Weather Woes in the Soybean Field

Published: October 18, 2018

Mother Nature proves almost every year that she has much to say about soybean yield and grain quality. This year many of Missouri's soybean fields were affected by drought. Although the severity and the length of the drought varied across Missouri, several of our important soybean growing regions experienced severe drought. The most worrisome effect from drought is yield loss, but there are several other effects that will be felt during harvest. Although summer was dry, fall has turned quite wet in some of the same regions. This wet weather can also effect harvest in ways other than delays.

Smaller seeds

Drought that occurs during seed-fill will decrease seed size. The effect on seed size depends on the timing, severity and length of stress. These small seeds are not much different from normal sized seeds for seed quality, seed composition, and appearance and should be accepted at elevators. Combines should be adjusted to keep harvest loss of these small seeds to a minimum.

Drought can be severe enough to kill soybean plants. If plant death occurred before seeds had matured, seeds will remain green. Green seeds are docked at the market. See US Grain Standards for more information https://www.gipsa.usda.gov/fgis/standards/810soybean.pdf. The green color may decrease during storage, but that is not a certainty.

Green stems

Soybean plants that have experience drought stress may not mature normally. In this phenomenon, stems remain green even though pods and seeds have matured. Sometimes stems remain green and leaves die, but remain on the plant. One effect from this occurrence is that we will wait too long to harvest. We may assume that the pods are not mature if we see green stems. Harvesting soybean plants with green stems is difficult and slows harvest. But, delaying harvest will increase pod shattering and yield loss.

Weak pods and shattering

Soybean pods are composed of two halves that are stitched together. The stitching is weak on wild soybean and pods easily split to disseminate seeds. Soybean breeders selected against shattering and normally the two halves of soybean pods are strongly held together. They rupture only during harvest or if plants are left in field long after maturity. Drought stress can result in weaker than normal sutures that hold the two halves together. This increases the possibility of shattering, especially if seeds swell and shrink in response to repeated cycles of wetting and drying.

For an individual soybean pod, the pod wall obtains almost full length before seed growth begins. Drought stress during pod wall growth can result in thinner than normal pod walls. This thin wall is prone to breakage. This phenomenon is less common than shattering from split pods and the tears in the pod wall are usually not large enough to allow seeds to fall. But, tears in the pod wall can allow water into the pod.

Sprouted seeds

Pod walls usually prevent immature and mature soybean seeds from absorbing water by shedding rain water. Unfortunately the current spell of frequent rains, drizzle, and/or foggy days and nights can bathe the soybean pod in enough water that the water soaks through the pod wall and wets the soybean seed. This can result in seed sprouting while still in the pod. Please see the linked article "Wet Weather Can Cause Seeds to Sprout on the Plant" to learn more about this phenomenon.

Sprouting rapidly decreases seed quality and grain will be docked at the point of sale. Grain that has sprouted seeds is difficult to safely store. Sprouted seeds release sugars and other compounds that speed fungal growth so that seed quality continues to deteriorate. Timely harvest and making sure that grain is dry may stop sprouting.

Poor grain quality

Much of what I discussed leads to poor grain quality. Seeds that are on plants that were killed by drought before maturity may be more susceptible to invasion by fungi. Tears in pods either within the pod wall or at the bottom suture allow water into the pod. If water is able enter the pod, so can fungi. The combination of water and fungi result in fungal growth. This growth may produce toxins, but mostly its presence reduces grain quality. US Standards for soybean states that the upper limit for damaged kernels (it uses this term for seeds) in Number 2 soybean is 3%. If damage kernels exceed 8% the grain lot will be graded Sample. Sample grade soybean lots are difficult to sell and there are few uses for them near most communities.

Timely harvest may decrease the amount of damaged seeds, but some of the damage occurred well before harvest. Scouting the field to find pockets of damage may help isolate damage so the better appearing grain is not contaminated. I understand that this is not easy and is time consuming. But, not having grain rejected at market is worth trying to avoid. Storing damaged grain is difficult because fungal growth may continue and the seed coat integrity has been broken. Grain moisture of grain lots with damaged seeds should be brought quickly below 13%. Be careful not to raise grain moisture by aerating with humid air. Carefully watch stored grain for signs of heating.