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Conventional 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 14 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?  Good questions, but the effect of planting date on statewide average corn yield is not clearcut.

If one reviews USDA-NASS crop progress reports for the past 20 years (USDA-NASS, 2013), 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 and 2 illustrate this relationship for two measures of planting progress; percent acres planted by April 30 or by May 15. 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 22 to 24% of the variability in yield trend departures from year to year. 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.

Here's the Conundrum

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 23% of the variability in statewide yields from year to year? Let's look more closely about this seeming conundrum.

It is true that corn grain yield potential declines with delayed planting after about May 1 (Myers & Wiebold, 2013, Nafziger, 2008; Nafziger, 2011). Estimated yield loss per day with delayed planting varies from about 0.3% per day early in May to about 1% per day by the end of May. Yield potential goes down 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, the good news is that planting date is only one of many YIFs for corn. What is important to understand is that yield loss due to delayed planting is relative to the maximum possible yield in a given year. In other words, if all the other YIFs work together to determine that the maximum possible yield this year for the optimum planting date is 220 bu/ac, then the consequence of a 10-day planting delay beyond April 30 (at 0.3% decrease per day) would be a yield potential of about 213 bu/ac (i.e., 220 bushel potential minus [10 days x 0.3%] due to delayed planting).

However, if all the other YIFs work together to determine that the maximum possible yield this year for the optimum planting date is only 150 bu/ac, then the consequence of a 10-day planting delay beyond May 1 (at 0.3% decrease per day) would be a yield potential of about 146 bu/ac (i.e., 150 bu/ac potential minus [10 days x 0.3%] 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. Figure 3 illustrates this often confusing concept.

In that graph, a delayed planting of corn (B) in an otherwise high yielding year may still be higher yielding than a crop planted on the optimum planting date (C) in an otherwise lower yielding year. Farmers know this to be true because some have had June-planted crops in recent years that ultimately yielded better than any crop they have ever had, because the remainder of the growing season following the delayed planting was exceptional.

For example, the crop years 2012 and 2009 represent early and late planting date years in Indiana.  About 94% of the state's crop was planted by May 15 in 2012, but only 20% of the crop was planted by May 15 of 2009 (Fig. 2).  Yet, the earlier planted 2012 crop yielded 38.7% BELOW trend yield for that year and the later planted 2009 crop yielded 9.3% ABOVE trend yield. Why? Important differences in YIFs between the years other than simply the planting dates.

Bottom Line

Let's not succumb quite yet to fearmongering triggered by the prospects of a delayed start to corn planting in 2013. “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 but quickly recovered within two weeks to a respectable pace and finished the season with the largest POSITIVE departure from trend yield in the past 20 years (Fig. 4). 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).