Manitoba Forest Summary

The extent of frost damage to a crop will depend on several factors. The species, stage, and
hardening of the crop, the soil type and soil moisture, the actual air temperature, the duration of freezing, and the
rapidity with which freezing takes place are important. A drop in air temperature of short duration will cause less
damage than a prolonged period the same low temperature.

Assessment of frost damage – The first signs of damage in canola can occur within the first 6 hours after the
event as plant wilting (severe) or speckling seen on the pods. In soybeans, it may take longer to see damage and
may be limited to killing upper leaves and have not had frost penetrate into the crop canopy. In corn, first signs of
frost will appear in 1 to 2 days, appearing as water soaked lesions on leaves.

Canola – information from Canola Council of Canada. Seeds with greater than 20% moisture will be damaged by
frost, as a reference, canola at 60% seed colour change is approximately 30% seed moisture.

If intention is to swath canola:

>Light frost (0C to -2 C) - Check in the afternoon and evening after the frost for wilting to assess frost
damage, there may be speckling on the stem and pods, but this is of little concern as long as the plant is
still alive and not wilting. If no wilting, leave the crop standing and check daily for seed colour change to
indicate swath timing.

> Heavy frost (> -2 C) - Assess the damage in early afternoon. Check pods for a white, wilted appearance.
Pod shatter and pod drop could begin within a day, especially with warm sunny afternoons. If pods are
desiccating rapidly, swathing right away will preserve as much yield as possible. If the pods are severely
damaged and are beginning to desiccate, swath during periods of dew or high humidity to reduce the
amount of pod shelling and pod drop.

If intention is to straight cut, see Canola Council information on Frost on Canola Left for Straight Combining

Soybean – Using the Manitoba Pulse and Soybean Growers soybean maturity guide on their website,
determine the growth stage of your crop using pictures and descriptions. In terms of yield loss, use the following
as a guide:

•  Frost during the R5 stage - reduce yield by 50%-70%.
•  Frost at the R6 stage - reduce yield by 20%-30%.
• Frost at the R7 stage - reduce yield by only about 5%.
•  Frost at R8 stage - no yield reductions expected.

Besides yield loss, there is also the increased concern of green seed issues. This occurs when there is rapid dry
down in the plant and seed – e.g. when frost occurs or when hot/dry weather causes premature and fast ripening.

Corn (Silage) – A killing frost occurs when temperatures reach at or below 0C for 4+ hours. In terms of yield
loss, use the following as a guide.
>Frost at R4 (dough) - kernel moisture of around 70% and too high of moisture to be ensiled successfully.
This crop will need to dry in the field for several days and monitored closely for moisture content.
>Frost at R5 (dent), stage 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%. Frost occurring during the R5 stage in silage corn will require
harvest right away before leaves start shedding or plant breakage occurs, to prevent yield loss

Full forest summary