Why knowing soybean growth stages really does matter
A common assumption is indeterminate soybeans are done creating yield by the end of R5 reproductive growth stage. However, yield can be affected until the end of R6, says Farm Journal Associate Field Agronomist Missy Bauer.
“You don’t want to walk away from soybeans too early,” she says. “Stress from moisture, heat, insects and diseases from R1 through the end of R6 effect the components of yield differently.”
- Stress at R1 through R4 could reduce the number of pods
- Stress at R3 through R4 could reduce the size of pods (one, two or three bean pods)
- Stress at R4 to R6 might cause beans to abort in the pod
- Stress at R5.5 to R6.5 will affect bean size
Scout soybeans for signs of stress through R6. If irrigating, watering up to R7 can typically increase yield.
Soybean Reproductive Growth Stages
R1: Beginning Bloom. At least one flower appears on the plant on any node on the main stem. The plant is 15" to 18" tall. Flowering always begins on the third to sixth node, depending on vegetative stage, and progresses up the plant and branches. Vertical roots grow rapidly and continue to R4 or R5. Secondary roots and root hairs continue to grow.
R2: Full Flowering. The plant is beginning full bloom and the appearance of flowers begins to slow. Nitrogen fixation increases rapidly and nodulation is maxed out. In determinant types, the top node should have nearly fully expanded trifoliate leaves. For indeterminate types, the top nodes might have trifoliates and blooms developing concurrently at each node. Especially for indeterminate types there will be small pods forming at the lower nodes.
R3: Beginning Pod Development. When one pod on one of the four upper nodes reaches 3⁄16" long, the plant is at R3. Plants are 23" to 32" tall. Typically, 60% to 75% of flowers abort, but stress can increase the loss. Temperature or moisture stress can limit pod numbers, beans per pod or bean size. Because of its long flowering period, the plant can compensate for some losses, but its ability to make up ground dwindles as it approaches R5.
R4: Full Pod. At R4, the plant reaches the full pod stage, pod growth is rapid and seed development begins. This is the most crucial stage for yield. The pods are filling at a maximum rate, making the plant most susceptible to stresses and causing it to move nutrients from other areas weakening it to invasion by diseases via the roots and foliage.
R5: Full Seed. Seed filling requires water and nutrients. The plant redistributes nutrients to provide half the nitrogen, potassium and phosphorus needs. The rest comes from nitrogen fixation and root uptake. At R5, the plant is less able to compensate for stresses. The seed is at least 1⁄8" long in one of the pods on the upper four nodes. Halfway through this stage, the plant reaches its maximum height, number of nodes and leaf area. Seeds accumulate dry weight.
R6: Full Seed. At the “green bean” stage, total pod weight peaks. Seed growth is rapid. Leaves on the lowest nodes start to yellow.
R7: Beginning Maturity. One normal pod on the main stem is mature in color (brown or tan). Dry matter is peaking in seeds. Seeds and pods appear yellow. The seeds are 60% moisture at physical maturity. Stress has little effect unless pods shatter or fall off.