Average recommended planting dates for corn in Iowa lie between April 12 and May 18 in order to achieve 98-100 percent yield potential, but this varies a bit on the specific location. Iowa soil temperatures ranged from approximately 57-63 degrees F. prior to the May 2, 3 cold, rain and snow.
While those pre-cold-snap temperatures are often considered good for planting corn because 50°F is the temperature necessary for corn seed germination, the weather that hit was not ideal for a corn seed to germinate and begin emergence.
Last year, on April 26, Iowa soil temperatures ranged from 58-64°F; however, by April 29 soil temperatures dropped to 49-53°F across the state. This swing in temperatures, coupled with rain at that time, resulted in many reports of lower relative yields for corn planted between April 22-26 than corn planted later. Fluctuations in soil temperatures are related to deformed mesocotyl growth, which can result in “corkscrewed corn” (Nielsen, 2012).
In a look at historical planting dates and yields, some interesting data appears. Although the trend line of corn planted by May 15 in Iowa is positive, three years were exceptional. In 1982, 1984 and 2008 only 45, 35 and 46 percent of the crop was planted by May 15, respectively; yields were above trend line by 20, 7 and 6 percent respectively. In 1991 and 1995, only 33 and 30 percent of the crop was planted by May 15, respectively. In these two years in which planting proceeded slowly, yields deviated from trend by only -3 percent and -6 percent.
The relationship between acres planted by May 30 and yield is not strong; this is likely due to the fact that the majority of corn acres are generally planted by the end of May. In 1982, corn was only 71 percent planted by May 30 and yields were above trend line by 20 percent; however, in 1991 and 1995, only 70 and 80 percent of corn was planted and yields were below trend line by -3 percent and -6 percent. [Obviously, 1991 and 1995 were really rough years and really late planted crops.]
Farmers having planted 1.2 million acres per day in critical windows of recent years, Iowa farmers are now equipped to plant many more acres per day than ever before. Waiting for soil temperatures to rise above 50°F and warmer weather in the forecast is most favorable for corn growth, development and yield (Elmore, 2013).
Delaying planting until conditions improve will encourage more uniform and faster emergence, greater emergence percentage and more rapid growth and development. Planting date is only one of many yield factors; weather conditions the rest of the season, management and genetics will likely be more substantial yield factors.
While our normal recommendation—based on multiple years and locations of data—is to plant early to maximize yield potential, keep in mind that soil conditions and forecasted weather affect planting recommendations. Early planting does not ensure high yields just as planting late does not foretell low yields.
Source: Warren Pierson and Roger Elmore, Department of Agronomy