ISSUE 10   July 17, 2008

SOYBEAN PLANT DEVELOPMENT

At this time of the year the soybean plants are starting to flower. Soybean plant development has two distinct growth phases. The first is the vegetative stage (V) that covers emergence and leaf development (Table 1). The second phase in the development is the reproductive (R) stage which starts with flowering and ends at the time the seed is mature. Plant stages are determined by the leaf, flower, pod, and seed development within the pods. A leaf is considered fully developed when the leaf at the node directly above it (the next younger leaf) has expanded enough so that the three leaflets, which make up the "trifoliate leaf", are fully visible. A node is a part of the stem where a leaf is attached. Although the plant will change from the vegetative stage to the reproductive stage, leaf development will continue and plant stature will still increase. Growth stages can overlap. On a field level a growth stage begins when 50% or more of the plants are in or beyond that stage.

Table 1. Growth stages of Soybean.

Stage

General Description

Specific Description

VE

Emergence

Plant is emerging

VC

Cotyledon

Unifoliate leaves fully developed

V1

First true leaf

The first trifoliate leaves are fully developed

V(n)

Leaf development

Additional trifoliates are fully developed

R1

Beginning bloom

One open flower at any node on the main stem.

R2

Full bloom

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

R3

Beginning pod

Pod is 3/16 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf.

R4

Full pod

Pod is 3/4 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf.

R5

Beginning seed

Seed is 1/8 inch long in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf.

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.

R7

Beginning maturity

One normal pod on the main stem that has reached its mature pod color (normally brown or tan, depending on variety).

R8

Full maturity

Ninety-five percent of the pods have reached their mature pod color.1

1. The crop will have to dry down to less than 15% before the soybeans can be harvested.

Table 2. Average number of days after planting for the soybean plant to reach different growth stages, Carrington 2004-2007.

 

Soybean Growth stage

VE

V1

R1

R5

R7

R8

Days from planting

0.0 maturity group

18

31

52

78

112

118

0.4-0.5 maturity group

18

30

54

81

119

124

Source: G. Endres.

Flowering of the soybean plant is initiated on the third to sixth main stem node and continues upward and downward from there. Pollination and fertilization are usually accomplished before the soybean flower opens.

The critical growth stage in soybean for water stress is from flowering to seed fill. Any stress from the R4-R6 (late pod development to early seed fill) will cause more yield reduction than if the stress takes place at another time in the plantís development.

Table 2 indicates that the start of flowering at Carrington was on average only two days later for a later maturing soybean variety. However, the plants reached full maturity six days later within the 0.4-0.5 maturity group compared with the 0.0 soybean maturity group.

Hans Kandel
Extension Agronomist - Broadleaf Crops
hans.kandel@ndsu.edu


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