By Lori Abendroth and Roger Elmore (Text written March 8, 2010)
Note: This article was written March 8, 2010. The statistical analysis and recommendations used and stated in this article may be changed slightly given further interpretation. The recommendations are not expected to be altered significantly though and are stated now to aid producers and agronomists this planting season.
Our research group at Iowa State University (ISU) has been working to update corn planting date recommendations based on new field data and combining that with what we understand of the environment and plant physiology. It has led to some exciting new findings and subsequent recommendations. As corn growers and agronomists know, corn planting dates have increasingly become earlier over the years. This change is largely due to advancements in equipment, seed treatments, hybrid stress tolerance, tile drainage, and reduced tillage. Combine these factors with a desire to maximize the length of the growing season and it is clear why we encourage earlier planting than before. Iowans plant approximately two weeks earlier today than during the 1970's.
Corn will not germinate below a soil temperature of 50°F, although seeds absorb water irrespective of temperature. Therefore, growers should plant when the soils are at or quickly approaching 50°F to avoid seed rot, poor emergence and/or poor stand establishment. Although planting too early can cause negative repercussions, in general, planting significantly after the recommended window has greater risk and potential loss for the grower.
Planting date recommendations are developed by conducting research at a number of locations and years to reduce the impact of site and weather variability that exists year to year. Weather impacts the yield response to planting date more than any other factor. Every year there are variables that can sway yields, such as: abnormally low or high number of heat units, late spring freeze, early fall freeze, dry conditions at pollination, etc. By having research at many locations and years, we minimize the impact of abnormal weather on final recommendations.
Our new recommendations are based on multi-year (2006, 2007, and 2009) and multi-location (7 research sites) data for a total of 21 site-years. We excluded data from 2008 primarily due to the exceptional amount of precipitation early in the growing season. A large amount of data was collected at the research sites including: plant population, leaf area, plant height, grain moisture, grain yield, and kernel weight. Here, we only discuss our findings in relation to grain yield.
The recommendations for maximizing yield are developed based on calendar date. At each location, our goal was to have five planting dates beginning on April 1 and ending on June 1, in 15 day increments. ISU farm managers and superintendents (listed at the end of this article) planted and managed these trials. Plots were planted as early as possible and adjustments were then made to the intervals between the remaining planting dates so that the trial always ended on 1 June. Once all the data was pooled and analyzed, three distinctive patterns or "regions" emerged for Iowa:
Recommendations are developed based on achieving a percentage of the maximum yield possible in relation to planting date. Each region had a different response curve, or optimum window of time. These "windows" were developed for each region by identifying the date that optimized yield on average and then expanding the window from there based on how narrow we want the window to be. Recommendations are given for achieving 95+% or 98+% maximum yield. The window of time that producers can expect to reach 98% to 100% yield potential in relation to planting date is narrower than the 95% to 100% window.
Northeast region (red):
This has the narrowest planting window due to the need to maximize the length of the available growing season. Grain yields begin to drop off more significantly here than the rest of the state if plantings are too late. We recommend planting between April 12 and May 2 (95-100% yield window) or between April 12 and 30 (98-100% yield window). The dataset is limited for plantings before April 12th in this region, which limits our ability to make recommendations prior to this date.
Northwest and central region (yellow):
This has a flatter yield response to planting date than the other regions. In other words, planting date does not appear as important of a management practice here as in other parts of the state. We recommend planting between April 15 and May 18 (95-100% yield window) or between April 15 and May 9 (98-100% yield window). The dataset is limited for plantings before April 15 in this region, which limits our ability to make recommendations prior to this date.
Southern region (blue):
The yield response in this part of the state is presumably related more closely to rainfall patterns and soil moisture than the length of the growing season since this typically is not a limitation as it is in the northern part of the state. We recommend planting between April 11 and May 13 (95-100% yield window) or between April 17 and May 8 (98-100% yield window).
Although planting date impacts yield and is an important factor, it is clear that an approximate 3- to 4- week window exists for growers in each Iowa region to plant their crop and realize 95% or greater yield. Other factors may limit yield, but in terms of planting date, growers should feel secure when planting within these windows. Interestingly, the regions and recommendations mimic fairly closely to what we see for average daily temperature patterns across the state during the growing season.
Thank you to the following ISU faculty and staff for their significant contributions to this project: Stephanie Marlay, Anthony Myers, Robert Foster, Dr. Philip Dixon, Jeff Butler, Mike Fiscus, David Haden, Ken Pecinovsky, Nick Piekema, David Rueber, Ryan Rusk, Jim Secor, and Kevin Van Dee.
Thank you to the following undergraduate and graduate research assistants of the ISU Extension Corn Production program who collected and managed the field and laboratory data: Lesa Andersen, Sarah Baune, Matthew Boyer, Matthew Brower, Tim Chwirka, Leslie Freehill, Nick Kastler, Wade Kent, Marcos Paulo da Silva, Derek Shalla, Eric Wilson, and Alicia Wulf.