The threat of a killing frost across WI has many growers concerned about the potential yield loss to soybean. The soybean fields that I have scouted over the last week are well into the the R6 growth stage (full seed: pod containing a green seed that fills the pod cavity at one of the four upper most nodes on the main stem with a fully developed leaf). In fact as of Monday 9/12/11 8% of the soybean varieties had reached physiological maturity (R7) at our Arlington site. On average a soybean plant remains in the R6 growth stage for 18 days; however the range can be as short as 9 days or as many as 30. As we move through the R6 growth stage and into R7 soybean seed moisture declines thus decreasing the risk of yield loss due to frost. Judd et al. (1982) found that seed in green pods which contain 65% moisture are injured at 28 °F whereas seed found in brown pods at 35% moisture was not injured at 10 °F. If frost damage is suspected remember that frost damaged soybeans will dry slower in the field and accuracy of moisture sensors may be suspect (may be 1-2% higher than the moisture meter reading) (PM 1635).
For more information on storage issues and uses of frost damaged soybean read page two of: Frost Damage to Corn and Soybean.
PM 1635: Frost Damage to Corn and Soybean. http://www.extension.iastate.edu/publications/PM1635.pdf
Judd, R., T.M. Tekrony, D. B. Egli, and G.M. White. 1982. Effect of freezing temperatures during soybean seed maturation on seed quality. Agron J. 74:645-650.