Spring planting progress of corn in 2020 has been much ahead of a typical year in South Dakota. Crop development, however, seems slow. For corn, we can use Growing Degree Days (GDD), also known as Growing Degree Units (GDU) or heat units, to estimate crop stage, using just daily average temperature.
For a corn plant, the development from seedling to maturity is related to the accumulation of heat units above a threshold of 50° F. These heat units, or GDDs, are calculated by subtracting the lower threshold value from the average daily temperature. The daily average temperature is a simple average calculated by adding the maximum plus minimum temperature and dividing by two.
In the spring of 2020, corn planting began relatively early in several areas of South Dakota. If soil temperatures remain above 50° F, time from planting to emergence is around 115-120 GDDs. South Dakota typically receives about 10 GDDS per day during mid-late April resulting in a time to emergence of about 12 days (120 divided by 10). A stretch of below normal temperatures with only 5 GDDs per day would correspondingly take 24 days to emerge.
The months of April and May saw alternating periods of warm and cool temperatures, so GDDs were slow to accumulate at times. Using a planting date of April 25 in southeastern SD (Turner county), the Figure 1 shows the accumulation of GDD through May 27. The green line shows the daily accumulation of GDD. The purple line is the 30-year average. The yellow-orange line is 2019, with all of these data given the planting date of April 25. This year, there was a warm period of above-average accumulation of GDD until early May, but after May 10 the current year fell below average. This year is still ahead of what 2019 would have been at the same time, assuming the same planting date.
As of May 26, 2020 for the Turner county area, a month after planting, accumulated GDDs were 271 as compared to an average of 319. The season is young yet, however, so there is plenty of time to catch up to average, especially given the outlook for warmer than average temperatures in early June.
In the north, in Brown county, we use an estimated planting date of May 15. In that case, GDD accumulation has been near average. For a planting date just 5 days earlier, on May 10, the accumulated GDD are the same as those planted on May 15, with 134 as of May 26.
Figure 2 illustrates the difference between planting dates as well. The left side of this image was planted on April 30, and the right side was planted on May 15. As GDDs drive corn development, this two-week difference in planting date can be seen early in the season.