What does summer solstice really mean in the life of a soybean plant?

Dan Davidson, Agronomist PhD, CPAg, CCA

Does soybean flowering begin on summer solstice (June 21) or is that just a myth? Shawn Conley, soybean specialist at the University of Wisconsin, calls it the “Summer Solstice Fallacy.” Yes, soybean flowering does seem to commence in a “big way” on the solstice regardless of where soybeans are grown. It also does not seem to matter whether they are determinate or indeterminate types. Indeterminate varieties continue to develop nodes and leaves on the main stem and branches throughout flowering, while determinate plants cease growth on the main stem at R1, but nodes and leaves continue to develop on branches until R5.

Flowering commences at R1 (beginning bloom) when there is one open flower at any node on the main stem. We automatically assume that soybeans start flowering on solstice because if we visit a field on June 22, we should see flowers. However, soybeans can start flowering much sooner, and the earlier you plant the sooner flowers will appear. If you are planting soybeans by the end of April and conditions are normal or warmer than normal, some flowers will appear by early June and the last flowers appear in early August, which covers a period of two months. I used to be a scout on the annual Pro Farmer Crop Tour held the third week of August. When we scouted soybeans fields to estimate pod count, we would frequently still see some colorful flowers at the top of the plant, much later than we expected.

Why is solstice such an important trigger? Larry Purcell, crop physiologist at the University of Arkansas explains, “The main effect of day length on soybean development is that of floral induction. Soybeans are referred to as short-day plants because short days (i.e., long nights or dark periods) initiate flowering (floral induction). The R1 stage begins when there is one flower at any node on the plant. Usually the first flower is located between the fourth and sixth nodes and daylength shorter than a critical value will induce the plant to flower.”

Shawn Conley recently posted in his CoolBean blog, “In soybean, floral induction occurs when soybean leaves can measure the night length (from dusk to dawn), and thus begins when unifoliolate leaflets appear at stem node 1 (V0) and a young trifoliolate leaf appears at node 2, with induction continuing thereafter in every subsequent leaf. If soybean is planted early enough, flower initiation can be triggered on the front of the summer solstice.” Conley added that the colder than normal April followed by a warmer than normal May can hasten the start of flowering. “So not only a warm spring, but also this exceptional heat wave in the last few days has hastened floral evocation. R1 is likely to be earlier in all North Central USA areas that have experienced both early soybean planting and a much warmer than normal spring.” You can read the full report here.

So, does early flowering mean more yield potential? Not necessarily, since soybeans flower over a very long window and produce many more flowers and pods than the plant can ever support. Some say it will abort as many as 60% of those structures over its lifetime. The benefit is that the plant will abort flowers or pods in order to fully support those that remain due to environmental stresses. And if there is a stress period with high loss rates, it will compensate and keep more flowers and pods produced afterwards. Think back to the drought of 2012: soybeans aborted flowers and pods produced early since the plant couldn’t support them. But after the rains return in early to mid-August, it kept and filled those later pods that survived, and yields were often near normal.

Remember, the key to higher soybean yields is to always provide the best environmental conditions for the plant to maintain and maximize the production of the pods and prevent the plant from aborting them prematurely.

What does early flowering mean to management? Soybeans that reach R1 followed by R2 three to five days later affects two things: how late you can apply a herbicide (some aren’t labeled for application at R1 or later) or when you apply a fungicide (since R3 could come sooner than the traditional calendar window). So, it pays to track when R1 and R3 commence because it can impact management actions on choosing your crop input.

Fortalis® is an essential crop input

Your soybean yields can also benefit from Fortalis®, an advanced crop enhancement foliar spray that works by mobilizing calcium already existing within plant tissues, resulting in higher pod retention. When applied as a tank mix, Fortalis® with a fungicide or insecticide contributes to better plant health and higher yield potential. It’s an essential crop input that you need to get the most out of your crop.

Dan Davidson is a PhD agronomist, part-time farmer and soybean expert from Nebraska who consults with soybean checkoff and industry on crop production, marketing, and product development projects.


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