Soybean management decisions depend on growth stage

July 22, 2010

WEST LAFAYETTE, Ind. - Heavy spring and summer rainfall in Indiana made for a sporadic soybean planting season. The crop ranges in development from just planted to flowering and podding, and it is important for farmers to be able to identify those growth stages before making management decisions.

"Soybean management is based on growth stage of the plant, time of year and pests, including weeds, insects and diseases," said Shaun Casteel, Purdue Extension agronomist. "Producers need to be able to accurately identify the growth stages so they can scout fields and make the best possible decisions."

Ninety-six percent of Indiana's soybeans had emerged as of July 4, and 23 percent were blooming. Casteel said farmers should take this time to scout fields, and he suggested the following tips to properly identify the first four stages of reproductive maturity:

* Growth stage R1, or beginning bloom, is when any open flowers are present on the main stem nodes. R1 begins approximately six to eight weeks after emergence and responds to both light and temperature. During this stage vertical root growth rate rapidly increases, and plants are about 65 days from the beginning of physiological maturity.

* The next step toward maturity is full bloom, or growth stage R2, which is identified by open flowers at one of the two uppermost nodes on the main stem. At this point the plant has accumulated about 25 percent of the total dry weight and 50 percent of the total node number, and rapid dry weight and nutrient accumulation begin and continue until maturity. Nitrogen fixation rate and nitrogen demand also increase.

* At R3, soybeans begin pod development. Pods are three-sixteenths of an inch in length and present on one of the four uppermost nodes of the main stem. Developing pods, withering flowers, open flowers and flower buds also can be found during this stage.

* Some soybeans in the state may have even reached full pod, or growth stage R4. Pod size increases to three-fourths of an inch and can be found at one of the four uppermost nodes of the main stem. Flowering is complete and plants are about 40 days from physiological maturity.

Pictures and more in-depth descriptions of all four growth stages are available in the July 9 issue of the Purdue Pest and Crops Newsletter at  and

Writer: Jennifer Stewart, 765-494-6682,

Source: Shaun Casteel, 765-494-0895,

Ag Communications: (765) 494-2722;
Keith Robinson,
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