Soybean frost damage is pretty extensive across North Dakota and parts of South Dakota.
Yield loss will depend on how far along your soybeans were and how cold it got in the canopy, says Hans Kandel, NDSU Extension agronomist,
A frost will not hurt soybean yields if the soybean growth stage is beyond growth stage R7 (one normal pod on the main stem that has reached its mature pod color), he says.
A frost when soybean plants are between R6 and R7 growth stage may or may not affect yield, depending on the temperature and duration of the freeze.
Research information from area universities show that yield are reduced when frost occurs at or before the R6 growth stage (pod contains green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf). Yield losses up to 50% can be expected for very late maturing or late-planted beans.
Kandel says to keep these things in mind when looking at frost damaged soybeans:
Seed on frost-damaged plants may mature and change color as early as or even earlier than non-frosted soybean plants. The leaves tend to remain on the frost-damaged soybean plants. Seed moisture may be slightly higher and seed size is usually reduced as the soybeans dry and shrink.
Beans that are still green and soft will shrivel after a frost. Stalks rapidly turn dark green to brown and will not recover. Beans in pods that have turned yellow will mature normally. Some green beans which already were close to color change may turn yellow after 30 to 40 days of storage. However green beans most likely will not change their color in storage.
Growers and researchers through the years have tried to use color keys such as yellow soybean leaves, yellow pods and brown pods to estimate soybean maturity and safety from frost. Generally these methods didn't work because of differences in varieties regarding symptoms of maturity. However, studies do show that "yellow" pods sprinkled with brown are the best clue of physiological maturity.
Open pods and check shrinking of beans and look for separation of beans from the white membrane inside the pod. This indicates the soybean plants are physiological mature and fairly safe from frost injury. Pods do not all mature evenly. Note that if one or two pods on any of the upper four nodes have turned brown and other pods are light yellow to tan, the soybean plants are fairly tolerant to a killing frost. In the event of a leaf-killing frost when pods are still light green or yellow, wait until the pods are mature in color before combining. The most significant effect of an early frost on soybean may be in the reduction in their value as a future source of seed.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.