The Planting Date Conundrum for Corn

R.L. (Bob) Nielsen
Professor Emeritus
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu
Twitter: @PurdueCornGuy

Early planting favors higher yields, but does not guarantee higher yields.
Statewide averages for planting progress and yield are not strongly related.
Planting date is but one of many yield influencing factors.

Conventional agronomic wisdom says that the prime planting "window" to maximize corn yields in much of Indiana opens about April 20 and closes about May 10. This "window" typically opens about one week later across the northern tier of Indiana counties (later warmup) and about one week earlier across the southern tier of Indiana counties (earlier warmup). For mostly weather-related reasons, actual corn planting throughout Indiana historically stretches from late April to late May or early June. In fact, over the past 20 years, half of Indiana's corn crop on average has been planted after mid-May (Fig. 1).

The 2024 corn planting season in Indiana to date has been slower than hoped for due to frequent rainfall and wet soils. For the week ending May 12th, USDA-NASS (2024) estimated that 36% of Indiana's corn crop had been planted. Considering the running 20-year average for this date is 46% planted, some are concerned that yield potential will begin to decrease significantly.

When analyzing statewide yield data relative to statewide planting progress, defining "late planting" can be a challenge. Since half of Indiana's corn crop, on average, is typically planted after May 15th (Fig. 1), I arbitrarily selected that date as a reference point to characterize "early" and "late" planting seasons. Years where a higher percentage of corn was planted after mid-May are considered "late planting" seasons and years where a lower percentage of corn was planted after May 15th are considered "early planting" seasons.

Then, I evaluated the relationship between percent departure from trend yield and percent of corn acres planted AFTER May 15th for Indiana over the past 30 years. Low yielding years are represented by negative departures from trend and higher yielding years are represented by positive departures from trend.

The data show there is indeed a "tendency" for lower yields statewide as more and more acres are planted after May 15th, i.e., a negative linear regression (Fig. 2). However, notice the negative relationship is not perfect. There were three late-planted years where 76 to 89% of the state's corn crop was planted AFTER May 15th but statewide average yields ended up 4.5 to 8% ABOVE trend (2009, 2013, 2022). There were also two earlier-planted years where only 31 to 38% of the crop was planted AFTER May 15th but statewide average yields ended up 6 to 10% BELOW trend (1997, 1999).

In fact, the planting date "effect" only describes about 13% of the overall year to year variability in yield over the past 30 years (calculated R² value = 0.13). Elmore & Rees (2019) documented the same absence of a strong relationship between statewide corn planting progress and departures from trend yield in Nebraska. Such a weak relationship reflects the fact that a number of other factors, in addition to planting date, also affect yield in any given year.

SIDENOTE: Scott Irwin, Univ. Illinois Ag Economist, has published several articles wherein he discussed the impact of late planting from the perspective of the U.S. corn crop (Irwin; 2022, 2023, 2024). In his multivariate regression analysis involving yield and weather data collected from 10 major corn producing states, he defined May 20th as the planting date after which substantial yield losses occur for corn, based on agronomic planting date trials. His analyses concluded that "late planting" was the third most important weather-related variable influencing average U.S. corn yields, behind July rainfall and temperature. Remember, however, the effects of summer rainfall and temperature on the corn crop are not independent of planting date because crop growth stage influences the effects of extreme weather.

Here's the Conundrum

Why is it that every corn agronomist worth their salt preaches about the importance of timely planting and yet the statewide statistical data suggest that planting date accounts for only 13% of the variability in statewide yields from year to year? Let's look more closely at this apparent conundrum.

It is true that RELATIVE grain yield potential of corn declines with delayed planting after about May 1 (Lauer, 2022; Licht & Clemens, 2021; Nafziger, 2014, 2017, 2019; Wiebold, 2022). Estimated yield loss per day with delayed planting varies from about 0.3% per day early in May to about 1% per day beginning about May 15. RELATIVE grain yield potential decreases with delayed planting because of a number of factors including a shorter growing season, greater insect & disease pressure, and higher risk of hot, dry conditions during pollination.

However, planting date is only one of many yield influencing factors for corn. What is important to understand is that the ABSOLUTE yield response to delayed planting depends on the yield potential for any given year.

In other words, if all the other yield influencing factors work together to determine that the maximum possible yield this year for an optimum planting date was 190 bu/ac, then the consequence of a 30-day planting delay beyond May 1 might be a yield potential of about 154 bu/ac. However, if all the other yield influencing factors work together to determine that the maximum possible yield this year for an optimum planting date was 240 bu/ac, then the consequence of a 30-day planting delay beyond May 1 might be a yield potential of about 194 bu/ac (i.e., higher yield than the maximum yield potential in a challenging year). Make sense?

Consequently, it is possible for early-planted corn in one year to yield more than, less than, or equal to later-planted corn in another year depending on the exact combination of yield influencing factors for each year. The accompanying Figure 3 illustrates the confusing concept of the previous paragraph. In that graph, delayed planting of corn in an otherwise high yielding year (A) may still be higher yielding than an earlier planted crop in an otherwise lower yielding year (B). Farmers know this to be true because many have had June-planted crops in recent years yield better than any crop they have ever had.............. because the remainder of the growing season following the delayed planting was extremely favorable for crop growth and development.

Another example: The 2009 and 2012 Indiana corn crops represent late and early planting date years, respectively. About 94% of the state's corn crop was planted as of May 15 in 2012, but only 20% of the crop was planted as of May 15 of 2009. Yet, the earlier planted 2012 crop yielded 38.6% BELOW trend yield for that year and the later planted 2009 crop yielded 9.5% ABOVE trend yield. Why? There were other important differences in yield influencing factors between the years other than simply the planting dates.

Bottom Line

Let's not succumb quite yet to fearmongering triggered by the delayed planting progress of 2024. We need only look back to the 2018 planting season for an example of a slow start to the planting season that was followed by a 2-week period in early May in which 60% of the state's corn acreage was planted. "Mudding in" a crop early to avoid planting late will almost always end up being an unwise decision.

When faced with prospects of delayed planting, one should certainly look for ways to expedite the planting process by eliminating unnecessary tillage trips or delaying some field operations (Nielsen, 2019; Thomison & Culman, 2019) so that you do not plant any later than absolutely necessary. One example of a field operation that can be delayed with little risk of yield loss is to forego pre-plant nitrogen fertilizer applications in favor of sidedressing the crop later. This choice is especially low risk if your planting operation includes 2x2 starter fertilizer at rates of 20 lbs/ac of nitrogen or greater.

Finally, since delayed planting by itself is no guarantee of lower ABSOLUTE grain yield, I see little reason to change any crop inputs because of delayed planting, other than possibly seeding rates. Significantly delayed planting generally coincides with warmer soil temperatures compared to early planting. Consequently, stand establishment may be more successful with delayed planting, resulting in established plant populations that are closer to actual seeding rates than the usual 90 to 95% success rate with earlier planting dates. So, you might consider slightly reducing your seeding rates if planting is delayed out towards late May or beyond.

Figures

Fig. 1. Average weekly Corn Planting Progress in Indiana for the past 20 years.
Data derived from USDA-NASS crop survey data.

Percent departure from statewide trend vs. percent corn acres planted AFTER May 15

Fig. 2. Percent departure from statewide trend yield versus percent of corn acres planted
AFTER May 15 in Indiana, 1994 - 2023. Data derived from USDA-NASS crop survey data.

Absolute vs relative planting date effect on yield

Fig. 3. The planting date conundrum relative to absolute yield potential: A late planted crop in an otherwise good
crop year (A) can yield better than an early planted crop planted in an otherwise challenging crop year (B).

More information at... The Kernel

Corny News Network

14 May 2024

URL: http://www.kingcorn.org/news/timeless/PltDateCornYld.html

The Planting Date Conundrum for Corn

R.L. (Bob) Nielsen
Professor Emeritus
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu
Twitter: @PurdueCornGuy

Here's the Conundrum

Bottom Line

Figures

Related Reading

END OF DOCUMENT

14 May 2024

URL: http://www.kingcorn.org/news/timeless/PltDateCornYld.html

The Planting Date Conundrum for Corn

R.L. (Bob) Nielsen Professor Emeritus Agronomy Dept., Purdue Univ. West Lafayette, IN 47907-2054 Email address: rnielsen at purdue.edu Twitter: @PurdueCornGuy

Here's the Conundrum

Bottom Line

Figures

Related Reading

END OF DOCUMENT

R.L. (Bob) Nielsen
Professor Emeritus
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu
Twitter: @PurdueCornGuy