Aug. 8, 2015

Persistently cool temperatures across most of the Corn Belt have dominated this season, raising the question of whether crops will reach physiological maturity before the first hard freeze.

The portion of the Corn Belt most vulnerable to an early freeze appears to be the Dakota's, Minnesota, Wisconsin, and Michigan. The greatest hard freeze vulnerability in Nebraska lies along the northern border, as well as in fields replanted late due to devastating storm damage.

Table 1 looks at Growing Degree Day (GDD) unit accumulation for each of Nebraska's research and extension center sites through August 5. The first date listed (May 10) is when 50% of the state's corn crop had emerged, as reported by Nebraska Agricultural Statistics Service. Additional dates are listed by 10-day intervals to provide information for producers who planted late or replanted. The data in parentheses indicates how far behind the accumulated GDD units are compared to a May 10 emergence date using normal temperatures.

GDD unit differences from normal are excessive using the May 10 and May 20 emergence dates. On August 5, normal GDD unit accumulations average 25 to 27 units per day.  If we take normal GDD accumulations from emergence to maturity and divide by the number of days, the average daily GDD units average 22-23 per day.  Therefore, corn emerged on May 10 is running two to three days behind what would be expected given normal temperatures.

Table 1 also examines the GDD unit accumulations using May 20, May 30, and June 10 emergence dates, then calculates the accumulated GDD deficits this growing season based on a May 10 emergence date (in parentheses).  Converting the accumulated deficits into days behind average using a standard 22 units per day during the growing season indicates corn is running five to eight days behind for the May 20 emergence, 13-17 days behind for a May 30 emergence, and 20-27 days behind for a June 10 emergence.

The number of GDD units one could expect to receive by the mean hard freeze date is listed in Table 2. To come up with the average seasonal accumulated GDD units, GDD accumulations from May 10 to August 5 were added to the normal GDD accumulations (baseline: 1981-2010) expected from August 5 through the mean fall hard freeze date.  The high and low values represent the lowest and highest GDD values accumulated from August 5 through the average hard freeze date during the 1981-2010 period.

Table 3 lists the seasonal GDD accumulations expected using accumulated units from the emergence date through Aug. 5 plus the normal GDD accumulations from Aug. 5 through the average hard freeze date.  Low and high values indicate the range of GDD accumulations using the 1981-2010 data period.   

Table 3 shows that corn emerged on May 30 will likely accumulate 200 fewer  GDD units than corn that emerged May 20; corn emerged June 10 would be expected to have 300-350 fewer GDD units.  Producers who didn't replant a shorter maturing variety have an above average probability of incurring hard freeze damage prior to physiological maturity.

In order to bring the GDD accumulated deficits back to normal during the next 60 days for corn that emerged May 10, temperatures would need to average 0.5-1.0°F above normal. Temperatures would need to average 2.0-2.4°F above normal for a May 20 emergence date, 4.5-6.3°F above normal for a May 30 emergence date, and 7-10°F above normal for a June 10 emergence date.  These temperature thresholds assume a shorter season hybrid was not planted.

The warmest 30 days during this growing season was mid-May through mid-June when temperatures averaged 2-4°F above normal. Barring the warmest September on record, corn that emerged at the end of May through early June will likely see a hard freeze before physiological maturity, unless producers moved to a variety that needed at least 300 fewer GDD units to reach maturity. This equates to a variety that takes 13 fewer days to mature than the variety best suited to a particular growing area.

Al Dutcher
State Climatologist