Soybean progress and flowering growth stages

By Laura Lindsey, OSU Extension Soybean and Small Grains Specialist, Adapted from C.O.R.N. 2022-21

Currently, most soybean fields in Ohio are at the flowering growth stage (R1-R2). (Some late-planted or re-planted soybean may still be at a vegetative stage.) Even as soybean plants begin to flower, they may only have 3-5 trifoliolates due to late planting and wet weather followed by dry conditions. However, even if plants have flowers and only a few trifoliolates, the plant will continue to add leaf area up to the R5 growth stage, which comes 4-6 weeks later. As long as the canopy is complete by the beginning of seed filling, the plant has the potential to reach full yield potential.

What does the soybean crop need to maximize yield during the flowering growth stage? While adequate soybean flowers are needed for subsequent reproductive development, soybeans are amazingly resilient to stress during flowering due to their ability to continue to develop flowers over several weeks. Flowering marks the beginning of rapid dry weight and nutrient accumulation rates. Therefore, duration of light interception and thermal energy/heat unit accumulation provide the potential for flower and pod retention and seed fill.

Misconceptions at the R1-R2 growth stage: There are several common misconceptions about soybean plants at the flowering growth stage.

Misconception: Compared to normal-sized plants, short, compact plants often produce more pods and higher yields.

Reality:  Short plants can produce high yield, but if dry soils or low plant populations limit plant height, pod numbers, and canopy cover, yields will usually be lower.

Misconception: Throughout the growing season, all soybean plants in a field should be uniform in size, weight and pod number; if they aren’t, yield will be lower.

Reality: Soybean plants grown at normal plant populations always develop a great deal of variability in size and pod numbers, even if they are uniform early in the season. Reasons for this are not well-understood, but larger plants adjust to compensate for smaller ones, and there is no indication that this causes lower yields. Smaller soybean plants do not act ‘weedy’- lower yields of larger plants- in a full stand.

Misconception: Flower abortion reduces yield.

Reality: Soybean naturally aborts 20-80% of its flowers. Soybean produces many more flowers over a long period of time than can be supported by the plant as a mechanism to avoid the impact of short periods of stress.

Misconception: Sugar can be applied to produce more flowers.

Reality: It has been proposed in the past that foliar applications of sugar can make the plant set more flowers; however, this is not the case.

For more information on soybean flowering, see this Science for Success video featuring my colleague Dr. Shaun Casteel from Purdue University: https://www.youtube.com/watch?v=oYqNNqsz_zU and also this Science for Success FactSheet: https://soybeanresearchinfo.com/wp-content/uploads/2022/01/Science-for-Success-Soybean-Growth-Stages-V3.pdf