The Van Trump Report

It’s Soybean Scouting Season – What You Need to Know

We have reached that time of year when it’s important to be scouting soybean fields for an array of yield-sapping pests and diseases. Catching problems early gives crops the best chance of minimizing yield loss, something that has become even more important amid razor-thin margins for many farmers. Just as important to identifying trouble in the field is understanding the growth cycle of soybean plants as this plays a crucial role in determining how to best tackle the issues at hand.  

The industry standard in growth staging for soybeans is an Iowa State University Extension publication, “Soybean Growth and Development (PM 1945)”, which can be ordered HERE. This method is commonly used in the field and grower publications. It counts the number of open trifoliolate leaves on the main stem. There is also the “Fehr and Caviness Method” which is primarily used in academic settings and journal articles. This one counts the number of nodes on the main stem. Although they use different techniques, the resulting growth stage determination is the same.

Germination: Germination begins with the seed absorbing 50% of its weight in water, a process called imbibition. Imbibition is dependent on soil temperature (optimum 60-70°F) and soil moisture (optimum 50%). The radicle (or primary root) grows from the swollen seed and elongates downward 9 The hypocotyl begins elongation upward toward the soil surface, pulling the cotyledons along.

Vegetative Stages:
VE, Vegetative Stage Emergence – VE stage occurs approximately 5-14 days after planting depending upon the soil temperature, which can be influenced by planting date. Technically, there are two methods used to determine the vegetative growth stages of soybean.

VC, Vegetative Stage Cotyledon – Occurs when the unifoliolate leaves have unrolled sufficiently so the leaf edges are not touching. Unifoliolate leaves are simple, consisting of a single leaf blade. Unifoliolate leaf nodes are opposite on the stem and are counted as the 1st leaf node. The cotyledons are the first source of nutrients and energy prior to photosynthesis. Notably, plants will not recover if damaged below cotyledons.

V1, One Open Trifoliate – The 2nd leaf node and all nodes to follow are singular and alternate on the stem. Nitrogen fixing root nodules begin to form on the roots through infection by Bradyrhizobium japonicum bacteria (introduced via inoculation or native in the soil). At this stage, agronomists recommend assessing stand count to determine if replanting is necessary.  

V2 – Marked by two open trifoliates, lateral roots are growing rapidly and active nitrogen fixation of the root nodules has most likely begun.

V3 to V4 – Once soybeans reach V4 (four open trifoliates), plants can typically recover from 100% defoliation at this stage with minimal risk of yield loss.

V5 – Five open trifoliates, or five nodes on the main stem with fully developed leaves beginning with the unifoliolate nodes. This stage marks the start of rapid dry weight accumulation.

V(n) – V stages continue with the unfolding of trifoliolate leaves. The final number of trifoliolate’s depends on the soybean variety and the environmental conditions.

Reproductive Stages:
R1, Beginning flowering – Marked by one open flower at any node on the main stem. Flowering on the branches begins after those on the main stem. Flowers can be purple or white. If a field has a history of white mold, this is the earliest growth stage to apply an effective fungicide.

R2, Full flowering – Open flower at one of the two uppermost nodes on the main stem with a fully developed leaf. Flowering will continue for 3-5 weeks. 20-80% of flowers produced will be aborted.

R3, Beginning pod – Pod is 3/16 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf. A plant can have all of the following happening at this stage: developing pods, withering flowers, new open flowers, and flower buds. Importantly, the size of the developing pods and seeds at one of the four uppermost nodes on the main stem with a fully developed leaf determines the R stage from R3 through R6. When staging plants, it is important to examine only these nodes as plants will simultaneously have a range of pods and seed sizes above and below these nodes. This is also the last stage to treat for white mold.

R4, Full pods – Pod is 3/4 inch long at one of the four uppermost nodes on the main stem with a fully developed leaf. At this stage, rapid pod growth is occurring and seeds are starting to develop. Flowering continues on the upper branch nodes. Peak nitrogen uptake rates occur between R4-R5.

R5, Beginning seed – Seed is 1/8 inches long in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf. Rapid seed filling begins, while root growth slows. Symptoms of many diseases, including white mold and SDS begin to show up at this growth stage.

R6, Full seed – Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf. Beans of many sizes can be found on the plant.

R7, Beginning maturation – Marked by one mature-colored pod anywhere on the main stem. Yellow pods are moving toward maturity; tan or brown pods (depending on variety) signal physiological maturity. Seeds at the R7 growth stage are at approximately 60% moisture.

R8, Full maturity – 95% of pods have reached mature pod color. Mature pod color does not necessarily indicate that beans are ready to harvest. 5-10 days of drying weather are typically required after R8 for soybean moisture to be less than 15%. Harvesting at 13% moisture is optimal for storage. Delaying harvest after optimal moisture is reached can result in yield loss due to shattering and shrinkage.

Farmers looking for low-cost tools that can help diagnose troubles in the fields also have numerous apps to choose from that are free or carry only a minimal charge. Precision ag specialists Charles Ellis and Kent Shannon at the University of Missouri maintain a pretty exhaustive list of ag-specific apps HERE. Ellis and Shannon have divided the apps into categories, including Agronomy, Crop Scouting & Precision Ag, Farm Management, and more. They also provide very helpful descriptions of what the apps do, compatible operating systems, and their cost. (Sources: Iowa State University, University of Wisconsin-Madison, Soybean Research and Information Network)

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