Corn and Cold September Nights

Cold September nights can cause widespread anxiety in corn producers.  The level of anxiety for most growers will be determined by the stage of maturity their grain corn crop has reached.

As temperatures drop to zero frost damage first occurs to the leaves of the corn plants.  This damage will eliminate any further photosynthesis, reduce grain filling and often have a negative effect on stalk strength.  Generally as long as air temperatures do not fall below –2 Celsius stalk tissues will remain viable and stalk constituents will be mobilized to fill the ear as much as possible.  If on the other hand temperatures fall below –2 Celsius both leaves and stalks may be damaged and no further photosynthesis or remobilization can occur.  This will terminate grain filling and kernel black layer (brown layer) will develop.  Table 1 outlines the potential risks to yield and quality for grain corn experiencing different levels of frost damage.

Table 1.  Estimated risks to grain corn yield and quality associated with late season frost damage.

Crop Growth Stage Frost Damage Estimated GrainYield Loss (%) Grain Quality Concerns
Mid-dough Complete plant


Mid-dough Leaves only


Early Dent Complete plant


Early Dent Leaves only


Half Milk Line Complete plant


Half Milk Line Leaves only


Note:  This table is meant as a guide; differences among hybrids, overall plant vigour at time of frost, and subsequent temperatures will all affect final grain yield and quality.

Generally growers will recognize the early dent stage as being the cut-off point where corn can withstand frost damage to the leaves and still produce a reasonable grain yield.  This stage is characterized by having virtually all kernels with small indentations in the crown of the kernel.  It should also be noted that grain quality concerns are based mostly on low test weights.  Our experience in Ontario does indicate that low test weight corn maintains relatively high livestock feed value.

The other question regarding cold nights revolves around the corn crops ability to continue grain filling after experiencing a cold night but were no frost damage occurs.  Dr. Thys Tollenaar conducted research at the University of Guelph where he measured 50% reductions in photosynthesis and rate of grain filling due to a cold night.  However when these plants were restored to higher temperature conditions they eventually resumed plant activities at rates similar to those plants that had never experienced the low temperatures.  In contrast to this finding is the observation that when corn plants experience near frost condition for 3-4 nights in a row the grain filling process can be significantly disabled for some time. 

In some situations frost damage will preclude harvesting the crop as grain and will force the grower to consider harvesting or selling it as silage.  There are important issues surrounding the management of the silage crop as well. Following a frost, silage corn frozen before reaching the half milk line on the kernel may be too high in moisture to properly ensiled.  In cases of frost, ideally corn harvest should be delayed until the whole plant reaches the desired moisture content for ensiling.

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