Agronomy Centre

Cold 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 and silage corn experiencing different levels of frost damage.

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

Crop Growth Stage	Frost Damage	Estimated Grain Yield Loss (%)	Grain Quality Concerns	Estimated Silage Yield Loss (%)	Silage Quality Concerns
Mid-dough	Complete plant	40	Severe	30	Severe
Mid-dough	Leaves only	25	Severe	20	Moderate
Early Dent	Complete plant	25	Moderate	15	Moderate
Early Dent	Leaves only	15	Moderate	5	Minor
Half Milk Line	Complete plant	10	Minor	0	None
Half Milk Line	Leaves only	0-5	None	0	None

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

Grain Corn

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.

Test weights will generally improve as drying progresses in the field, but this can be a relatively slow process, usually requiring months not weeks to see significant improvement.  The increased quality in the crop will always have to be weighed against losses due to lodging.

The other question regarding cold nights revolves around the corn crops ability to continue grain filling after experiencing a cold night but when no frost damage occurs. Research has shown 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 conditions for 3-4 nights in a row the grain filling process can be significantly disabled for some time.

Where frost has shut the plant down completely prior to black layer, moisture loss from the kernel will be slower than normal.  Under normal conditions the flow of starch into the kernel helps to displace moisture, when this process is disrupted the moisture will need to move out through the kernel wall.

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.

Silage

Silage Moisture Concerns  

Following a frost, silage corn frozen before reaching the half milk line on the kernel may be too high in moisture to be properly ensiled.  In the case of a severe frost where the leaves and stalks are dead the plant will quickly move to a brownish color that normally would indicate plant moistures that are suitable for silage.  This is rarely the case.

The biggest risk in silage corn that has been pre-maturely frozen is harvesting when the silage is too wet for good fermentation and storage.  In this situation the visual cues such as half-milk line or some ratio of green to brown plant material are less useful.  Although the leaves may be dead most of the moisture is in the stalk and cob.  The most reliable way to get silage moistures correct is to take a chopped sample, carefully seal it in a plastic bag and take it to a lab for oven drying to get an accurate moisture test.  Koster Testers and microwaves can also be used but they often underestimate moistures by about 3%, that is, what appears to be 68% moisture may actually be 71%.  Typically, silage dries down at about 0.5% per day, this is a reasonable guide for frozen corn plants as well. Growers will notice plant tops and leaves will be lost in the days and weeks after a killing frost, this does represent a decline in potential yield, but harvesting earlier to capture this yield must be weighed against the real risk of fermentation and storage problems if harvested too wet.

The most important element in managing frozen corn fields is to get the harvest moisture correct for your storage system.  For horizontal bunk silos silage needs to be in the 65% to 70% range, bag silos 60% to 68% and tower silos 62% to 67%.  If possible, segregate very immature silage into different bunks, bags or silos so that it can managed separately from silage that is closer to normal in maturity and quality.

Nutrient Quality

Corn that has fully dented but not reached the half milk line stage can still make good quality silage. Kernels will have less starch in them than normal but higher levels of sugar in the stalks that has not moved to the kernels will help contribute to overall energy levels.

Corn at the dough stage will have lower starch and relatively higher fibre levels. OMAF (Ontario Ministry of Ag and Food) numbers suggest that dough stage silage can be 3 percentage points lower in digestibility and 8 percentage points higher in Neutral Detergent Fibre (NDF). Careful quality analysis will need to be conducted to assess immature silage nutritional factors and properly balance it in a ration.

Other Concerns

Frost may kill some of the bacteria that contribute to a natural fermentation process. Experience suggests that using a silage inoculant on frost damaged corn ensures a more reliable fermentation.  In some cases, if the frost has killed the leaves but roots and lower stalks are alive nitrates may accumulate in the lower stalks.  Silage in this situation should be tested for nitrate levels.  Increasing cutting heights can reduce the possibility of high nitrate levels when lower stalks are the areas of N concentration.