Soybean growth stages and the R3 fungicide decision

How to stage a soybean plant

Work on the main stem only; branches do not count. For vegetative stages, count nodes that carry a fully developed leaf, meaning its leaflets have unrolled and the edges are no longer touching. VE is emergence, VC is the unifoliolate stage, V1 is the first unrolled trifoliolate, and so on to V(n). Mid-season a new node appears roughly every 3 to 5 days. Emergence needs about 150 growing degree days, which can take 5 days in warm soil or 40 in cold soil.

For reproductive stages, look at flowers, pods or seeds at the top of the plant, not the bottom. R1 uses any main-stem node, R2 the two uppermost nodes, and R3 to R6 the four uppermost nodes with a fully developed leaf. Indeterminate varieties, which is almost everything grown in the Corn Belt and Ontario, start flowering in the lower or middle canopy and keep adding nodes while they flower, so the top nodes always lag the rest of the plant.

A field is at a stage when at least 50 percent of plants have reached it. Pull 10 plants from several spots, stage each one, and use the majority.

Stage-by-stage table

Pod and seed sizes are the whole test. Carry a ruler the first few times. A 5 mm pod is about the length of a grain of rice and easy to overlook under a flower that is still attached.

Recognising R1, R3 and R5 in the field

R1 is easy to miss because the first flowers on an indeterminate plant open in the lower or middle canopy, often under leaves. Part the plant and check the lower main-stem nodes. One open flower on one plant is not R1 for the field; half the plants need one.

R3 is the stage growers most often get wrong, usually by calling it early. Go to the top of the plant, count down four nodes that carry a fully developed leaf, and look for a pod at any of those four nodes. Pods lower on the plant will already be longer than 3/4 inch; they do not count. If the top four nodes have only flowers, the plant is R2, even if the lower stem is full of pods. If they already carry 3/4-inch pods, the field is R4 and the ideal fungicide window is closing.

R5 is a seed test, not a pod test. Open a pod from one of the top four nodes; a seed 1/8 inch long means R5. From here to R6 the plant sets seed weight, and leaf loss now costs more than leaf loss earlier.

Why R3 is the fungicide timing

Three things line up at R3. First, Iowa State trials from 2008 to 2010 with strobilurin, triazole and premix products found a larger yield response from R3 applications than from R1 applications, with mean R3 responses of 2 to 4 bu/ac (0.13 to 0.27 t/ha) at most sites and 5 to 6 bu/ac (0.34 to 0.40 t/ha) at one site, and a chance of breaking even that ranged from 7 to 99 percent depending on site-year and price. Second, most leaf diseases that respond to a fungicide, including frogeye leaf spot and Septoria brown spot, cause yield loss only once they reach the middle and upper canopy, which happens during pod set; Nebraska Extension puts the most effective frogeye window at R3 to R5. Third, an R3 application protects the canopy through R4 and into R5, the period extension guides identify as the start of the most yield-critical window and the one most sensitive to moisture stress and leaf loss.

The same logic applies to insecticide tank-mixes, with one difference: an insecticide should be driven by counts, not by stage. In Iowa State's seven-location 2012 trial, fungicide and insecticide treatments were all applied at R3, soybean aphid never reached threshold at any site, disease stayed in the lower canopy, and yield effects were minimal. R3 is when to be in the field counting, not a reason to spray on its own.

White mold is the exception. Infection happens through senescing flowers, so the Crop Protection Network's efficacy ratings for white mold are based on R1 to R2 timing, and efficacy drops at R3 or later or once symptoms are visible. A field with white mold history and a closed canopy at flowering needs an earlier decision than the rest of the farm.

Why staging matters for labels

Labels write their restrictions in growth stages and days. A post-emergence herbicide may be labelled up to a V stage or a plant height; a pre-harvest herbicide may name R7 or a seed moisture; most soybean fungicides carry a pre-harvest interval in days and some add a growth-stage limit. Mis-staging by one R stage is a week in July, which can be the difference between a legal application and an illegal residue. Rates, intervals and stage cut-offs are on the label registered in your state or province. Ontario growers should check the Health Canada PMRA label (PCP number); a US registration does not carry into Canada.

The common staging errors are reading the lowest pods instead of the top four nodes, calling a stage from the first plant rather than half the field, and counting branch nodes in a thin stand. In an uneven field, stage the good and poor areas separately; a single pass timed for the best ground may be too late for the rest.