The Soybean Plant | NC State Extension Publications

The soybean (Glycine max) is native to East Asia and has been grown for thousands of years. Soybean plants are on average 3 to 5 feet tall and can have up to 20 nodes. The plant has the ability to produce 600 pods per plant, but on average there are 50 to 100 pods per plant that set seed. Each pod contains on average three seed. Soybean yields are directly dependent on the number of plants per acre, the number of pods per plant, the number of seeds per pod, and the size of the seed.

Soybean varieties are classified based on their requirement to initiate reproductive development and their morphological growth habit. Soybeans are photoperiod sensitive, short-day plants, meaning that days must be shorter than a critical value to induce flowering. Soybean varieties are classified into maturity groups according to their response to photoperiod. Soybean varieties are also classified based on their growth habit. In varieties with a determinate growth habit, the onset of reproductive growth results in the termination of vegetative growth. Indeterminate varieties, however, start flowering several weeks before they terminate vegetative growth. Most Southern varieties are determinate, while most Midwest varieties are indeterminate.

Soybeans are legumes and, like most other legumes, have the ability to supply their own nitrogen. Nitrogen fixation begins with the formation of a nodule on the root. Nodules are produced from Bradyrhizobia bacteria in the soil that invade the root and multiply within the root cells. The soybean plant supplies the bacteria with nutrients and energy, and in return the bacteria convert atmospheric nitrogen (N2) in the air to nitrates (N03-) the plant can then use.

Understanding how soybeans grow and develop is critical to effectively managing the crop for increasing yields.

A descriptive system has been developed to describe the growth stage of a soybean plant. The system most commonly used was developed by W.R. Fehr and C.E. Caviness in 1977. Understanding and being familiar with soybean growth stages are useful when discussing proper management throughout the year. Soybean development can be divided into vegetative (V) and reproductive (R) stages (Table 1-1). Each stage starts when at least 50% of plants in that field are at that stage.

The vegetative stages begin with emergence (VE), which occurs when elongation of the hypocotyl brings the cotyledons out of the soil. After emergence, a pair of unifoliate leaves on the first node unroll just above the cotyledons and start the VC stage. Following VC, trifoliate leaves begin to unfold. The number of nodes with the trifoliate leaf fully developed and unrolled is referred to as V(n). A leaf is considered fully developed when the leaf at the node directly above it has expanded enough that the edges of the leaflets are not touching. The vegetative stages proceed from V1 through V(n).

The reproductive stages begin when the first flower is present on the plant (R1). The first flower is typically toward the bottom of the plant. As the plant moves into full bloom, it enters into R2. The reproductive stages include pod development (R3 and R4), seed development (R5 and R6), and finally maturity (R7 and R8).

Stage

Stage No.

Abbreviated Stage Title

Description

Image

Vegetative Stages

VE

Emergence

Cotyledons above the soil surface.

VE growth stage

VC

Cotyledon

Unifoliate leaves unrolled sufficiently so the leaf edges are not touching.

VC growth stage

V1

First-node

Fully developed leaves at unifoliate nodes.

V1 growth stage

V2

Second-node

Full developed trifoliate leaf at node above the unifoliate nodes.

V2 growth stage

V(n)

nth-node

n number of nodes on the main stem with fully developed leaves beginning with the unifoliate nodes. n can be any number beginning with 1 for V1, first- node stage.

V(n) growth stage

Reproductive Stages

R1

Beginning bloom

One open flower at any node on the main stem.

R1 growth stage

R2

Full bloom

Open flower at one of the two uppermost nodes on the main stem with a fully developed leaf.

R2 growth stage

R3

Beginning pod

Pod 5 mm (3/16) long at one of the four uppermost nodes on the main stem with a fully developed leaf.

R3 growth stage

R4

Full pod

Pod 2 cm (3/4) long at one of the four uppermost nodes on the main stem with a fully developed leaf.

R4 growth stage

R5

Beginning seed

Seed 3 mm (1/8) long in a pod at one of the four uppermost nodes on the main stem with a fully develop leaf.

R5 growth stage

R6

Full seed

Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf.

R6 growth stage

R7

Beginning maturity

One normal pod on the main stem that has reached its mature pod color.

R7 growth stage

R8

Full maturity

95% of the pods that have reached their mature pod color. 5 to 10 days of drying weather are required after R8 before the soybeans have less than 15% moisture.

R8 growth stage

The descriptions focus on the top of the soybean plant, so they are applicable to both determinate and indeterminate varieties. Some of the stage descriptions may seem awkward, but they were intentionally chosen to be interpreted the same by most, if not all, users. The most ambiguous of these stages is R7, which was originally intended to identify physiological maturity. While physiological maturity (when dry matter accumulation ceases) is fairly easy to determine in other crops, its more difficult in soybeans. There is no obvious visible signal that indicates physiological maturity has been reached, but Fehr and Cavinesss description works fairly well for determinate varieties in the South.

The number of days between stages varies depending on the maturity group and variety planted, but there are a few trends that usually hold true.

Soybean development is also influenced by temperature, day length, soil moisture, and other environmental conditions. Therefore, the timing of growth stages will be different for different varieties, planting dates, and climates.

VE growth stage.

VC growth stage.

V1 growth stage.

V2 growth stage.

R1 growth stage.

R2 growth stage.

R3 growth stage.

R4 growth stage.

R5 growth stage.

R6 growth stage.

R7 growth stage.

R8 growth stage.

Soybeans were first classified into maturity groups (MGs) in the early 1900s. Today there are 13 major groups ranging from MG 000 to MG X, with lower-numbered maturity groups representing earlier maturing varieties. These groupings were based on adaptation within certain latitudes. A variety is classified to a specific MG according to the length of time from planting to maturity. Maturity group belts run east to west in North America. Historically, lower number MGs were grown in the extreme northern United States and Canada, and they progressively got higher as you moved south to the Gulf Coast states.

The most recent classification of MGs was carried out in 2017 by Mourtzinis and Conley (Figure 1-1) by aggregating MG-specific yield data from variety performance trials across the United States. Data were collected from 27 states over a period of 14 years to develop the MG zones.

Figure 1-1 shows that most of North Carolina is in the group V zone. This implies that a group V variety would be considered a mid-season variety for most of the state. A group IV variety would be considered an early-season variety, and a group VII variety would be considered a late-season variety. Most of the state could grow all three maturity groups successfully.

Because of soybeans ability to adapt to a wide range of conditions and North Carolinas flexibility in planting date, varieties with maturity group designations outside of the optimal range can still be grown. North Carolina growers successfully plant a range of maturity groups from late III's to early VIII's.

Typically, varieties in earlier maturing groups develop fewer leaves and reach R1 earlier. This means a group V will mature and quit growing earlier than a group VI will, if planted at the same time. Historically, group IV and earlier maturing varieties are indeterminate in growth habit, while group V and later maturing varieties are determinate varieties; but recently, later maturing indeterminate varieties have been released. Whether one growth habit is an advantage or a disadvantage, compared to the other, is arguable.

Each major maturity group is further divided 10 times to designate the relative maturity rating for a soybean variety. The relative maturity is expressed as a decimal. For example, a 4.1 will mature earlier than a 4.7 even though they are in the same major maturity group. Most seed companies use the relative maturity rating to classify their varieties.

Figure 1-1. Soybean maturity zone map of the United States. Data from Mourtzinis, S., and S. Conley. (2017). "Delineating soybean maturity groups across the United States." Agronomy Journal 109: 1-7. 10.2134/agronj2016.10.0581

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Publication date: Jan. 6, 2022AG-835

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The Soybean Plant | NC State Extension Publications