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). Over the past 10 years, the pace of corn planting has typically accelerated beginning about April 20 and tapers off toward the end of May.
Recent rains across Indiana, although not excessive, have delayed the start of the 2022 corn planting season. As of April 24, the USDA-NASS estimated that only 1% of the state’s corn crop acreage was “in the ground” (about 3 weeks behind the 10-year average). Continued rainfall events this past week will keep most planters in the shed and the current short-term forecast for even more rain threatens to further delay planting around the state. The fearmongers and pessimists among us are already worrying about the consequences of a delayed planting season and the risk that imposes on the crop’s yield potential in 2022.
But, hold on, let’s think about this… How absolute are the negative consequences of late planted corn? How accurately does planting date predict statewide corn yield 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 simply not clearcut.
Analysis of USDA-NASS crop progress reports over the past 31 years (USDA-NASS, 2022) indicates there is NOT a strong relationship between planting date and absolute yield or even percent departure from trend yield on a statewide basis for Indiana.
Statistically, planting dates only account for about 10-11% of the variability in trend yield departures from year to year. 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.
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 10% 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. 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. Relative grain 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 yield influencing factors for corn. What is important to understand is that the absolute yield response to delayed planting is relative to the maximum possible yield in a given year.
In other words, if all the other yield influencing factors 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 yield influencing factors 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 yield influencing factors for each year. Farmers know this to be true because many have had June planted crops yield better than any crop they have ever had, because the remainder of the growing season following delayed planting was extremely favorable for crop growth and development.
Finally, since delayed planting by itself is no guarantee of lower grain yield, there is little reason to change any crop inputs because of delayed planting, other than seeding rates. 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 to late May or beyond.