ISU Research Suggests Changes in Corn Planting Dates and Populations

ISU Research Suggests Changes in Corn Planting Dates and Populations

Iowa map of planting dates 
Figure 1: Planting date recommendations for Iowa, based on 95+% yield potential. Current recommended ranges are April 11-May 13 (blue); April 15-May 18 (yellow); April 12-May 2 (red).

April 2, 2010

As corn hybrid genetics and seed treatments continue to improve, producers are often encouraged to consider increasing their plant populations and, more recently, to try earlier planting dates. Recent research from Iowa State University (ISU) supports these changes.

Planting Date Research

In 2006, 2007, and 2009, planting date studies were conducted at seven research sites for a total of 21 site years. The five planting dates were at 15-day increments from April 1 to June 1 each year. Data collected at each site included plant population, leaf area, plant height, grain moisture, kernel weight, and grain yield, the latter being of ultimate interest to producers.

The results indicated three planting regions in Iowa (see Figure 1) where 95+% maximum corn yields could be obtained with the ISU recommendations. They were April 11-May 13 for the southern tier of Iowa, April 15-May 18 for the middle tier, and April 12-May 2 for northeastern Iowa. (Read the full report of the ISU research conducted by Lori Abendroth and Roger Elmore.)

While this research was conducted in Iowa, it also may apply to Nebraska. Typically, we aim to begin corn planting in southern and south central Nebraska around April 20-25 while the northern tier of Nebraska aims for May 1-5. With better genetics, we may be able to push these windows a littler earlier.

Soil Temperature

See 1- and 7-day average soil temperatures at CropWatch Soil Temperature Update. We recommend using the 7-day average rather than looking at a single day as soil temperatures can fluctuate quickly this time of year.

Seven-day average soil temperatures reported April 2 ranged from 45.3 at Smithfield to 50.9 at Monroe.

Corn germinates when soil temperatures consistently reach 50ºF; when soil temperatures are quickly approaching 50ºF, corn can be planted. Today's corn seed is already treated with fungicides to help avoid damping off problems; however, planting too early when soils are not quickly approaching 50ºF can lead to problems such as seed rots, damping off, and poor emergence and poor plant populations.

In a wet year like this year, it may be difficult to get in earlier than our typical planting dates, but we bring this research to your attention in the event you wish to try it in your own fields. Our on-farm research producers working with UNL Extension educators and specialists are planning to try it. Our goal is to have as many producers as are interested use two planting dates: as early as they can get in the field and at least two weeks later.

Planting Population Research

Studies from ISU conducted from 2006-2008 across 32 sites showed maximum corn yields occurred with final plant populations between 34,500-37,000 plants/acre, a figure which may sound high to what is typically planted here. Looking at net return (using 200 bu/ac grain yield and $3.25/bu grain price), best returns occurred with 30,000-35,000 plants/acre. There was quite a bit of variability across the locations and the recommended plant populations were related most to soil type and the amount of rainfall during the growing season.

While this research was conducted at ISU, there may again be implications for Nebraska. Ultimately, ISU researchers Lori Abendroth and Roger Elmore found that a 5% increase in typical plant population appeared to be a good place to start if you're considering a change. This year our on-farm research producers working with UNL extension educators and specialists plan to study corn planting populations so we can obtain some Nebraska research. We are planning the following comparisons:

  • Irrigated on 30-inch rows with (A) 28,000 (B) 32,000 (C) 36,000 (D) 40,000 seeds/acre
  • Irrigated on 36-inch rows with (A) 28,000 (B) 32,000 (C) 36,000 seeds/acre
  • Rainfed/Dryland (do 3 or 4 seeding rates) with (A) 18,000 (B) 22,000 (C) 26,000 (D) 30,000 seeds/acre


ISU research is showing that we can plant earlier to maximize the length of the growing season and we can increase our planting populations which will increase the amount of plant material present to capture sunlight and produce grain. We don't have much data from Nebraska yet, but expect to after this year's research studies.

All of our on-farm research comparisons are conducted full field length, replicated, and randomized throughout the field to obtain valid scientific field data. For more information about on-farm research, to contact an extension educator working with on-farm research, or to obtain the plot protocols for these studies, visit the UNL Farm Research Web site. Conducting research on your farm is a great way to determine if a particular practice or product is right for your operation.

A special thanks to ISU Agronomy Specialist Lori Abendroth for providing her research findings and reviewing this article.

Jenny Rees
Extension Educator in Clay County

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A field of corn.