This blog entry features an excerpt from an article by Nebraska state climatologist Al Dutcher, which was posted in the University of Nebraska-Lincoln Cropwatch weekly e-newsletter Friday, August 8. Al Dutcher focused much of the article on Nebraska conditions, but the details presented here apply to other areas of the Corn Belt as well in regard to possible Growing Degree Day shortages for full corn maturity this season.--Bryce
Corn GDDs Suggest Freeze Vulnerabilities
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.
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.
Converting 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.
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.
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 degrees Fahrenheit above normal. Temperatures would need to average 2.0-2.4 degrees F above normal for a May 20 emergence date, 4.5-6.3 degrees F above normal for a May 30 emergence date, and 7-10 degrees 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 degrees 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.
The full report, with detailed information tables, is at this link: http://tinyurl.com/…
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