Soybean Harvest

Impact of Fall Frost on Soybeans

A frost event late in the season can benefit crops such as 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 to crops will depend on the growth stage and severity of frost. It is initially important to select a variety with a maturity group suited to your region. 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

Frost is generally referred to as temperatures of 0.0°C or lower. A “hard” or “killing” frost is considered to be temperatures of -2.2ׄ°C or lower, which can result in plant death.

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 is better
  • Cloud cover – more nighttime cloud cover is better

Impact on Soybeans

The impact of fall frost on soybean yield and quality is influenced by the growth or maturity stage. The maturity stage in turn depends on the variety maturity group and the planting date in spring. Though the risk of frost typically increases across Manitoba at the end of August or beginning of September, soybeans approaching maturity at this time are at a lower risk of yield and quality loss.

It is a good idea to assess your field prior to a potential frost to understand the staging and maturity of your beans  and the potential impact.

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

What will happen to soybeans following a 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 in 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.

Reading the Risk Maps

Prior to selecting soybean varieties and planting in the spring, it is important to know your risk of both spring and fall frost events in Manitoba. Risk maps provide the calendar date(s) in which you can expect the last spring or first fall frost (Figures 1-3) in your area.

According to Figure 2, there is a 25% (or one in four year) chance that the first fall frost will occur as early as September 13 in the Pembina Valley, September 7 in the northern Interlake region, and September 1 in areas of the Parkland region.

Average Date of the First Fall Frost

Figure 1: Average first fall frost dates at a 50% or one in two year risk level.
Figure 2: First fall frost dates at a 25% or one in four year risk level.
Figure 3: First fall frost dates 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.