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Soybean Growth and Development

By: Dale Risula, P.Ag., Provincial Specialist, Special Crops

* Adaptations from, The Minnesota Soybean Field Book, Editor in Chief Dr. J. Michael Bennett Professor Emeritus, Rhetoric University of Minnesota

Soybeans are dicotyledonous plants exhibiting epigeal (above the surface) emergence. In the germination process, the cotyledons are pushed through the soil to the surface by an elongating hypocotyl. Energy is required to push the large cotyledons through heavy soils, so soybeans generally emerge best if they are planted no deeper than 3.8 cm. (1.5 inches). When they are above ground, the green cotyledons open and supply the developing leaves with stored energy, while capturing a small amount of light energy.

The first leaves to develop are the unifoliate leaves. Two “single” leaves appear directly opposite one another above the cotyledons. Subsequent leaves are trifoliate leaves comprised of three leaflets.

There are two growth phases characteristic of soybean development. The first is the vegetative stages (V) covering development from emergence through flowering (Table 1). Following that are the reproductive (R) stages from flowering through maturation (Table 2). Plant stages are determined by classifying leaf, flower, pod and/or seed development. Staging also requires node identification. A node is the part of the stem where a leaf is (or has been) attached. Leaves are considered fully developed when the leaf at the node directly above it (the next younger leaf) has expanded enough so that the two lateral edges on each of the leaflets have partially unrolled and are no longer touching.

Table 1. Vegetative (V) growth stages

 Stage Description
 
VE Emergence
Cotyledons above the soil surface
VC Cotyledon –
Unifoliate leaves unrolled sufficiently so that the leaf edges are not touching
V1
First-node – Fully developed leaves at unifoliate node
V(n) nth-node
Here, the "n" represents the number of nodes on the main stem with fully developed leaves beginning with the unifoliate leaves.
From Fehr and Caviness

Table 2. Reproductive (R) stages

Stage Description
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 a fully developed flower
R3
Beginning pod – Pod 3/16" long at one of the four uppermost nodes on the main stem with a fully developed leaf
R4
Full pod – Pod 3/4" long at one of the four uppermost nodes on the main stem with a fully developed leaf
R5
Beginning seed – Seed 1/8" long in a 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 colour
R8
Full maturity – Ninety-five percent of the pods have reached their mature pod color. Five to ten days of drying weather are required after R8 for the soybean moisture levels to be reduced to less than 15 percent
From Fehr and Caviness

Temperature and precipitation changes can greatly affect the height of soybeans without having a large effect on early reproductive growth stages such as flowering. Post-emergence herbicides are labelled for application that coincides with soybean growth stages. To avoid injury, growers should identify soybean development by growth stage rather than by their height or the date they fill the rows.

Soybean crop with volunteer canola
Because soybeans are poor competitors with weeds, it is critical to employ weed control measures during early stages of development. The critical weed control stages are from VE to V3 (third trifoliate). During V4 to V5 (first flowering) you can assess nodulation and make your last herbicide applications. At R3 (early podding) the plants take up nutrient for biomass. R4 to R5 are full-pod to early-seed stages, which are important stages affiliated with yield, so adequate moisture is important here. At R5 your soybeans are approximately 30 days away from reaching the “safe from frost” stage. At R6 (full seed) the plant reaches maximum height and node development. At R7 (early maturity) the plants are safe from frost. Full maturity is reached at R8, where plants will have approximately 30 per cent moisture content and harvest should start in approximately seven to 10 days.

Become familiar with soybean growth stages to help your soybean growing experience.


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