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Soybean Reaching R1 (07/08/21)

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Soybean plants in our area are transitioning from the vegetative growth stages to the reproductive phase of their crop development cycle. Thus far, the 2021 growing seasons has been dry with warm to hot growing conditions. The higher than normal Growing Degree Day units have accelerated the crop development and the production of the first soybean flowers. Flowering was observed in some of my research plots during the last days of June. The soybean plant usually begins the reproductive period after the 4-6 trifoliolate leaf stage (Photo 1) has been reached. The first flower can be found lower on the stem and this stage is called reproductive one (R1) and is depicted in photo 2.

Photo 1. Soybean trial at Casselton ND, at the V5-V6 growth stage with 88% canopy cover and transitioning to the R1 growth stage, July 1, 2021 and Photo 2. Soybean plant at R1 at Casselton, ND, July 1, 2021
Photo Credit:
Jose D. M. Bais

Full flowering or bloom (R2 growth stage) is reached when many nodes on the stem have flowers and a flower is open at the one of the two uppermost nodes on the main stem. Soybean is mostly self-pollinating with natural crossing occurring at a rate of less than 0.5 percent. Pollination usually occurs the day before the full opening of the flower. The stigma is receptive to pollen about one day before opening of the flower and remains receptive for two days. Shortly after pollination, the pod formation phase starts. The pod development stage (R3) is reached when the pods are 3/16 inch long at one of the four uppermost nodes on the main stem with fully expanded trifoliolate leaf. Lack of moisture during the pod development and grain filling growth stages will result in lower yield. Description of soybean growth stages can be found in the Soybean Growth and Management Quick Guide at https://www.ag.ndsu.edu/publications/crops/soybean-growth-and-management-quick-guide.

Table 1. Soybean Reproductive 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 leaf.

R3

Beginning pod – Pod 3/16 inch long at one of the four uppermost nodes on the main stem.

R4

Full pod – Pod ¾ inch long at one of the four uppermost nodes on the main stem.

R5

Beginning seed – Seed 1/8 inch long in a pod at one of the four uppermost nodes on the main stem.

R6

Full seed – Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem.

R7

Beginning maturity – One normal pod on the main stem has reached its mature pod color.

R8

Full maturity – 95% of the pods have reached their mature pod color.

 

The soybean plants will continue to develop new leaves during the flowering period. As weeds are still emerging, it is important to recognize that herbicides can only be applied before a designated growth stage is reached. It is important to always follow labeled herbicide use instructions to prevent any crop damage or yield loss.

 

Hans Kandel

Extension Agronomist Broadleaf Crops