Understanding Soybean Growth and Development

Yield potential of a soybean crop is determined by the interaction between the genetics of the seed and its growth environment. So it’s important to better understand soybean growth and development. Two broad stages, vegetative (V) and reproductive (R), define soybean growth and development. The stages are reached when 50% or more of the plants have reached or are beyond that particular stage.

Germination, emergence, and early development
Soybean seeds begin to germinate at approximately 50°. Germination and emergence accelerate as soil temperatures climb to 77°. Planting soybeans in soils less than 50° will slow germination, and seedling vigor may be reduced.

With adequate moisture and favorable soil temperatures, the radicle is the first to emerge. Following primary root initiation, the hypocotyls begin elongation toward the soil surface, pulling their weighty cotyledons through the soil surface. This process requires a lot of energy, which is why it’s recommended that soybeans be planted no deeper than 1.5 inches.

Emergence typically occurs 1 to 2 weeks after planting, depending on soil moisture, soil temperature, and planting depth. Crusty surfaces restrict emergence, resulting in swollen hypocotyls. Shortly after VE, the unfolding cotyledons expose the growing epicotyl (young leaves, stem, and the growing point).

The subsequent expansion and unfolding of the unifoliate leaves marks the VC initiation stage, which is followed by the numbered Vn stages. The cotyledons are very important to the young plant, as they store all the energy reserves the plant will need for establishment. Quickly emerging plants usually do not consume all of the food reserves and end up with thick, fleshy cotyledons at the unifoliate (V1) growth stage. Plants that require several weeks to emerge may have noticeably smaller cotyledons or may lose their cotyledons all together. These plants are at a substantial disadvantage with nearby healthy plants. Loss of both cotyledons at or soon after the VE stage may result in an 8%-9% grain yield loss.

V3-V6
Plants at V3 typically range from 17 to 23 cm high, and have four nodes with unfolded leaflets. Plants at V5 can have six nodes with unfolded leaflets. V5 is about 1 week from R1, or first flower. At V6, the unifoliate leaves and cotyledons may have fallen from the plant, and the root system is expansive. New V stages are developing approximately every three days. At the V6 stage, a 50% loss of leaf area would equal about a 3% yield loss.

Vegetative Stages

• VE (emergence)
• VC (cotyledon stage)
• V1 (first trifoliolate)
• V2 (second trifoliolate)
• V3 (third trifoliolate)
• V(n) (nth trifoliolate)
• V6 (flowering will soon start)
  

Reproductive Stages

• R1 (beginning bloom, first flower)
• R2 (full bloom, flower in top 2 nodes)
• R3 (beginning pod, 3/16" pod in top 4 nodes)
• R4 (full pod, 3/4" pod in top 4 nodes)
• R5 (1/8" seed in top 4 nodes)
• R6 (full size seed in top 4 nodes)
• R7 (beginning maturity, one mature pod)
• R8 (full maturity, 95% of pods on the plant are mature)
  

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