Staging Soybeans – Early Considerations

Delbert Voight, Jr. PennState Extension

With many fields of soybeans actively growing, there is some general information one needs to become familiar with what the plant is going through. This article will discuss how to growth stage soybeans in the early season.

This article is this week’s Agronomy Highlight for Monday, June 29 at 8:00 a.m. The Agronomy Highlight is an opportunity to ask the authors questions and get updates from around the commonwealth. You can ask a question about this article before the Agronomy Highlight discussion. You can join us on zoom or by calling +1-646-876-9923, and when prompted enter the webinar ID: 210 945 454. The discussion will also be live-streamed Mondays at 8:00 am and stored for later viewing on the Field and Forage Crops Facebook page .

VE is identified by first inspecting for the presence of the cotyledons (those are the thick leaves that came up first, lose one of those and that could be 2% of your final yield! Yocum 1990). Then look at the first leaf that is called the unifoliate (this means single leaf). It is rounded and appears just above the cotyledons. Next, begin counting the number of trifoliate (three leaflets on one stem) leaves that have emerged above the unifoliate leaf. Each trifoliate is a V stage. So, if one counts 2 trifoliates then the stage is V2. Plants have two main stages V and R. The V stands for vegetative, that is to say, the stage that the plant germinates emerges and puts leaves on the main stem. Then the R stage refers to the reproductive stage. This stage marks a change in the plant from simply growing green leaves (vegetative) to begin to prepare the metabolism and prepare to produce a pod. At the transition from V6 to R1 it is a great time to now gather 20 to 30 uppermost leaves for tissue testing to ensure there is no hidden hunger for macro and micronutrients. At this point in many cases, both V and R stages exist with soybeans and they will continue to put new leaves on while flowering. Typically, they will stay in the V stages for about 40 days or more. So, the plants having an observed flower in the area are observing plants in the R1 stage.


Soybean cotyledons in early vegetative stage. Photo: Del Voight

Now you know what I am referring to pertaining to stages. At the V2 stage, the nodules are rapidly forming on the roots. It is a great idea to go check to ensure that 10-15 nodules are visible on the roots. With about 8 to 10 on the main stem. If the nodule is squeezed, they should be pink inside indicating that they are indeed producing the

nitrogen required by the soybean plant. Remember it takes about 3-4 lbs of N per bushel of harvest soybeans so these nodules must fix about 300 lbs of N for a 60 bu/acre harvest. If there are issues it could be pH related, nutrient relates, inoculation or virgin soil issues, or other maladies. I keep a quick pH kit in my truck and many times check pH near the seed. Low pH many times hampers the nodule formation on the roots. If you do find limited to nonexistent nodules there is still time, however, it is becoming limited. If the plant becomes N deficient, an application of @ 50lbs of actual N (preferably as dry fertilizer) might prove useful. Another key aspect at this time is that at each joint of the stem there will be axillary buds (simply a bud at a stem joint). This is a great advantage, unlike corn that has one growing point, the soybean plant will have a numerous amount of growing points at each and every one of the joints on the plant once the axillary buds form, adding to its ability to survive stress, hail, and other maladies in season. Short of you going out and clipping off the plants, or a disease or insect takes out the stem below the cotyledons very little can really destroy the plant’s regenerative ability. Even groundhogs leave just enough that the beans regenerate from the axil buds to have a continual supply of fresh growth.


Soybean just entering the reproductive stage. Photo: Del Voight

As the vegetative stages continue things begin to change once the first flowers emerge. Any flower that you can identify on the plant indicates the R stages or reproductive stages. As the change to putting more leaves on and beginning to pollination occurs demand for nutrients becomes extremely important. At his time of year, it is not uncommon to see potassium deficiency.

 

 


Soybeans showing typical potassium deficiency symptoms. Photo: Del Voight

Another key, and as a general rule of thumb, is that once the R2 or full flower stage is reached by doubling the height of the plant at that stage is roughly about the height the plant will be at harvest. At R2 a lot of other changes occur, and this stage and the rest of the season will be addressed in a future article on FCN.

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