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The two main requirements for seed germination are adequate moisture and warm (around 50°F) temperatures. In corn and soybean, those two requirements are often met during grain fill and development, so why doesn’t grain normally germinate before maturity?
Among other mechanisms, corn and soybean crops maintain a hormonal balance in the seed that helps prevent sprouting before maturity. As seed approaches maturity, the hormone which inhibits germination declines until the point at which seed is mature and possesses the ability to germinate. However, while still attached to the plant, seed is usually protected from reaching the moisture threshold required for sprouting. Sometimes though, plant characteristics and environmental conditions can align to the point at which mature seed attached to the plant can sprout, an occurrence known as vivipary.
Corn: Premature sprouting of kernels prior to harvest primarily occurs when moisture becomes trapped in the husk, allowing kernels to absorb water and germinate (Figure 1). Kernel sprouting on the cob prior to harvest is most likely to occur when black layer has been reached, kernel moisture has dried to less than 20 percent, and kernels are then exposed to moisture. It is more common in warmer, southern regions. Corn products that tend to have an upright ear at maturity with an open husk may be more susceptible to kernel sprouting. Continuous rainfall at harvest favors kernel sprouting.
While it is more common after physiological maturity, immature kernels on ears that have been subject to damage have also been found to sprout. It is believed that physical damage to immature kernels may disrupt the hormone balance within the kernel and allow for early germination. Factors that contribute to premature sprouting of kernels include erect ears, stalk breakage/lodging, bird and hail damage, ear molds, wet weather, and flooding conditions.
Soybean: Generally, the soybean pod wall prevents mature soybean seed from absorbing water by shedding rain and moisture. When the pod is split open, however, water is permitted to contact and enter the seed. A combination of greater than 50% moisture and air temperatures above 50°F can result in seed germination. Sometimes, even continuous exposure of intact pods to moisture – frequent rain, ongoing drizzle, foggy conditions – can result in so much moisture that it soaks through the pod wall and wets the seed, resulting in germination.
Figure 1. Corn kernel sprouting is usually limited to several rows of kernels at the butt end of the ear because this is where water can be trapped in the husk. Sprouting can also occur from uncovered kernels near the ear tip or on the ear in general under the right conditions.
Wetting and drying cycles can cause seeds can expand and contract. If enough force is put upon the seams during expansion, pods can rupture because they do not expand as much as the seeds. Splitting is most common when seeds begin to expand (R4 to R6 growth stages) under stressful conditions and finish filling during very wet conditions. When seeds dry and contract, they sometimes fall from the open pods, a process also known as shattering. However, when the seeds remain swollen and stay in the pod, they can germinate if conditions favor adequate moisture and warm temperatures.
Impact and Management: Typically, the overall negative effects of seed sprouting are minimal; however, seed quality will be compromised. Timely harvest and quick drying of the grain can stop germination and reduce grain deterioration.
Sprouted corn kernels are usually lighter, lower in test weight, and more susceptible to mold and mycotoxin development. These quality issues may result in the grain being discounted when marketed. Once harvested, if a high number of kernels are affected, grain can be dried at higher temperatures to prevent any further growth of the seedlings. Prior to storage, screen grain to remove green growth or damaged kernels. Core the stored grain after filling to remove additional fines or broken kernels from the center of the bin.
In soybean, the sprouted seed will often dry and fall from the pods or be blown out the back of the combine. In fields where there has been sprouting, adjustments can be made to the combine to better remove the light, sprouted seed at harvest. Worst case is slightly lower test weight grain and increased foreign material (from the dried sprouts).
Sources: Holshouser, D. 2015. Wet conditions may lead to soybean seed sprouting. Virginia Ag Pest and Crop Advisory. https://blogs.ext.vt.edu/ag-pest-advisory/. Nielsen, R. 2012. Premature corn kernel sprouting (aka Vivipary). Purdue University Corny News Network. http://www.agry.purdue.edu/. Rees, J. 2018. Why are corn kernels germinating on the ear? University of Nebraska Cropwatch. https://cropwatch.unl.edu/2018/whats-causing-corn-kernel-germination-ear/. Ross. J. 2016. Splitting pods and sprouting soybean seed in the pods. University of Arkansas Extension. http://www.arkansas-crops.com/2016/08/24/splitting-sprouting-soybean/. Wiebold, B. 2016. Wet weather can cause seeds to sprout on the plant. Integrated Pest Management. University of Missouri. https://ipm.missouri.edu/IPCM/. Websites verified 10/1/18. 181005085300