Soybean Flowering and Reproductive Stage

BY Dairyland Seed Agronomy Team

Soybean flowering can occur from the V2 (Second Trifoliate) which is the third node to V5 (Fifth Trifoliate). Once soybean plants start the flowering process, they are in the reproductive phase (R), and all growth phases from here on out will be listed in the R stage (example R1, R2, etc....) Soybeans will continue to put on vegetative growth during the reproductive stage and some agronomists will list both the vegetative and reproductive stages V7 or R1 for growth stages.

Reproductive Stage	Definition	Average Number of Days
R1	Beginning Bloom- One open flower at any node on the main stem.	3 days
R2	Full Bloom-Open flower at one of the two uppermost nodes on the main stem.	10 days
R3	Beginning Pod-Pod is 3/16" long at one of the four uppermost nodes on the main stem.	9 days
R4	Full Pod- Pod is 3/4" long at one of the four uppermost nodes on the main stem.	9 days
R5	Beginning Seed- Seed is 1/8" long in the pod at one of the four uppermost nodes on the main stem.	15 days
R6	Full Seed-Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.	18 days
R7	Beginning Maturity- One normal pod on the main stem that has reached its mature pod color.	18 days
R8	Full Maturity- 95% of pods have reached their mature pod color.

The number of days between Reproductive stages (R stage) can be influenced by its maturity group e.g. (1.0 vs 3.0) as well as weather conditions.

Soybeans can abort up to 80 percent of the flowers that they produce. This may be advantageous because if the first few flowers or blooms do not pod, the plant will start podding higher up the stem and off the ground making combining easier. Flower height, is also a concern for beans planted just prior to, or shortly after, the Summer Solstice. An example of this would be replanted or double cropped soybeans after winter wheat or peas.

Nutrient uptake technically starts at V2, when soybean plants start to add nodules to their roots, after it is infected with the soil borne Rhizobium japonicum bacteria. This bacteria causes the nodules to start fixing their own nitrogen. The plant does this by taking in non-available nitrogen (N2) in the air, and the bacteria in the nodules converts it to a nitrogen product the plant uses. Nodules will continue to form on the soybean root shortly after R5 (Beginning Seed) to R6 (Full Seed).

Less mobile nutrients like P and K are being taken up by the root system, which is growing .25 to .75 inches per day. Once a soybean plant enters the reproductive phase, the plant’s nutrient uptake increases dramatically. Applying micronutrients during the early to mid-reproductive growth phase is thought to be akin to someone grabbing a mid-morning snack or a cup of coffee, it jumpstarts the system to be more productive.

This nutrient demand will peak shortly after R5.5, with redistribution of remaining collected nutrients being moved to the seed crop from R5.5 to R8

As always if you have questions regarding this or any other agronomy topic contact your Dairyland Seed DSM or Agronomist.