Soybean Growth and Management Quick Guide (continued)

A-1174, June 1999
Reviewed and Reprinted August 2004

Reproductive Stages

1. Beginning Bloom (R1)

At least one flower is located on the plant at any node on the main stem. Plants have lengthened to 15-18 inches tall and are vegetatively structured as a V7-V10 plant. Soybean flowering always initiates on the third to sixth node on the main stem depending on vegetative stage when flowering begins. This flower initiation will progress up and down the plant. Branches eventually also flower. Within each raceme, the flowering will occur from the base to the tip, so basal pods are always more mature. Once again dominance of the primary racemes is seen over secondary racemes on the plant; however, secondary racemes can develop just to the side of primary racemes on the same axil. Vertical roots are rapidly growing and will continue until R4-R5, as are secondary roots and root hairs nearer the soil surface.

Figure 6. R1 soybean. (46KB color photo)

2. Full Bloom (R2)

Soybeans are around 17-22 inches and are seen as V8-V12 in vegetative growth at the beginning of full bloom. An open flower is seen at one of the two top nodes of the main stem. At least one of these two upper nodes shows a fully developed leaf. At this stage, the soybean has accumulated about 25% of its total dry weight and nutrients and has obtained about 50% of its mature height. About 50% of the total mature node number have been established. Very rapid N, P, K and dry matter accumulation is now occurring and will continue through V6. Roots can reach across 40-inch rows. The appearance of new flowers on the plant begins to slow between R2.5-R3 and will be complete by R5. Major lateral roots have turned downward in the soil and nitrogen fixation by root nodules is increasing rapidly. Defoliation of the plant of 50% at this stage will reduce yield by 6%.

Figure 7. R2 soybean. (33KB color photo)

3. Beginning Pod (R3)

Plants can be up to 23-32 inches tall and can be at the V11-V17 stage. A pod on the upper four nodes is 3/16 inch long. Temperature or moisture stress at this time can affect yield through total pod number, bean number per pod or seed size. Partial compensation with only temporary stress can occur in soybeans, but as the plant matures from R1 to R5.5 this ability to compensate will decrease. Very favorable conditions will result in greater pod number per plant at this time. Since 60-75% of most flowers typically abort on soybean, any stress that increases this abortion will greatly influence yield. Half of most flowers are lost before pods begin developing and the other half are due to pod abortion. However, the long flowering period of soybeans is one reason these plants can compensate so well.

Figure 8. R3 soybean. (46KB color photo)

4. Full Pod (R4)

This stage shows rapid pod growth and the beginning of seed development at the beginning of the full pod stage. Dry weight of pods is greatly increased from R4-R5. There is a pod at this stage which is at least � inch long on at least one of the four upper nodes of the main stem. This stage is the most crucial period for seed yield. Any stress from R4-R6 causes more yield reduction than at any other time. Late pod formation at R4.5 to early seed fill at R5.5 is most critical. Yield reduction at this time is mainly from fewer pods. This is a critical period to consider irrigation, if needed, to reduce yield losses. The last flowering will occur at the main stem tip (through R5).

Figure 9. R4 soybean. (34KB color photo)

5. Beginning Seed (R5)

Seed filling during this stage requires much water and nutrients from the plant. Redistribution of nutrients in the plant occurs with the soybean providing about a half of needed N, P and K from the plant's vegetative parts and about a half from N fixation and nutrient uptake by the roots. Leaf loss of 100% at this stage will reduce yields by 80%; the plant is less able to compensate from stress and vegetative damage. Stress can actually lower yields by reducing pod number and the number of beans per pod, and to a lesser extent by reducing seed size. This stage has seed at least 1/8 inch long in one of the pods on one of the four upper nodes of the main stem. About halfway through this stage, the plant attains its maximum height, node number and leaf area. Nitrogen fixation peaks and begins to drop and the seeds continue a steady period of dry weight accumulation. Toward the end of this stage, the nutrient accumulation in the leaves peaks and then begins the process of redistributing to the seed. Seed accumulation will continue until shortly after R6.5, with about 80% of total seed dry weight being accomplished.

Figure 10. R5 soybean. (40KB color photo)

6. Full Seed (R6)

This stage is also known as the "green bean" stage or beginning full seed stage, and total pod weight will peak during this stage. Growth rate of the beans is rapid but will slow by R6.5 and peak at R7. This stage initiates with a pod containing a green seed that fills the pod cavity on at least one of the four top nodes of the main stem. Rapid leaf yellowing will begin right after this stage until R8, or all leaves have fallen. Within this stage, three to six trifoliolate leaves may fall from the lowest nodes on the plant prior to leaf yellowing. Root growth is complete at around R6.5.

Figure 11. R6 soybean. (38KB color photo)

7. Beginning Maturity (R7)

This stage begins with one normal pod on the main stem which obtains the mature color (brown or tan). Dry matter begins to peak in individual seeds. This is visually seen when all green color is lost from both the seeds and pods (they appear yellow). Seeds contain about 60% moisture at physiological maturity. Stress at this stage or later has almost no effect on yield unless pods are dropped to the ground or seeds are shattered from the pods. Also, any lodged plants may reduce actual yield (due to reduced light interception) and harvested yield (harvest losses). At this stage, the crop is safe from a killing frost.

Figure 12. R7 soybean. (31KB color photo)

8. Full Maturity (R8)

On the soybean plant, 95% of the pods have reached their mature color and only five to 10 days of good drying weather after this stage will be required to have the soybeans at less than 15% moisture, or harvest moisture. Soybeans will lose moisture rapidly with warm and dry weather at this point but should be harvested soon to prevent losses. Long-term storage of soybeans should be at 13% or less moisture. When harvesting leave short stubble to ensure full harvest. A 3� inch stubble still leaves 5% or more of the crop and a 6� inch stubble, 12% or more.

Figure 13. R8 soybean. (24KB color photo)

For more information on this and other topics, see: www.ag.ndsu.nodak.edu


Photography for this publication was provided through Iowa State University Photo Service, J.C. Herman, editor. Stage designation format was derived from the Iowa State Extension special report no. 53, "How a Soybean Plant Develops" prepared by S.W. Ritchie, J.J. Hanway, H.E. Thompson and G.O. Benson. Also, qualification on production management suggestions were summarized from lectures from J.R. George through Iowa State University. Special thanks to the suggestions and editorial review from T. Helms, Associate Professor, NDSU.

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A-1174, June 1999
Reviewed and Reprinted August 2004