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Temperature affects crop growth and development. Accumulation of heat during the growing season can be used as a predictor of plant developmental progress. Growing Degree Days (GDD’s) is a calculation to express the heat accumulation. GDD’s are calculated using the maximum and minimum daily air temperature to determine the average daily temperature. From the average temperature, the base of 50° is subtracted to arrive at the daily GDD’s. There are temperature limits used when calculating GDD’s because little or no growth occurs when the temperature is greater than 86°F or less than 50°F. So when the maximum temperature is above 86°, then 86 is used as the maximum temperature and when the minimum temperature is below 50°, then 50 is used as the minimum temperature for the day. Daily GDD’s are summed for the season beginning May 1.

GDD’s and Corn Growth Stages. The average calendar dates for corn reaching tasseling and kernel growth stages in Minnesota are given in Table 1. The GDD’s that have accumulated by those calendar dates are also given in Table 1. The GDD’s will vary from the south to north in Minnesota with about 10% fewer GDD’s in northern Minnesota than are given in Table 1.

Vegetative development (leaves, stalk and roots) normally occurs between May 1 and July 19 while 1200 GDD’s accumulate. During grain filling, 1190 GDD’s accumulate. These numbers of GDD’s are not the absolute values required to produce mature grain. When corn is planted late, tasseling and grain filling occurs later in the season when average air temperatures are lower and GDD accumulation is less. Later planted corn yields less and fewer GDD’s certainly contributes to lower grain yields.

Table 1. Average calendar dates of corn growth stages and the GDD’s accumulated during grain filling stages.

2004 GDD’s
The average tassel date for the 2004 corn crop was July 25, which moves the grain filling period later – July 25 to September 18. On average, there are 644 GDD’s during July. As of July 25, 550 had occurred, leaving 94 if average temperatures occur for the rest of July. There are 584 that, on average, occur during August and another 275 for the first 18 days of September. The total of these is 948 that are expected to occur during grain filling this year if normal temperatures occur.

If normal temperatures occur, what effect will fewer GDD’s have on grain yield?
Figure 1 shows the relationship between GDD’s during grain filling and Minnesota state average corn yields for 1982 to 2003 (average yields and GDD’s for each year are given in table form in Table 2). The average GDD’s during grain filling for those years was 1052. Generally, grain yields were higher in years when there were more GDD’s that occurred during grain filling. The exceptions to that were the dry years of 1983 and 1988 when there were 1200 and 1150 GDD’s during grain filling and yields were 84 and 74 bushels per acre, respectively. The highest yields (above 140 bu/a) occurred during 1998, 1999, 2000, 2002 and 2003 when GDD’s during grain filling was more than 1100.

There were four years with fewer than 925 GDD’s during grain filling. For all of these years, the average planting date was later than May 16. The 1993 season was wet and cold with a very late planting season and a high percentage of the crop did not reach full maturity; the state average yield was 70 bushels per acre.

For years with about 1000 GDD’s during grain filling, state average yields ranged from 110 to 140 bushels per acre. Average planting dates for these years ranged from May 9 to May 18, which reduced yield potential.

If normal temperatures occur for the rest of this growing season (without moisture stress), the 948 GDD’s should produce good yields. If above average temperatures occur and the GDD’s reach 1050 or more, there should be even better corn grain yields with good test weight and quality. We need normal to above normal temperatures!

Figure 1. Minnesota average corn yields and GDD’s during grain filling, 1982-2003.

Table 2. Minnesota state average corn yields and growing degree days during the grain filling period, 1982-2003.