Germinating Trouble: Soybean Pre-Harvest Sprouting

By Daniela Carrijo and Adriana Murillo-Williams

After a dry summer, rain events over the past two weeks have jeopardized soybean harvest in many places in the Commonwealth. Many soybean fields had just reached harvest maturity before the weather took a turn. These fields now show high levels of pod splitting (Figure 1), pre-harvest sprouting (PHS), mold (Figure 2), and other grain quality issues. These issues were likely exacerbated because the rain events were preceded by hot, dry weather.

Pod splitting happens when the dry grain quickly absorbs moisture and swells, causing the pod to rupture. The grains become exposed and susceptible to shattering, which leads to pre-harvest losses before and during combining (header losses). In addition, as the grains are exposed, they can more easily absorb moisture, leading to PHS and mold growth, among other grain quality issues.

Why is PHS occurring?

During seed formation, the growth regulator abscisic acid (ABA) is involved in the accumulation of storage reserves and desiccation tolerance and is the primary inhibitor of precocious germination. In soybean seeds, ABA levels are high early in development and decrease as the seed approaches physiological maturity (1). Therefore, mature seeds will germinate when moisture, temperature, and oxygen availability allow it.

Sprouting generally happens when mature grain reaches 50% moisture, and the temperature is above 50 F.  Seed germination leads to the breakdown of reserve molecules like lipids, proteins, and sugars into smaller molecules (2), decreasing grain quality and storability. Sprouted and moldy soybean grains are considered part of the "total damaged kernels," along with frost-damaged, heat-damaged, and other sources of damaged kernels. According to the U.S. Soybean Standards for Grain, grade 1 soybeans are allowed a maximum of 2% total damaged kernels (3).

Below are general recommendations for managing soybean fields that have been affected by the recent rain events:

1. Assess the level of damage – what are the expected yield and grain quality losses?
2. Talk with your grain buyer and insurer about discount schedules for total damaged grain.
3. Harvest affected fields as soon as possible and adjust combine settings to minimize harvest losses or eliminate damaged grain depending on the most prominent issue.
4. Before storing, dry grain to 13-12% moisture to prevent further sprouting and spoilage.

Literature cited

• Gosparini C.O., Busilacchi H.A., Vernieri P., and Morandi E.N. (2007). Endogenous abscisic acid and precocious germination of developing soybean seeds. Seed Science Research 17(3):165-174. doi:10.1017/S0960258507785872
• de Paiva Gonçalves, J., Gasparini, K., de Toledo Picoli, E.A., Dal-Bianco Lamas Costa, M., Luiz Araujo, W., Zsögön, A.; Mendes Ribeiro, D. (2024). Metabolic control of seed germination in legumes. Journal of Plant Physiology 295:154206  org/10.1016/j.jplph.2024.154206
• US Standards for Soybeans. USDA Agricultural Marketing Service.