Soybean Maturities

Vegetative growth stages in soybeans (VC, VE, V1, V2, … VN) are numbered according to how many fully-developed trifoliate leaves are present. The reproductive stages begin at flowering (R1-R2) and include pod development (R3-R4), seed development (R5-R6), and plant maturation (R7-R8). Growth stages can overlap. Determine the growth stage of a crop when 50% or more of the plants are in or beyond that growth stage in question

Beginning Bloom (R1)

Beginning bloom (R1) is a time of rapid growth. During this reproductive growth stage, at least one flower is located on the plant (Figure 1). Soybean flowering always initiates on the third to sixth node of the main stem. The initial flowering node depends on the vegetative growth stage when flowering begins. Flowering will continue up and down the main stem and then eventually move to the branches. Each raceme, or group of flowers, will occur from the base to the tip. Consequently, the pods at the base of the plant are usually more mature than those at the tips. The vertical roots are rapidly growing along with secondary roots and root hairs. This root growth will continue until R4-R5. Plants are 15 to 18 inches tall and are at a vegetative stage somewhere between V7 to V10. Stress, such as defoliation or root damage that occurs during the early reproductive stages (R1 to R5.5) can affect growth rate and may have an impact on yield potential.

Soybean Maturities R1
Figure 1. Soybean plant during the Beginning Bloom (R1) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Full Flower (R2)

Full flower (R2), also know as full bloom, is distinguished by an open flower at one of the two top nodes on the main stem (Figure 2). One or more of these upper nodes has a fully developed leaf and approximately 50% of the total number of nodes has developed. New flower development slows down between R2.5 to R3 with completion typically taking place by R5. At the beginning of full flower, soybean plants are in the V8 to V12 vegetative growth stage and are around 17 to 22 inches tall. Roots can reach across 40-inch rows. Major lateral roots have turned downward in the soil and nitrogen fixation by root nodules is increasing quickly. Rapid accumulation of dry matter along with nitrogen (N), phosphorus (P) and potassium (K) is occurring and will take place until V6. During this stage the soybean plant will accumulate 25% of total dry weight and nutrients and about 50% of the mature height. The biggest yield reducing stress during full flower is defoliation, which can occur from various sources including insect damage, disease, or hail. Fifty percent defoliation at this stage can reduce yield by 6%.

Soybean Maturities R2

Figure 2. Soybean plant during the Full Flower (R2) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Beginning Pod (R3)

At the beginning pod (R3) growth stage, one of the four uppermost nodes is 3/16 inch (5 mm) (Figure 3). Stress during this growth stage may affect yield potential by decreasing total pod number, bean number per pod, or seed size. Typically, soybean plants can compensate, at least partially, from temporary stress. One reason for the ability to compensate is the long flowering period; however, the soybean plant loses this ability as the soybean plant matures from R1 to R5.5. During R3, 60- 75% of the flowers can abort and as many as 50% of the formed pods may abort. Stress during this reproductive stage may increase those abortion rates and decrease yield potential. Conversely, favorable conditions may increase pod number per plant and increase yield potential. At this time, soybean plants can be 23-32 inches tall and may be at the V11-V17 vegetative growth stage.

Soybean Maturities R3
Figure 3. Soybean plant during Beginning Pod (R3). Picture courtesy of Palle Pedersen, Iowa State University.

Full Pod (R4)

At the beginning of the full pod (R4) growth stage, one of the four uppermost nodes will have a pod that is ¾ inch long (Figures 4 and 5). At first, rapid pod growth and seed development take place, followed by finalization of pod number. Pod dry weight increases from R4-R5. This stage marks the beginning of the critical period for determining seed yield potential. Stress during R4-R6 can cause more reduction in yield potential than at any other growth stage. The most critical time is during pod formation in late R4.5 to early seed fill at R5.5. Reductions in yield potential can occur from fewer pods. If needed, irrigation during this critical time may help reduce potential yield loss. As a tip to help determine the end of this critical period, the last flower occurs at the main stem tip through R5.

Soybean Maturities R4

Figure 5. Soybean pods during the Full Pod (R4) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Beginning Seed (R5)

The reproductive stage termed beginning seed (R5) represents a seed that is 1/8 inch long in a pod at one of the four uppermost nodes (Figure 6). Seed fill during this growth stage demands large amounts of water and nutrients. The soybean plant redistributes stored nutrients with half of the needed N, P, and K coming from the plant’s vegetation and the other half coming from N fixation and nutrient uptake by the roots. During this growth stage N nutrient accumulation in the leaves and N fixation both peak and then start to drop as the seeds use nutrients. Dry matter accumulation continues and will stop halfway between R5 and R6. Maximum height, node number, and leaf area are obtained approximately half way through R5. Seed accumulation continues until about R6.5 when about 80% of total seed dry weight should have been reached. The plant is less able to compensate due to stress and vegetative damage at this growth stage. Leaf loss of 100% at this stage can reduce yield potential by 80%. Reductions in yield potential typically occur due to lower pod number and number of beans per pod. Seed size may also reduce yield potential but it is not as common at this stage.

Soybean Maturities R5

Figure 6. Soybean pod and seeds during the Beginning Seed (R5) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Full Seed (R6)

Full seed (R6) may also be known as the “green bean” stage or beginning full seed. The stage starts with a pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem (Figure 7). The beans have a rapid growth rate that slows around R6.5 and peaks at R7. Total pod weight peaks during R6. During R6, three to six trifoliate leaves may fall from the lowest nodes just before leave yellowing starts. Following the R6 growth stage, rapid leaf yellowing begins through R8 or until all leaves have fallen. Halfway through R6, root growth is complete. R6 also marks the transition from the end of the critical period for reductions in yield potential (R4.5 to R5.5) and the start of the period when stresses have very little effect on yield potential (R7).

Soybean Maturities R6

Figure 7. Soybean pod and seeds during the Beginning Seed (R6) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Beginning Maturity (R7)

Beginning maturity (R7) signifies that one pod on the main stem has achieved the brown or tan mature color (Figure 8). Eventually the seed and pods appear yellow and all green color is lost, which denotes the peak of dry matter accumulation in individual seeds. When physical maturity is achieved, seeds contain about 60% moisture. Stress during the beginning and full maturity growth stages does not affect yield potential unless one or more of the following occurs: pods drop to the ground, seeds are shattered from the pods, plants lodge reducing light interception, or losses occur during harvest. Finally, when the soybean plants get to beginning maturity they are also safe from a killing frost.

Soybean Maturities R7
Figure 8. Soybean plant during the Beginning Maturity (R7) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.

Fulll Maturity (R8)

The soybean crop can be called fully mature (R8) when 95% of the pods have achieved their mature color. (Figure 9). Typically, 5 to 10 days of good drying weather after R8 is all that is needed for the soybeans to reach harvest moisture of less than 15%. This is a guideline; the rate of moisture loss can vary based on conditions. For example, warm, dry weather will lower the soybean moisture faster and wet weather will obviously slow moisture loss down. For long-term storage, soybeans should be stored at 13% moisture or less. Prior to harvest the final plant population should be assessed.

Soybean Maturities R8

Figure 9. Soybean plant during the Full Maturity (R8) growth stage. Picture courtesy of Palle Pedersen, Iowa State University.