Corn Production

Corn planting dates

How do planting dates impact final yield? The importance of planting date is covered in the 2001 Iowa State University Extension publication, Corn Planting Guide. Planting date research was conducted across the state in 2006 (see 2006 research map). Research was also conducted from 2002-2005 at two locations, the Iowa State University (ISU) Northeast Research and Demonstration (R & D) Farm at Nashua and the ISU R & D Farm at Crawfordsville.

Historically, corn planting dates have moved earlier and earlier, as hybrids and technology improve (see map in the Planting date trends article). Planting extremely early is discouraged due to wet or cold (below 50° F) soils. The condition of the seedbed is always an important consideration for proper seed germination and seedling establishment, regardless of what date is on the calendar. An advantage that corn has compared to soybean with earlier planting dates is that its growing point is below ground until the sixth leaf. With the growing point under ground, the plant is able to sustain cold temperatures with minimal long-term freeze damage.

ISU Research

Three years of planting date research (1998-2000) at Lewis, Ames, and Nashua are summarized in the Corn Planting Guide. Planting between April 20 and May 5 resulted in 100 percent yield potential, although a 99 percent yield potential could still be achieved with a planting date up to May 20. A significant yield reduction occurred only once when the planting date was extended to late May or June.

An earlier planting window was used in the more recent research, with dates beginning as early as March. This research was also featured in the article Polymer coatings for early planted corn; results here are the average of polymer coated and non-coated seed within each planting date. At Crawfordsville, four planting dates were used (Table 1). Corn planted between March 15 and April 15 yielded similarly. A significant yield loss did not occur until corn was planted on May 1 (183 bu/acre). Data from the Nashua location show the optimum planting window shifted a bit later (Table 2). The highest yields were achieved with planting dates between April 5 and May 5. A planting date that was too early (between March 20 and April 5) or too late (between May 5 and May 20) yielded significantly less.

Planting Date (+ / - 3 days) Yield (Bu/Acre)*
March 15 208 a
April 1 210 a
April 15 205 a
May 1 183 b

Table 1: ISU SE R & D Farm (Crawfordsville) planting date research conducted by K. Van Dee and J. Jensen. Yield response to planting date averaged across 2002 to 2004. *Yield values with any letter in common are not significantly different from one another.

Planting Date Window Yield (Bu/Acre)*
March 20-April 5 186 b
April 5 -April 20 198 a
April 20-May 5 196 a
May 5-May 20 183 b

Table 2: Northeast Research and Demonstration Farm (Nashua) planting date research conducted by K. Pecinovsky. Yield response to planting date averaged across 2003 to 2005. *Yield values with any letter in common are not significantly different from one another.

Overall, past and present research has shown a small yield loss with very early planting dates and a larger yield loss with late planting dates. This late planting yield loss is again confirmed by research at Nashua, Crawfordsville, and 2006 data (not shown). Although a yield loss is still possible if corn is planted too early, the optimum planting window may be earlier than previously thought.

To understand the overall response to planting date requires several years of research to eliminate year to year variability. Environmental conditions can easily reduce yield of corn planted one year and not the next, dependent on when stresses occur in the plants development. Therefore, 2006 results are not shown here in detail as more years need to be accumulated first. In general, the 2006 data showed a positive yield response to early planting dates with yields dropping off past mid-May; the optimum window was generally between April 15th and May 10th.

Recommended planting window

We believe that corn can be planted prior to April 20th and reach its maximum yield potential in all parts of the state. This should provide a sense of security to producers who need to plant earlier than in the past to get all their corn acres planted. Demographically, producers in southern counties can begin planting about two weeks before northern counties. Of course, embedded within this planting date recommendation is the assumption that soil conditions are favorable and that good hybrids have been selected. Planting date is simply one criterion among many that will help a producer reach maximum yield potential.

So why have planting date recommendations shifted?

The difference in yield responses among the years cited here and previous recommendations may be due to location or weather differences. For example, in some years, planting early versus late may help to avoid moisture stress around pollination. Or a cold spell in the spring could hamper early plantings one year while the next year it might not be a factor. Therefore, it is important to remember that planting date recommendations can change significantly depending on which years are compared. Although it appears that producers can plant earlier and still achieve maximum yield potential, it is only through more research that this can be verified to reduce any variability that may be impacting the overall recommendation.

Portions of this text, written by Lori Abendroth and Roger Elmore, originally appeared in the Integrated Crop Management extension newsletter on pages 61-62 of the IC-496 (4) -March 13, 2006 issue.