A killing frost is a risk at any point during September. Of course, that risk increases the later into the month it gets. Nonetheless, being able to identify maturity staging is key in also knowing the risks of a killing frost prior to physiological maturity.

As seen in the table below, you generally do not want to see a killing frost before 1/2 milkline to avoid major yield losses. Of course, best case scenario is R6, or physiological maturity/black layer, when the grain has filled completely and there will be no penalty to yield or quality (weight).

Table 1: Measurements for each reproductive stage of corn development and how a killing frost would affect yield at that stage.

A killing frost occurs when temperatures dip to 0 Celsius for four hours or -2 Celsius for minutes. However, a killing frost can still occur with temperatures above freezing, especially in low and unprotected areas when there’s no wind. When grain at early to mid-dent experiences a killing frost, yield will be penalized, test weights will be low and these plants will require a long in-field drying period.

A visual inspection of frost-damaged corn should be made the morning after the frost, after the sun has risen and the crop has begun to thaw. At this time, cell contents will begin to leak out and can be seen and smelled. Determine how much of the leaf tissue has been damaged and if the ear shank is frozen. If the shank is frozen, there will not be further movement of sugars to the grain.

For details on how to move forward after a frost during each reproductive stage of corn growth, read this excellent paper from Purdue University.

Fields that have reached the dent stage (R5), days to maturity can be estimated using the chart below from Iowa State University. Estimate a corn field is at ¼ milk line (R 5.25). It will take approximately +/- 30 days to reach physiological maturity (R6), depending on temperature, available moisture and hybrid maturity.

To estimate using current daily temperatures, use the following GDD calculation:

Daily Corn GDD (°C) = ((Daily Max Temp °C + Daily Min Temp °C)/2) – 10 °C

With the following constraints:

If daily Max Temp > 30 °C it’s set equal to 30 °C;

If daily Max or Min Temp < 10 °C, it’s set equal to 10°C.

Table 2: Progression of milk line during R5 with approximate percent moisture, dry matter, growing degree day and days for each substage. Iowa State University.

Depending on current staging, we could be looking at anywhere from 20-30 days until black layer is reached, but a number at the latter end is most likely for many grain corn producers in Manitoba. While we are concerned with how long until each crop reaches physiological maturity and beating the first killing frost, keep in mind the challenges that may arise as a result of drying a wet crop.

“Reaching maturity is important as it means that the crop has maximized the amount of weight that it has packed into its kernels. However, reaching maturity is only part of the issue, as the crop must dry in the field to the point that it can be harvested and be economically dried for storage and marketing. Predicting the rate of drying in the field is more complex than predicting corn development. Factors that affect the rate of field drying include: the initial moisture content of the grain, air temperature, relative humidity, rainfall, dew, wind speed and kernel characteristics.” Joel Ransom, NDSU Crop & Pest Report, September 12, 2019

Article written by Morgan Cott, Agronomy Extension Specialist – Special Crops with Manitoba Crop Alliance