Dan Arkels, 100-bushel Yield Challenge winner in 2014, planted his Yield Challenge entry April 17, 2016. These plots are already flowering. It might seem sort of early for soybeans to flower considering that we normally attribute the start of flowering to the timing of the summer solstice on June 21.
Soybeans actually start flowering 7 to 10 days before summer solstice—just a couple flowers—but it starts early and if you have a watchful eye you can see it. Therefore, when planting early, it makes sense that a few soybean flowers show up on these plants. Kip Cullers (remember he set the 160-bushel world record for soybeans) had soybeans flowering two weeks before the solstice when he planted beans in April and then intensely managed them. So this occurrence is not unheard of.
Soybeans are photoperiod sensitive—they respond to day length or more particularly night length and begin to flower when nights become longer, which starts June 21. However, early planted soybeans or soybeans ahead in development will shoot flowers before summer solstice. And while summer solstice sets off flowering, it starts rather slowly and builds over the next six to eight weeks, winding up around August 10.
Soybean development is impacted by temperature and night length:
- Before flowering, temperature drives vegetative growth and later night length drives flowering.
- After flowering begins, temperature drives growth through pod development—the warmer the summer, the faster the plant develops.
- Soybean maturity will be delayed in soybeans that are grown near high intensity roadway lighting because plants perceive the artificial light as an extended day.
- Although soybeans respond to day length (photoperiod), flowering will be delayed past solstice when the crop is planted late. However, because it’s warmer, flowering will be faster which will compensate for the late start.
The goal of any producer should be to produce and fill as many pods as possible. However, it begins with the number of nodes followed by the number of flowers set. The number of flowers is determined by the number of nodes on the main stem and branches with flower-bearing nodes. The greater the number of nodes and branches, the greater the flower-bearing potential. In other words, the more physically spread out flowers are on a plant, the greater the final production of pods.
Soybeans have an almost unlimited potential to produce flowers, however most of these flowers abort and never produce pods. Eliminating or mitigating stress ensures a great number of flowers will survive to produce pods. I was taught that for pods to survive and become seed-bearing, they need to reach a length of at least a quarter of an inch.
So with summer solstice near, note when your soybeans begin to flower. Did it start before, around or a week after solstice? Of course this will be determined by planting date—whether you planted very early or late. If you planted early, before May 1, I would expect to see a few flowers before June 21—and this is a good indicator of a potentially high yield crop.
Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at email@example.com or call him at 402-649-5919.