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Soybean vegetative (V) growth stages in soybeans are numbered according to the number of fully developed trifoliate leaves (Figure 2). Reproductive (R) stages begin at flowering and progress through pod development, seed development, and plant maturity. Vegetative growth stages begin overlapping reproductive stages at about R1. A new growth stage is established when 50% or more of the plants meet the requirements of the growth stage.
Soybean seed begin to germinate after absorbing approximately 50% of the seed’s weight in water. Over 5-10 days (depending on temperature, moisture, product, and planting depth), the radical emerges, followed by the hypocotyl (stem), and the cotyledons. The first two unifoliate leaves begin to emerge (Figure 1). Emergence process can be stimulated by small amounts of fertilizer placed in a band to the side and slightly below the seed, especially in cool soils. However, this may delay the onset of the nodulation process as the plant will use soil available nitrogen (N) before it allows rhizobia to colonize the root system. Effective nodulation occurs is with the application of a commercial inoculant to the seed or furrow.
Unifoliate leaves are fully expanded and the cotyledons supply all nutrient needs for the first 7-10 days after emergence. Loss of one cotyledon during this time has little effect on plant growth. Loss of both cotyledons at or soon after VE may reduce yield potential by 8-9%. The first trifoliate emerges and opens at V1.
Active N fixation begins to occur as plants reach 6-8 inches tall with three nodes and two unfolded leaflets. When the plants switch from using soil-available N to fixed N, a brief yellowing of the crop may be observed. Adding N at this stage generally inhibits N production by nodules. Since lateral roots are developing rapidly in the top 6 inches of soil, cultivation should be shallow to minimize root pruning.
Axillary buds that will develop into flower clusters (racemes) begin to develop in the top stem. The total number of nodes the plant can produce is established at V5. Axillary buds that develop on indeterminate soybean plants can help plants to recover from bud damage. This is typically the time that iron chlorosis deficiency symptoms become visible in impacted fields.
Figure 1. Early soybean seedling.
Lateral roots should overlap in rows 30 inches or less. A loss of 50% of the leaves at this stage may reduce yield potential only about 3%.
Plants have at least one flower at any node on the main stem. Soybean flowering begins on the third to sixth node, continues up and down the main stem, and eventually moves to the branches. Vertical roots as well as secondary roots and root hairs will continue rapid growth until R4-R5.
An open flower develops at one of the top two nodes of the main stem. The plant has accumulated about 25% of its total dry weight and nutrients and about 50% of its mature height. Dry matter, N, phosphorus (P), and potassium (K) levels are rapidly accumulating. Roots now may reach across 40 inch rows and lateral roots have turned downward into the soil. Nitrogen fixation by root nodules is increasing rapidly. Up to 50% loss of plant leaves from hail, insects, or disease at this stage may reduce yield potential by 6%.
A pod on the upper four nodes is 3/16 inch long. Heat or moisture stress at this stage can reduce pod numbers, bean number/pod, or seed size, which may reduce yield potential. The ability for soybean plants to recover from temporary stress decreases from R1-R5.5. Favorable growing conditions during this period may result in greater pod numbers and increased yield potential.
At this stage pods are growing rapidly and seeds are developing. At least one pod on one of the four upper-most nodes has a 3/4 inch long pod. Stress during this period can cause more reduction in yield. Irrigation at this time may help reduce the potential for yield loss.
Figure 2. Soybean growth stages. Plants that are indeterminate in growth habit continue vegetative growth after flowering. The rate of development is directly related to temperature and moisture. Picture courtesy of University of Illinois.
At least one seed in one pod on one of the four upper nodes is at least 1/8 inch long. About half the nutrients required for seed filling come from the plant’s vegetative parts and about half from N fixation and nutrient uptake by the roots. Nitrogen fixation peaks and stress at this stage can reduce pod numbers, the number of beans/pod, seed size, and yield potential. Plants attain maximum height, node number, and leaf area.
This “green bean” stage marks the beginning of the full seed stage. At least one of the four top nodes of the main stem has a pod with a green seed that fills the pod cavity. Total pod weight peaks. Leaves begin to yellow.
One pod on the main stem reaches its brown or tan mature color. Seed dry matter begins to peak. Seeds and pods lose all green color. At this stage, soybean plants are safe from a killing frost. Yield potential may be reduced if pods are knocked from plants or seeds are shattered from pods.
When 95% of the pods have reached their mature color, the plants are considered fully mature. Typically, 5-10 days of good drying weather after R8 will be needed to reach harvest moisture of less than 15%.