“Denting by Labor Day” – An Important Benchmark

Joe Lauer , Corn Agronomist

The next benchmark to watch for in corn development is “Denting by Labor Day.” Most farmers feel that if their corn crop can start denting by Labor Day, the crop should mature before a killing frost. Unfortunately many areas of the state will not make this benchmark during 2004. Temperatures continue to be unseasonably cool.

The 2004 growing season at Arlington from May 1 to August 24 is 160 GDUs behind the coolest season on record for the last 30 years (Figure 1). GDU accumulation so far is 1543 GDUs. The second coolest year for this period is 1997 which had accumulated 1703 GDUs. Normally we accumulate 1975 GUDs by August 24. Since we usually only accumulate about 18 GDUs per day at this time, it means we are about 24 days ([1975-1543]/18) behind the 30-yr normal.

The 2004 growing season at Marshfield from May 1 to August 24 is the second coolest in the last 30 years. GDU accumulation so far is 1476 GDUs (Figure 2). The coolest year for this period is 1992 which had accumulated 1342 GDUs. Normally we accumulate 1844 GUDs by August 24. Since we usually only accumulate about 16 GDUs per day at this time, it means we are about 23 days ([1844-1476]/16) behind the 30-yr normal.

Some are concerned about the numerous cool nights that have occurred and the impact on corn leaf photosynthesis. Research by University of Guelph (Ontario) shows that photosynthesis and grain filling rate can be reduced up to 50% when temperature falls to 35 F. However, when higher temperature conditions are resumed, plant activities are at similar rates to those that never experienced low temperatures. If corn fields can escape any serious frost damage during cold nights, grain filling should carry on once normal temperatures return.


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