Forecasted overnight temperatures across the province are expected to dip as low as 7°C. In some areas this morning, temperatures dipped down to 4°C. When overnight temperatures dip below 5°C, there is a risk for a light frost in low-lying areas.
While there’s little risk for a killing frost (-2.2°C) with these forecasted temperatures, low overnight temperatures can impact seed fill and maturation. Cool temperatures (<10°C) during seed filling stages can result in reduced seed size and delayed maturity. Optimal temperatures for soybean ripening are 19 – 20°C and minimum temperatures for seed ripening are 8 – 9°C.
Some soybean fields are only entering R6 (full seed) stages this week. On average, soybeans will spend roughly 20 days at the R6 growth stage. With these low temperatures, we can expect this period to be extended even longer, putting soybeans at risk of experiencing a frost in September.
Soybean Maturity and Low Temperatures →
Fall Frosts: Risk and Yield Impact
On average, the first fall frost occurs between Sept. 9 to Sept. 24, depending on region. One in four years, a frost may occur as early as Sept. 1 to Sept. 16.
Light frosts (0-1°C) may kill top leaf growth but generally won’t penetrate the crop canopy. Plants will continue to mature, but will be delayed and green seeds will occur within pods that were frosted. Hard frosts (<-1°C) will cause damage to green stems, pods and seeds, reducing yield and quality. When entire plants are killed, seed fill stops.
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.
R5-R6 (Worst case scenario) – Green and immature beans will shrivel and remain green. Up to 50% yield loss at R6.
R6.5 – Green-yellow beans will have a mixture of green seed that will not mature and yellow seed that will mature. Up to 30% yield loss at R6.5.
R7 – Mature, yellow beans will continue to dry down slowly with minimal yield and quality loss. Considered ‘safe’ from frost and less than 10% yield loss expected at R7.
Impact of Early Fall Frosts on Soybeans →
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