Top Crop Manager

Impact of frost on grain corn

by Pam de Rocquigny, MAFRD Cereal Crops Specialist

Sept. 15, 2014 - Impact of frost will depend on the temperatures reached and what stage the crop is at. 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 40-45 per cent and days to maturity is approximately 13-18 days away.

The stage R6, or physiologically maturity, is reached when the milk line disappears and the starch has reached the base of the kernel. Kernels have reached maximum dry matter accumulation and kernel moisture can range between 30 to 35 per cent (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 no impact to yield or quality.

Table 1: Relationship between corn growth stages to moisture content and yield loss

Growth Stage    Moisture Content%      Yield Loss (%)   

Dough (R4)

60-70 40-70

Dent (R5)

50-55 25-40

Dent (R5.5) (1/2 Milk Line)   

40-45 5-10
Maturity (R6) 30-35 0


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.