Impact of Fall Frost on Soybeans

A frost event late in the season can benefit pulse crops like faba beans and lentils, acting as an aid for maturity. But frost is generally a concern for other long season crops like soybeans. The level of frost damage depends on the growth stage and severity of frost.

It is initially important to select a soybean variety suited to your maturity zone in Manitoba. This is to ensure the crop reaches maturity prior to any major frost events in the fall. However, weather anomalies have been known to occur and it is important to understand the potential risks.

Severity of Frost

A light frost (0 to –1°C) may kill top leaf growth but will not penetrate the canopy. Plants should continue to mature but will take longer and there may be some green seed within pods where leaves were killed. A hard frost (< –1°C) will cause damage to green stems, pods and seeds, reducing yield and quality and may kill the entire plant. When entire plants are killed, seed fill stops.

R5-R6 (Worst case scenario) – Green and immature beans will shrivel and remain green.
R6.5 – Green-yellow beans will have a mixture of green seed that will not mature and yellow seed that will mature.
R7 – Mature, yellow beans will continue to dry down slowly with minimal yield and quality loss.

Factors that influence frost severity:

Duration of cool temperatures – longer duration of freezing temperatures can cause more damage.
Soil moisture – moisture in the soil will retain heat.
Canopy thickness – narrow, thick rows maintain heat longer than wide rows.
Wind speed – stronger wind can reduce the severity of frost.
Cloud cover – more nighttime cloud cover can reduce plant damage.

Impact on Soybeans

The impact of fall frost on soybean yield and quality is influenced by the development stage or maturity of the crop. Later-seeded soybeans and longer-season varieties are at greater risk of damage. The risk of frost is also elevated toward the end of August or beginning of September. If frost is in the forecast, check your soybeans before and after the potential frost event to assess whether any damage was incurred.

For images and full details on the percentage of yield loss expected from frost at different development stages, refer to the MPSG bulletin on Soybean Maturity and Low Temperatures.

Frost Risk Maps

Figure 1. Date of the first fall frost at 0°C at a 50% (or one in two-year) risk level.

**Figure 2.** Date of the first fall frost at 0°C at a 25% (or one in four-year) risk level.

**Figure 3.** Date of the first fall frost at 0°C at a 10% (or one in 10-year) risk level.

Additional Resources

Assessing Early-Season Frost Damage in Soybeans. The Bean Report. No. 2. June 2, 2015.