should be available to the replanted corn, as long as a flooding event has not occurred.

Details on herbicide options, insect management options and disease considerations are addressed in other articles of this newsletter.

Table 3. 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 4. Number of plants per acre based on the number of plants counted in either 50 feet of row in 30-inch row widths