The late planting season and relatively cool summer have increased the potential for frost damage to occur in not yet mature soybean this fall. Soybean plants are easily damaged by temperatures in the 28 to 32⁰F range.
Research information from Wisconsin has shown that all varieties tested had reduced yields when frost occurred at or before the R6 growth stage. The R6 growth stage is defined as ‘full seed’ – the stage at which time pods contain green seeds that fill the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf and occur in more than half the plants in the field. Earlier-maturing varieties sustained economic yield losses from frost at more advanced growth stages than later-maturing varieties. The greatest yield losses occurred when frost damaged plants at the R5 growth stage. The number of beans per plant and reduced bean size contribute to overall yield loss.
Generally speaking, soybean fields planted to narrow row spacing (6 or 7 to 12 inches) may have slightly more tolerance to light frosts than soybean planted in wider rows (30 to 36 inches). The heavy plant canopy of the solid-seeded, closely drilled beans tends to hold the soil heat better and therefore protects the plants to some degree.
With a dense canopy a light frost (30 to 32 F, during a short period) may kill the upper leaves but the frost may not penetrate into the lower canopy. The killed leaves initially turn blackish. Once the upper leaves have been damaged an additional frost event will most likely penetrate deeper into the canopy and do damage to the leaves and stems.
If the plants have reached the R7 growth stage, yield reductions due to below freezing temperatures may be limited. The R7 growth (beginning maturity) occurs when one normal pod on the main stem has reached its mature pod color. A frost between R6 and R7 may or may not affect yield, depending on temperature and duration of freeze.
Frost-damaged soybeans generally can be harvested as long as the plants reached the R6 growth stage at the time the killing frost occurred. Frost-damaged soybeans may have higher moisture content and possibly are more difficult to thresh. Adjust the combine by reducing the concave clearance and adjust the cylinder speed if needed. Remove as much chaff and green plant material as possible before storing beans.
Severely frost damaged plants may dry down more slowly. The beans will need to be dried to a safe moisture level for storage (12% for 6 months). Electronic moisture meters may underestimate the moisture content in green and immature soybeans and actual moisture content may be 1.0 to1.5 percentage points higher.
Green and immature soybeans are considered damaged seed at the elevator. Elevators may discount loads with green and or immature soybeans and in some cases may reject entire loads if the damage levels are high.
(Source – http://www.farms.com/news/frost-damaged-soybean-81540.aspx)