Soybean yields ultimately depend on the number and weight of the seeds harvested per acre. Soybean yield is determined by nodes per acre (plants per acre x nodes per plant), pods per node, seeds per pod, and seed weight.

To maximize soybean yields, it is important to have an adequate plant population. The established plant stand must be able to canopy quickly to maximize light interception and limit weed emergence and competition. It is also important to balance plant population with the optimum return on investment. As plant populations increase, soybeans tend to grow taller, leading to lodging which reduces harvestable yield.

Soybean yields correlate to the number of nodes per acre. Nodes are the part of the plant stem where the trifoliate leaves attach to the stem and where flowers develop. Row width and plant spacing can directly influence branching in soybeans. Soybeans are photoperiod sensitive, meaning the plants initiate flowering in response to long nights. Indeterminate soybeans, grown by most Midwest farmers, continue vegetative growth after flowering begins.

Planting earlier gives soybeans the opportunity to put more nodes on each plant by increasing vegetative growth. Early planting also exposes the seed and seedling to challenges due to cold soils and pathogens, so a comprehensive seed treatment is recommended for early planting.

Pods per node is a key component affecting the weight of seed produced. A node is the junction on the soybean stem where a petiole and trifoliate leaf is attached. Flowers and resulting pods form at the nodes of the plant. Plant stress, whether abiotic or biotic, can greatly impact flower fertilization, flower retention, and pod development.

One of the main yield determining factors in soybeans is the total number of pods. 70% of a soybean plant’s yield comes from nodes 6 to 13, which are present at the R3 growth stage. Favorable conditions late in the growing season, particularly adequate moisture, can lead to more pod production and retention.

How do we increase nodes, flowers, pods, and ultimately, seeds per acre?

 Narrower rows lead to increased plant-to-plant spacing down the row, but also tend to increase soybean height. This can lead to more nodes per acre compared to the same seeding rate at wider row widths.

 Earlier planting increases vegetative growth before flowering occurs. It also moves initiation of the flowering time to earlier in the summer.

 Presence of foliar diseases and insects during flowering can cause pod abortion by decreasing plant health.

 A fungicide application at the R3 growth stage is a PFR Proven™ Practice.

Growing conditions shortly after flowering drive the number of seeds per pod. Stressful conditions have the potential to arrest seed development. Seed fill begins at the R4 growth stage and is completed by R6. Root growth slows as seed growth begins, leading to increased moisture sensitivity. Healthy soybean plants should average two and a half seeds per pod.

Late-season moisture can increase seed size by extending the seed fill period. Envir onmental factors drive seeds per pound.

Variation in seed size and individual plant yields means that yield estimates in soybeans are not as powerful as in corn. Yield estimation is more accurate the closer to harvest you are; the R5 growth stage is the earliest you can estimate yield in soybeans.

To estimate yield, count the number of plants in 1/1,000th of an acre, which is 17.5 ft. in 30 in. row soybeans. Choose ten plants at random and count and average the number of pods. Then count and average the number of seeds per pod on one representative plant.

There are 3,000 soybean seeds per pound on average; this number can vary widely depending on the size of the seed. This estimation needs to be completed several times throughout a field to get a more accurate representation of yield.


https://www.agry.purdue.edu/ext/soybean/Arrivals/10SoyDevt.pdf
https://cropwatch.unl.edu/setting-yield-soybean-and-avoiding-end-season-plant-stress-unl-cropwatch-aug-23-2013
https://www.ag.ndsu.edu/cpr/plant-science/soybean-nodulation-07-02-15