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Soybean seed possess several mechanisms that prevent germination before physiological maturity. The primary mechanism is a balance between two hormones; gibberellic acid, which promotes germination and abscisic acid, which inhibits germination.1 The level of abscisic acid peaks during seed filling and decreases as seeds near maturity allowing germination under certain conditions. Generally, temperatures need to be greater than 50 °F and seed moisture concentration above 50 percent for mature soybean seed to germinate.
Frequent rains, continuous drizzle, and heavy fogs soak pods to the point where water soaks through the pod wall and seeds begin to swell. Pods do not expand much with moisture, but may be large enough to stay intact and split after seeds have germinated.2 Swollen seeds, not yet sprouted, can cause pods to open and allow even more moisture to reach seeds (Figure 1). Once seed moisture increases over 50 percent, germination begins and continues as long as conditions stay wet. Elongation of the embryo root is the first visible indication germination has begun (Figure 2).
Figure 1. Swollen soybean seed within the pod. Photo courtesy of W.J. Wiebold, University of Missouri.
Soybean seed sprouting can be more frequent if dry conditions prevailed during pod and seed development (R4 to R6) growth stages. Drought stress results in small pods and may be followed by wet conditions during seed fill stages. As a result, expanding seed size due to favorable moisture causes seeds to outgrow and split pods.2 Open pods allow seeds direct exposure to moisture.
Figure 2. Germinated soybean seed within the pod. Photo courtesy of W.J. Wiebold, University of Missouri.
Premature seed sprouting may impact grain quality due to the following factors:
The amount of sprouted soybean at harvest is usually low and seed quality effects are typically minimal. Drier conditions are expected to cause sprouted seed to dry up and possibly fall out of pods before combining. Lower test weight or an increased level of foreign material (dried sprout pieces) are possible when loads are graded. In severe cases, light, sprouted seed could be blown out the back of the combine with air adjustments.