Soybean Development – Reproduction Phase

Soybeans are flowering, so it’s a good time to review reproductive stages.

Soybeans begin to flower around June 21 or summer solstice. A few flowers may appear before then, but the bulk of flowering begins in June. The last flowers will appear on the top of the plant in early August.

The most important reproductive stages are R1, R2 and R3. These are easy to identify. Stage R1 soybeans have an open flower at any node on the main stem. See an open flower pictured in the illustration below from University of Illinois Extension.

Stage R2 is when an open flower appears at any of the two uppermost nodes. To identify this stage, start at the very top of the soybean plant and look at each node moving down the plant. If one of the two uppermost nodes has an open flower, your soybeans are in the R2 growth stage.

Growth stage R3 is when you just have the start of pod development at one of the four uppermost nodes. These early flowers and pods are depicted in the illustration.

Here is a summary of reproductive stages in soybeans:

• R1: Beginning bloom – One open flower at any node on main stem, usually occurs at V7 to V10
• R2: Full bloom – One open flower at one of upper two nodes on main stem
• R3: Beginning pod – A pod 0.5 cm long at one of the four uppermost nodes on main stem; flowers appear rapidly
• R4: Full pod – A pod 2 cm long, rapid pod growth, beginning of seed development, flowering at one of four uppermost nodes. Plants very sensitive to stress.
• R5: Beginning seed – Seed 0.3 cm long inside pod at one of four uppermost nodes, pod number set, seed number determined
• R6: Full seed – Seed fills pod cavity at one of four uppermost nodes, seed weight approaches maximum, leaves begin to turn yellow
• R7: Beginning maturity – Once pod on main stem has reached mature color, seed size is set, plants are showing physiological maturity when 50 percent of leaves turn yellow.
• R8: Full maturity – 95 percent of pods are mature color, leaves have dropped off, five to 10 days before harvest.

Eric Ifft is an Illinois Customer Business Advisor for Bayer CropScience.