The effects of frost damage on grain quality is directly proportional to crop maturity and amount of killed leaf tissue. Soybeans that have only reached R6 growth stage may be severely affected by freezing nighttime temperatures. Soybeans that have at least one fully mature pod could have minor yield loss with some quality concerns. Yield loss and/or quality concerns are minimized once soybeans have 95% mature pods.
The effect of late-season frost on a soybean crop depends upon the growth stage of the soybean plants. Temperatures that range from 30°F to 32°F can easily damage the top leaves on a soybean plant. When air temperatures drop lower than 30°F, the entire plant can be killed. Fields with narrow-spaced rows (15 inches or less) seem to survive frost damage better than wide-spaced rows (30 inches and greater) because of the limited air movement within the canopy.
Frost damaged soybeans can generally be salvaged as long as plants reach the R6 growth stage by the time the killing frost occurred. The R6 growth stage occurs when the soybeans completely fill one pod at one of the upper four nodes of the main stem on 50% of the plants in the field. In dense, green soybeans, frost or freeze damage kills the upper leaves but rarely penetrates deeply into the canopy when temperatures remain above 30°F. However, once the upper leaves have been damaged, any subsequent freeze events are likely to penetrate deeper into the canopy. Once the plant reaches the R7 growth stage, yield reductions due to frost or freeze injury will be minor. R7 growth stage is characterized by when one pod on the main stem has attained its mature color on 50% of the plants in the field (Table 1).
Impacts from frost prior to soybean plants reaching maturity can include some or all of the grain being green, lower quality seeds, lower yield potential, and variable moisture content. An early frost prior to maturity may slow field dry down. Frost damaged soybeans may have a higher moisture content and be more difficult to thresh. Placing the harvested seeds in an on-farm bin with steady aeration for two to four weeks can help reduce moisture levels and may begin to turn some of the green seeds to a normal mature color. Soybean seeds in on-farm storage should be checked regularly for spoilage. Soybean seeds can be dried in a grain dryer at 130°F or lower temperatures.
1 Berglund, D. Assessing frost damage in soybeans. North Dakota State University Extension. http://www.ag.ndsu.edu;
2 Staggenborg, S., Dhuyvetter, K., Fjell, D., and Vanderlip, R. 1996. Fall freeze damage in summer grain crops. Kansas State University Extension. MF-2234. http://www.bookstore.ksre.ksu.edu;
Staton, M. 2013. How to manage frost-damaged soybeans. Michigan State University Extension. http://msue.anr.msu.edu.
Web sources verified 8/24/18.