Extensive research over many locations and years in Minnesota has shown the yield advantage of early-planted corn. And Minnesota corn growers have responded by planting corn earlier. Since 1968, the average planting date for Minnesota's corn acreage has been earlier by about 1/2 day per year. The average planting date for corn was May 20 in 1968 and was May 3 in 2005. State average corn yields and date when 50% of the corn crop was planted (defined as the average planting date) were analyzed to examine the relationship between planting date and state average corn yields.
While early planting is important to achieve good corn yields, it does not guarantee high yields because many other factors affect yield. However, early planting is an important management practice that is a "stage setting" yield factor. In replicated research where planting date is a variable and all other factors are held constant, there is a definite yield advantage to late April and very early May planting dates for corn in Minnesota. In addition, maturity occurs earlier in the fall allowing more calendar time for partial (or complete) field dry down or calendar time to harvest and complete fall tillage practices.
Generally, early-planted corn yielded higher than trend line corn yields. Corn yields decreased almost 1/2 bushel per acre with each one-day later for planting date from April 28 to May 21 during the 38 years from 1968 through 2005.
Corn Planted Prior to May 1
There were four years when the average planting date was prior to May 1 - 1987, 1998, 2000 and 2003. State average yields were above the trend line for all of these years, which were good growing seasons for temperature and rainfall. For these four years, corn yields averaged 10.8 bu/a above the trend line.
Corn Planted Between May 1 and May 5
There were 8 years when the average planting date was between May 1 and 5. For these 8 years, there were four with yields higher than trend, three with yields close to trend, and one with a significantly lower than trend yield (1976 which was a drought year following 1975 which was also short on moisture). For this planting period, yields for the eight years averaged 2.2 bu/a higher than trend yields.
Corn Planted Between May 6 and May 10
There were six years that the average planting date occurred between May 6 and 10; state yields averaged 2.6 bu/a lower than the trend line for these six years. Of these six years, 1988 was a drought year with yields 41 bu/a below the trend and 1992 which was a cold and wet season with yields 9 bu/a below trend.
Corn Planted Between May 11 and May 15
There were nine years that the average planting date occurred between May 11 and 15 and for these years the state yields averaged 1.2 bu/a lower than the trend line. The major low yielding year in this group was 1974 when a season ending frost occurred on Labor Day weekend.
Corn Planted After May 16
Of the 11 years when the average planting date was after May 16, there were five years with higher than average yields, three with average yields, and three with lower than trend yields. The lowest yielding years were 1975 (dry year with yields 17 bu/a below trend), 1983 (dry year and yield was 20 bu/a below trend) and 1993, which was a very late planting season and remained wet and cold such that yields averaged 56 bu/a below the trend yield. For these 11 years, state average yields were 3.1 bu/a below trend yields.
While early planting of corn sets the stage for excellent yields, there can be above average yields when planting is late such as occurred in 1968, 1969, 1970, 1972 and 1986. These years were good growing seasons characterized by average temperatures and good rainfall distribution.
On average, corn is now planted 23 days earlier compared with planting dates of the mid 60's. Based on several years of research in Minnesota, 23 days earlier has resulted in substantially higher corn yields. This analysis shows that corn planted prior to May 1 has averaged more than ten bushels per acre higher than trend yields. Corn yields decreased almost ½ bushel per acre with each one-day later planting date from April 28 to May 21 during the 38 years of 1968 through 2005. Early planting is important to achieve high corn yields.