Delayed crop development due to decreased heat units coupled with forecasted cool nighttime temperatures over the next week have many questioning the fate of the WI soybean crop. The soybean fields that I have scouted over the last week range from the R5.5 growth stage to 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). The minimum temperature range required for soybean seed ripening is 46.4 to 48.2 °F; whereas the optimal range is 66.2 to 68°F (Holmberg 1973). A survey of the literature suggests that though cool temperatures during grain fill may adversely affect yield, the more serious impact to soybean would likely be increasing the number of days to physiological maturity and the threat of a killing frost (Image 1).
Image 1. Average date for first killing frost in Wisconsin.
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. The risk to WI growers is that the longer we remain below the optimal temperature range for seed development the longer our crop may remain in the R6 growth stage. As we move through the R6 growth stage and into R7 soybean (physiological maturity) 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.
Though chilling may adversely impact soybean yield a more important issue may be the effect of chilling on seed quality of yellow hilum soybean. Morrison et al. (1998) found that seed coat discoloration in yellow hilum soybean increased with the accumulation of daily minimum temperature ≤ 59 °F during seed development.
Holmberg S. A. 1973. Soybeans for cool season climates. Agric. Hort. Genet. 31:1-20.
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.
Morrison, M. J., L. Pietrzak, and H Voldeng. 1998. Soybean seed coat discoloration in cool-season climates. Agron J. 90:471-474.