When frost occurs in the early fall before corn has reached physiological maturity (black layer), there is always concern about impact to yields and quality. The stage of crop, minimum temperature reached, relative humidity and duration of cool temperatures all contribute to the impact frost will have on the crop. Generally speaking, a light frost is considered below 0 but above -2°C, where a heavy frost is -2°C and greater.
In corn, grain yield and quality losses become less of a concern the closer the corn is to physiological maturity.
At R5, or the dent stage, crop impacted by either a light or heavy frost will be harvestable but there will be an impact to yield and quality (see Table 1). Within R5, kernels are often staged according to the progression of the milk line, i.e. ¼, ½, ¾. At ½ milk line (R5.5), moisture content of kernels is 35-40% and days to maturity is approximately 13-18 days away.
The stage R6, or physiological maturity, is reached when the milk line disappears and the starch line has reached the base of the kernel. Kernels have reached maximum dry matter accumulation and kernel moisture can range between 30 to 35% (but does vary by hybrid and environment). The formation of the black layer serves as a visual cue that the plant is mature. At this stage, frost will have minimal impact to yield or quality.
Table 1: Relationship between corn growth stages and calendar days to maturity, yield loss, and other kernel characteristics
A killing frost (-2°C) any time prior to physiological maturity (R6) will kill the entire plant which will stop kernel development. However, if the frost is not a killing frost and the leaves/stalks and husks are still green afterwards, grain filling will continue until maturity.
Even though the leaves may be impacted, the plants will continue to scavenge nutrients from the remaining plant material to help complete growth and maturity. However, the crop will still need the necessary heat units to aid in maturity. If the necessary heat units aren’t received, a premature black layer may form, ending further grain fill, potentially impacting yield but more likely quality.
Submitted by: Pam de Rocquigny, Provincial Cereal Crops Specialist, Manitoba Agriculture
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