Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


May 24, 2004


Corn Replant Decisions

Some will likely be having to make corn replant decisions in the next week or two. Corn planted this week generally yields about 10% less than corn planted in early May. A corn stand of 20,000 per acre yields about 20% less than a stand of 30,000. Therefore it will likely pay to replant corn now with a stand of 20,000 or less. Cost of replanting and the uniformity of the remaining stand need to be taken into account. Occasionally inter-seeding into an existing stand to increase the population can work, but this is risky, especially if the existing stand is larger than V2. Usually it is better to make a decision one way or the other. The following table can be used to help make decisions. Assume an additional 2% yield reduction if there are small gaps of 1-3 feet, and an additional 4-6% yield loss with numerous gaps of 4-6 feet in length. Corn hybrids planted this week should be about 5 days shorter than a full season hybrid. The following chart and more information are available in Iowa State University Extension publication Pm-1885 "Corn Planting Guide," which is available at ISU Extension offices.

Stand     April 20 -     May 13 -     May 26 -     June 10 -     June 24 -
X 1,000   May 5          May 19       June 1       June 16       June 28

28 - 32   100             99           90           68            52
   24      94             93           85           64            49
   20      81             80           73           55            42
   16      74             73           67           50            38
   12      68             67           61           46            35


Stalk Borers

Stalk borer migration from grassy areas to adjacent corn plants begins at about 1,300 Growing Degree days (GDD) Base 41. As of the end of the day on May 23, the GDD Base 41 along Highway 34 (Burlington - Mount Pleasant area) was 1,287. We are accumulating GDD at about 15 - 30 per day, depending on daily temperatures. So migration will begin about today along Highway 34 and the migration will then move northward. See pages 81 - 82 of the May 29, 2000 ICM Newsletter for scouting and management details. A tracking of GDD in the Burlington to Dubuque area is at http//



We are rapidly approaching or are at the beginning of the time for the Late Spring Soil Nitrate Test. This test can be used to determine side-dress N application rates or to provide a retrospective look at the appropriateness of the N rate already applied. Even if the entire planned amount of N has been applied, the test can provide insight indicating if the amount was too much, just right, or too little. If it is too little, there is still time to add. With the frequent rainfall received in many areas, be sure to adjust test results as appropriate. If, over several years, the test numbers come back consistently high, it provides some confidence that rates can be reduced without reducing yields. And, of course, if the numbers consistently come back "just right," it tells you to keep on doing what you are doing. Information on taking the test and interpreting the results, including adjusting for wet conditions, is in Iowa State University Extension publication Pm-1714 "Nitrogen Fertilizer Recommendations for Corn in Iowa," which is available at ISU Extension offices. The form to accompany the sample to the lab is at http//>.


If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: May 24, 2004
Contact: Virgil Schmitt

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