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How much corn will be planted late?

Updated: 04/29/2013 @ 10:46am

The spring rains that have improved soil moisture conditions in many areas have been welcomed as favoring a return to more normal corn yields in 2013 following the drought of a year ago. At the same time, persistent and heavy precipitation (including snow in Northern areas) that has delayed the start to corn planting raises concerns that late planting will have a negative impact on yield potential. 

Planting date, of course, is not the only factor and probably not the most important factor impacting corn yields. There are ample historical examples of late-planted crops that yielded near or above trend value, early planted crops that yielded below trend value, and timely planted crops that yielded both above and below trend value. Still, timeliness of planting is an important consideration for yield potential at this stage of the season. Here, we address the likelihood that a larger-than-average percentage of the corn crop will be planted late this year.

The first challenge in this analysis is to define late planting. The impact of planting date on potential corn yield has been clearly identified by agronomic research. That research generally shows, with all other conditions equal, that optimal yield potential is maintained over a fairly wide window of planting dates, but it declines at an increasing rate for planting dates after the optimal window. Research in central Illinois finds that, all else equal, average corn yields are not found to be substantially different for planting dates ranging from early April to mid-May. Yields generally decline at an accelerating rate for planting dates after mid-May. However, since planting dates have generally become earlier over time, yield response to planting date is nonlinear, and planting occurs at different times in different regions, defining late planting over time for the U.S. is not straightforward. In previous analysis we have quantified late planting as the percentage of the U.S. crop planted after May 30 in years prior to 1986 and after May 20 since 1986. That quantification balances the results of agronomic research and regional considerations and is used here.

Based on planting progress data as reported in the USDA's weekly Crop Progress report for the major corn producing states and the previous definitions of late planting, we calculate that an average of 15% of the U.S. crop was planted late in the 42 years from 1971 through 2012. The percentage of the crop planted late ranged from 1% in 1977 to 47% in 1995. Other years with more than 25% of the crop planted late included 1993, 1996, 2002, and 2009. Other years with less than 5% of the crop planted late included 1971, 1980, 1985, 2000, and 2012.

For the current year, the USDA reported 4% of the crop had been planted as of April 21. That leaves 29 days to plant the crop before late planting begins (May 20). For late plantings this year to equal the long-term average of 15%, 81% of the crop needs to be planted in that 29-day period. Reaching 85% planted by May 20 depends on how many days are suitable for planting and how much of the crop can be planted in each suitable day. The likely number of suitable days for planting through May 20 can be projected based on the average number of suitable days during that period in the past. We have not assembled that data for the U.S., but the data have been calculated for Illinois for the period of 1967 through 2012. Based on estimates of days suitable for fieldwork reported in the Illinois Weather and Crops report, about 50% of the days from the last 10 days of April through the first three weeks of May were suitable for fieldwork. Assuming those estimates are also reasonable for other corn-producing states, history suggests there will be about 14.5 days suitable for fieldwork from April 22 through May 20, 2013.

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