Soybean harvest is slowly starting across the state (2% complete, according to the USDA Kansas Agricultural Statistics report from September 30), slightly below the 5-year average. Soybean seed filling conditions were quite variable across the state and greenness remains in some fields.
Green stem syndrome in soybean is a condition by which the stem remains green while the seeds are mature and ready to harvest. In parts of the state, there are many fields of soybeans with brown pods but green stems (Figures 1, 2, and 3). A hard freeze will kill the leaves and stems, but it still may take a while for the leaves to drop if leaves are still green.
Producers can either harvest these soybeans now if the seed moisture is dry enough, or wait until the leaves have dropped and the stems have dried down. In most cases, it would be best to harvest sooner rather than later to reduce losses from shattering and lower seed quality. Harvesting beans before the leaves have dropped can be messy and gum up the combine, but at least the yield level will be maintained. Harvesting soybeans with green stems can be challenging. Make sure harvesting equipment is sharp and in top condition, and take it slow in the field.
Figure 1. Green stems and brown pods (seeds are mature) characterize green stem syndrome in soybean. Infographic developed by Ignacio Ciampitti, K-State Research and Extension.
Figure 2. Green stem syndrome in soybeans. Infographic developed by Ignacio Ciampitti, K-State Research and Extension.
Causes of green stem syndrome
What causes this unusual situation? It is most likely due to a combination of early-season stress, low pod counts, and improved late-season growing conditions.
In a normal situation, soybeans will accumulate carbohydrates and proteins in the leaves and stems up until seeds begin to form (R5). The leaves provide the photosynthates needed by the newly formed seeds as they begin filling. As the seeds continue to get bigger, their need for photosynthates will eventually become greater than what the leaves can provide through photosynthesis. As this occurs, the plants will move carbohydrates and nutrients from the leaves and stems into the seeds. This can be referred to as “cannibalization” of the vegetative tissue (rapid senescence and defoliation), but it is a normal process. This eventually causes leaves to turn yellow and drop, and stems to turn brown and die.
The fewer the number of seeds, due to abiotic or biotic stresses, the lower the demand for photosynthates produced by leaves and stems. If demand is low enough, the leaves and stems are never “cannibalized” for their carbohydrates and protein. As a result, the leaves and stems will remain green longer than normal, even up through physiological maturity of the beans. Late-season rainfall can make the problem worse by keeping the plants alive after the seeds have dried down. It will take either a frost or chemical desiccant to kill the leaves and stems in this situation. If the leaves are still green and intact when pods have turned brown and have reached 13-14% moisture, it is usually an indication of mid-season stress around flowering/pod set and low yield potential – at least relative to the amount of foliage produced.
Figure 3. Green stem syndrome in soybeans and suggested harvesting operations. Infographic developed by Ignacio Ciampitti, K-State Research and Extension.
What can be done for harvesting purposes?
Eventually, freezing temperatures will kill the leaves and dry down the stems. Otherwise, the utilization of desiccants to kill leaves and drop the stem moisture down is a viable option, but only if the producer wants to harvest the field soon, before a freeze is likely to occur. If the stems and/or leaves are still green when the field is harvested, the best option is to harvest slowly and make sure the harvesting equipment is sharp and in excellent condition.
We recommend scouting your field right before harvest to better understand what environmental conditions led to the green stems. As always, make sure to time your harvest for the optimum seed moisture content in order to maximize the final grain volume to be sold. More information on soybean dry down rate can be found in eUpdate Issue 767, September 27, 2019 at http://bit.ly/2nfhAWe
Ignacio Ciampitti, Cropping Systems Specialist