What’s the deal with soybean stages? And why do they matter anyway?

Fungicides, herbicides and insecticides all have an optimum application window, says Stephanie Porter, a sales agronomist for Burrus Seed. To get the best bang for your pesticide buck, farmers need to accurately identify soybean crop stages. 

Staging soybeans starts with understanding growth patterns.

Soybeans are day-length sensitive, Porter explains. They begin to flower around summer solstice, as nights become longer.  

Why is staging soybeans so challenging? Soybean growth stages overlap, explains Adam Day, Soy Envoy and Northern Partners Cooperative agronomist. They’re identified when the majority of plants move to the next phase.

Here’s a look at how to identify reproductive growth stages in soybeans, courtesy of Porter and Day:

Bloom stages
• R1, beginning bloom.  Soybean plants have at least one open flower at any node, Porter explains. A node is the part of the stem where a leaf is attached.

• R2, full bloom. One of the two highest nodes on the main stem has an open flower.  “At R2, you’ll have flowers in full bloom up and down the main stem,” Day notes.

• What’s changing? “At this stage, soybeans are taking more nutrients out of the soil and pushing them into the leaves at a rapid pace,” Day adds.

• What should you consider? Consider fungicide treatments for diseases like white mold as plants switch from R1 to R2, Porter says. Infection can take place as flowers senesce.

Pod development
• R3, beginning pod. Pods begin to form on one of the four uppermost nodes and have a fully developed leaf.

• What’s changing? “The plants will keep growing more flowers and more leaves at a rapid pace,” Day says.

• What should you consider? Agronomic decisions made at R3 could have a direct impact on your bottom line. “R3 is when retailers start talking fungicides, insecticides and plant growth regulators,” Day says. “We make lots of applications at R3 because we still have yield potential to gain.”

Your goal is to protect the middle of the soybean plant. “That’s where your yield is,” Porter explains.

Recent research supports a slight timing adjustment for fungicide applications. Preston Brown, Beck’s seed adviser, says pushing fungicide applications back to R3 provides a better return on investment.

“You want to wait until the last possible minute to add that protection, depending on what you’re spraying for and the environment,” Porter says. Pod and stem blight could creep in during pod development and pod fill, she explains, while bean leaf beetles or aphids could cause damage at this stage.

Day recommends keeping any threat — insects, disease or stress — from impacting the “solar panels,” or leaves, on soybean plants.

• R4, full pod. Soybean plants have pods all the way up and down the plant, with three-quarter-inch pods at the top of the plant.

• What’s changing? “That’s when flower production slows down,” Day explains.

BEGINNING SEED: At R5, soybeans reach their maximum height, leaf surface and number of nodes, Day explains.

Seed development
• R5, beginning seed. Seed is an eighth inch long in the pod at one of the four top nodes on the main stem.

• What’s changing? Soybeans reach their maximum height, leaf surface and number of nodes, Day says. “It’s done growing, and it’s moving to pod fill.” 

• R6, full seed. Green seed fills a pod at one of the four uppermost nodes.

• What’s changing? The heavy lifting is done, and the plant is reaching maturity.

Maturity
• R7 to R8, beginning to full maturity. One pod on the main stem turns tan to brown, and the rest of the pods follow.