13.2 Soybean Growth Staging (V1-Vn, R1-R8) & Physiology

13.2 Soybean Growth Staging (V1-Vn, R1-R8) & Physiology

Soybeans (Glycine max) are unique in their growth habits and development patterns compared to grasses like corn. Understanding soybean growth stages is essential for determining optimal timings for herbicide, fungicide, and insecticide applications, as well as predicting the crop's response to environmental stress. The staging system is divided into Vegetative (V) and Reproductive (R) stages.

Growth Habits: Determinate vs. Indeterminate

Before detailing the stages, it is important to distinguish between the two primary growth habits of soybeans, which largely follow geographic zones:

1. Indeterminate Soybeans: Primarily grown in northern latitudes (Maturity Groups 000 through IV). These varieties continue their vegetative growth (adding new nodes and leaves) well into the reproductive stages. They typically reach only half their final height by the time flowering begins (R1).
2. Determinate Soybeans: Primarily grown in southern latitudes (Maturity Groups V through VIII). These varieties cease vegetative growth (node addition and stem elongation) shortly after flowering begins. The terminal bud completes its growth, and the plant focuses almost entirely on reproductive development.

Vegetative Stages (V Stages)

Vegetative stages in soybeans are determined by counting the number of fully developed nodes on the main stem. A node is considered "fully developed" when the leaf at the node immediately above it is fully expanded (the leaflets have unrolled and the edges are no longer touching).

VE: Emergence

Emergence occurs when the cotyledons are pulled through the soil surface by the elongating hypocotyl. This usually happens 5 to 10 days after planting. The cotyledons supply the seedling with stored energy and nutrients for about 7 to 10 days. Damage to the cotyledons from insects or hail can severely stunt seedling growth.

VC: Cotyledon Stage

The VC stage occurs when the unifoliate leaves (the first true leaves, which consist of a single leaflet) are fully expanded. The plant is still heavily reliant on the cotyledons, but photosynthesis is beginning.

V1: First Trifoliolate

The V1 stage is reached when the first trifoliolate leaf (a leaf consisting of three leaflets) is fully expanded. This is an important milestone, as the plant is now primarily dependent on photosynthesis. Root nodules, formed by the symbiotic bacteria Bradyrhizobium japonicum, begin to develop on the roots. These nodules will eventually fix atmospheric nitrogen, but they are not yet actively providing substantial nitrogen to the plant.

V2 to Vn: Subsequent Trifoliolates

The stages progress as V2, V3, V4, etc., with each stage corresponding to the number of fully expanded trifoliolate leaves on the main stem. By the V2 or V3 stage, the root nodules become active and begin supplying fixed nitrogen to the plant.

Soybeans are quite resilient during the vegetative stages. If the main growing point is damaged by hail, the plant can generate new growth from axillary buds located at the base of the lower leaves. While significant defoliation can reduce the photosynthetic area, the plant can often compensate if the damage occurs early in the season.

Reproductive Stages (R Stages)

The reproductive stages are based on flowering, pod development, seed development, and plant maturation.

R1: Beginning Bloom

The R1 stage occurs when at least one open flower is present at any node on the main stem. Flowering typically begins on the third to sixth node from the bottom and progresses upward and downward. In indeterminate varieties, the plant is still growing vigorously. Soybeans are highly responsive to photoperiod (day length), and flowering is triggered as nights begin to lengthen after the summer solstice.

R2: Full Bloom

R2 is reached when there is an open flower at one of the two uppermost nodes on the main stem that has a fully developed leaf. The plant is accumulating dry matter rapidly, and nitrogen fixation in the root nodules is operating at peak capacity. Stress at this stage can lead to flower abortion, but soybeans produce far more flowers than they can support, so some abortion is normal.

R3: Beginning Pod

R3 occurs when a pod is at least 3/16 inch (5 mm) long at one of the four uppermost nodes on the main stem with a fully developed leaf. The plant is transitioning from flowering to pod development. Moisture stress at R3 can reduce the number of pods per plant, impacting yield.

R4: Full Pod

R4 is defined as a pod being at least 3/4 inch (19 mm) long at one of the four uppermost nodes on the main stem with a fully developed leaf. This is a critical period for yield determination. The rapid accumulation of pod dry weight begins. Severe stress during R4, particularly heat or drought, can lead to significant pod abortion. Indeterminate soybeans have reached their maximum height and node number by late R4.

R5: Beginning Seed

R5 occurs when the seed is at least 1/8 inch (3 mm) long in a pod at one of the four uppermost nodes on the main stem with a fully developed leaf. This stage marks the beginning of rapid seed filling. The plant's demand for water and nutrients (especially nitrogen, phosphorus, and potassium) is at its highest. Because root growth has slowed, the plant begins to remobilize nutrients from the older leaves and stems to the developing seeds. Moisture stress during R5 is devastating to yield, primarily by reducing seed weight and causing seed abortion.

R6: Full Seed

At R6, the seed contains "green seed" that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf. Total plant dry weight reaches its maximum. The seeds are rapidly accumulating dry weight and protein. The lower leaves may begin to yellow and drop as senescence begins.

R7: Beginning Maturity

R7 is reached when at least one normal pod on the main stem has reached its mature color (usually brown or tan). The seeds within that pod have reached physiological maturity (maximum dry weight). Stress at this point has little impact on yield, although severe weather can cause lodging or pod shattering.

R8: Full Maturity

The R8 stage occurs when 95% of the pods have reached their mature color. The crop requires several days of good drying weather to reduce seed moisture to harvestable levels (ideally 13-15%). The plants are completely defoliated, and the stems are dry.

Yield Physiology

Soybean yield is a function of the number of plants per acre, pods per plant, seeds per pod, and seed weight. Soybeans have an incredible ability to compensate for poor stands by branching out and producing more pods per plant. However, stress during the reproductive stages, particularly R4 through R6, severely limits the plant's ability to maintain pod numbers and fill the seeds, making these the most critical periods for crop management.