21 April 2010
URL: http://www.kingcorn.org/news/timeless/PltDateCornYld.html

Corn Planting Date is Important, But....

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

onventional 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). 

Very little corn, if any, has been planted in Indiana yet this spring as of 17 April. By itself, this is not much cause for concern because typically only a very small percentage of acres are ever planted by this date in Indiana. However, the specter of delayed planting is clearly on the horizon because little other spring fieldwork has been completed due to the frequent and sometimes excessive rainfall in recent weeks. For some growers, tillage operations, herbicide applications, and nitrogen fertilizer applications must be completed first before they can consider planting their crops.

What are the consequences of a delayed start to planting? How important a predictor of statewide corn yield is planting date anyway?  Does late planting in and of itself guarantee lower than normal yields?  Interestingly, the planting date effect on statewide average corn yield is not clearcut.

If one reviews USDA-NASS crop progress reports for the past 20 years (USDA-NASS, 2011), there is NOT a strong relationship between planting date and absolute yield on a statewide basis for Indiana. Specifically, departures from annual trend yields are not strongly related to corn planting progress. Figures 1 through 3 illustrate this relationship for three measures of planting progress; percent acres planted by April 30, by May 15, and by May 31. Even though one can statistically define a mathematical relationship between departure from trend yield and planting progress by April 30 or May 15, the relationship only accounts for 11 to 12% of the variability in yield trend departures from year to year (Fig's 1 & 2). In other words, a number of yield influencing factors (YIFs) in addition to planting date also affect the ultimate absolute yield for a given year.

So what's the deal?  Why is it that every corn agronomist known to man preaches about the importance of timely planting and yet the statewide statistical data suggest that planting date accounts for only 12% of the variability in statewide yields from year to year? Let's look more closely about this seeming paradox.

It is true that corn grain yield potential does indeed decline with delayed planting after about May 1 (Nafziger, 2008; Nafziger, 2011). The estimated yield loss per day varies from about 1 bu/ac/day early in May to nearly 2 bu/ac/day by the end of May. Yield potential goes down with delayed planting because of a number of factors, including a shorter growing season, insect & disease pressure, and moisture stress during pollination.

However, the good news is that planting date is but one of many YIFs for corn. What is important to understand is that yield loss to delayed planting is relative to the maximum yield possible in a given year. In other words, if all the other YIFs work together to determine that the maximum possible yield this year is 200 bu/ac, then the consequence of a 10-day planting delay beyond May 1 (at 1 bu/ac/day) would be a yield potential of 190 bu/ac (i.e., 200 bu/ac potential minus 10 bu/ac due to delayed planting). However, if all the other YIFs work together to determine that the maximum possible yield this year is only 160 bu/ac, then the consequence of a 10-day planting delay beyond May 1 (at 1 bu/ac/day) would be a yield potential of 150 bu/ac (i.e., 160 bu/ac potential minus 10 bu/ac due to delayed planting). 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 YIFs for each year. That is the reason why statewide average corn grain yields frequently vary by plus or minus 10% from the expected trend yield from year to year.

For example, the crop years 1997 and 2009 represent early and late planting dates in Indiana (Fig. 2).  About 80% of the state's crop was planted by May 15 in 1997, but only 20% of the crop was planted by May 15 of 2009.  Yet, the earlier planted 1997 crop yielded 8.3% BELOW trend yield for that year and the later planted 2009 crop yielded 8.1% ABOVE trend yield. Why? Important differences in YIFs between the years other than simply planting date.

Bottom Line

Let's not succumb quite yet to fearmongering triggered by the prospects of a delayed start to corn planting in 2011. “Mudding in” a crop early to avoid planting late will almost always end up being an unwise decision. While important, planting date is only one of many yield-influencing factors for corn.

Another reason that it is probably too early to fearmonger about the anticipated late start to planting is that growers have the machinery capacity to "catch up" quickly once the weather and soil conditions become favorable for planting. The 1992 planting season began as one of the slowest (Fig. 1) but quickly recovered within two weeks to a respectable pace (Fig. 2) and finished the season with the largest POSITIVE departure from trend yield in the past 18 years. We also know from past years' experiences that, on average, 50% of the state's corn crop is typically planted over about an 21-day period (Fig. 5). Furthermore, it is not unheard of for growers to plant 45 to 50% of the state's crop in a single week given good working conditions (Fig. 6).

References

Nafziger, Emerson. 2008. Thinking About Corn Planting Date and Population. The Bulletin (No. 2, Article 7, April 4), Univ. of Illinois Extension. [On-line]. Available at http://www.ipm.uiuc.edu/bulletin/article.php?id=890. [URL accessed Apr 2011].

Nafziger, Emerson. 2011. Corn Planting: Optimism on Hold. The Bulletin (No. 3, Article 9, Apr 22), Univ. of Illinois Extension. [On-line]. Available at http://bulletin.ipm.illinois.edu/article.php?id=1469 [URL accessed Apr 2011].

USDA-NASS. 2011. Crop Progress and Condition. USDA, National Agricultural Statistics Service. [On-line]. Available at http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID=1048. [URL accessed Apr 2011].

Fig. 1. Percent departure from trend yield versus percent of corn acres planted by April 30 in Indiana, 1991-2008.

Fig. 2. Percent departure from trend yield versus percent of corn acres planted by May 15 in Indiana, 1991-2008.

Fig. 3. Percent departure from trend yield versus percent of corn acres planted by May 31 in Indiana, 1991-2008.

Fig. 4. Departure from trend yield versus the date when at least 50% of Indiana's corn crop was planted, 1983 - 2010.

Fig. 5. Corn planting progress in Indiana during the years 1991 through 2010.

Fig.6. The fastest single week of corn planting progress in Indiana for individual years from 1991 through 2010.

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