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Soybean Physiological Development and
the Danger of Frost
Compliments of
Agronomy Services Department
When delayed planting or a cool season occurs, many soybean
producers are anxious to know if their soybean crop will be damaged by an early frost.
Since soybeans go through many vegetative and reproductive stages, let’s review these
growth stages to help determine the chances of early cold weather affecting the crop at a
particular growth phase.
Vegetative growth stages are identified as growth occurring
when no evidence of bloom is found on the plant. Reproductive stages are marked by bloom
occurring on the plant. The reproductive stages are as follows:
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R1 – Beginning
Bloom
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R5 – Beginning
Seed
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R2 – Full Bloom
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R6 – Full Seed
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R3 – Beginning Pod
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R7 – Beginning
Maturity
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R4 – Full Pod
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R8 – Full Maturity
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A description of R4 through R8 follows:
R4 stage of development is identified when one of the four
uppermost nodes with at least one fully developed leaf on the main stem contains a pod
that is ¾ inch long. Some pods on the lower part of the plant may be full sized with
beans beginning to fill the pods.
R5 stage is also known as the beginning seed stage. At this
stage some flowers are opening at the extreme top of the plant while some pods contain
beans which are 1/3 inch in length. The plant attains its maximum height, node number and
leaf area at this stage.
R6 stage is referred to as the full seed stage. The largest
pods are still green and the beans fill the pod cavity. Beans of all sizes are found on
the plant at this stage, but no pods exhibit mature color. (Figure 1)
R7 occurs when some pods have completely lost all green
color, although not all pods exhibit this color loss. Rapid leaf yellowing starts at this
stage and leaf loss is nearly complete at the end of the R7 stage. (Figure 1)
At the R8 stage beans are fully mature. 95% of the pods
have reached their mature color. Under normal conditions, 5 to 10 days are required to
reach 15% moisture when the R8 stage is reached. (Figure 1)
The time period to complete each stage is dependent on such
factors as moisture conditions, amount of sunlight received, day and night time
temperatures and fertility.
Under normal conditions, a group 1.5 bean will need the
following time to complete each stage:
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R5 – 16 to 18 days
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R6 – 8 to 10 days
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R7 – 8 to 10 days
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If a killing frost occurs during the R6 stage, yields and
seed quality will probably be adversely affected. If the frost occurs into the R7 stage,
yields remain intact and oil and protein levels remain close to normal. However, seed
germination and vigor may be seriously damaged if the plants are killed any time prior to
R8 stage.
If a frost takes the beans during the R7 stage, field
drydown is the best technique to use, allowing the green beans to dry naturally and lose
much of their color. Processors discount for green beans in the crop, and the discount is
normally at its highest during the harvest pressure timeframe. Storage of soybeans with
some green color in an aeration environment can help reduce the amount of green beans over
time.
R6 R7
R8
Figure 1
Iowa State University of Science and Technology Special Report
No. 53 |
