Last year, on April 26, Iowa soil temperatures ranged from 58 to 64 degrees F; however, by April 29 soil temperatures dropped to 49 to 53 degrees 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 to 26 than corn planted later. Fluctuations in soil temperatures are related to deformed mesocotyl growth, which can result in "corkscrewed corn."
According to the USDA/NASS latest weekly survey, which was as of April 28, 2013, approximately 2% of Iowa corn acres were planted by that date. This is somewhat similar to 2011: by April 24, only 3% was planted and, by May 1, only 8% was planted. Within two weeks after that though, approximately 84% of Iowa's corn acres (11.9 million acres) was planted, bringing the total planted to 92%. Even with this later-than- usual planting corn yield was about 3% above trendline in 2011.
Even with later-than-usual planting, yield in 2011 was about 3% above trend
One way to look at yields at different planting dates is to compare state average yields -- as reported in deviations from the trendline -- to the percent of the crop planted at different times during the spring. This is the way Purdue corn agronomist Bob Nielsen ("The Planting Date Conundrum For Corn") and University of Illinois agronomist Emerson Nafziger ("Planting Delays and Corn Prospects") looked at corn yield in their recent articles.
For example, in Iowa the trendline for acres planted by April 30, compared to yield relative to trendline, is actually negative (Figure 1). This suggests that delayed planting tends to increase yield potential although the trend is a weak one. The negative trend for acres planted by April 30 is likely due to the negative effect of 2012 and the three years of near trendline yields with 60% to 70% of Iowa's corn acres planted. What is clear is that earlier planting does not always guarantee high yields, and late planting does not always foretell low yields.
Although the trendline of corn planted by May 15 was positive, three years were exceptional. In 1982, 1984 and 2008 only 45%, 35% and 46% of the crop was planted by May 15, respectively; yields were above trendline by 20%, 7% and 6% respectively (Figure 2). In 1991 and 1995, only 33% and 30% 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% and -6% (Figure 2).
Every decision that you make influences the size and scope for corn yields. From the corn hybrid you select to the seeding rate and row width you choose. Download our FREE report over Maximizing Your Corn Yield.