Does Soybean Photoperiod Sensitivity Impact Variety Selection?

There are two distinct growth stages in many crops; the vegetative and reproductive stages. In soybeans the vegetative stage usually lasts between 30 to 50 days after emergence. Once a flower appears on the plant the reproductive phase has begun. What makes soybean unique is that vegetative growth (leaf number, node number, and dry weight) continues as flowers are formed. Flowering is triggered by day length although temperature also plays an important role. Soybeans are therefore called “photoperiod sensitive”. “Photoperiod” is a term that simply means the period each day during which a plant receives illumination (day length).

 

Soybean flowering is initiated as the days get shorter which is after the summer solstice on June 21st. Under ideal growing conditions, early planted soybeans can begin flowering before the summer solstice but do not ramp up to full flower until later. The initiation of flowering is not the only impact of photoperiod on soybeans. Many of the reproductive phases of the plant are also impacted by day length. The effect also seems to be cumulative, the shorter the days get, the faster the soybean plant will go through its growth stages. A Wisconsin study showed that the time spent from the R1 growth stage (first flower) to R6 (full seed) was much shorter for late planted soybeans compared to an early planting. Soybeans planted May 1st spent 60 days from the R1 to R6 growth stage while the same varieties planted June 1st spent only 45 days going through the same growth stages.

 

How fast a soybean plant goes through its growth stages will depend not only on day length but is also impacted by moisture, plant health, and temperature. This means the length of time it will take for a soybean plant to mature is not as easy to predict as other crops like corn. The variety, environment, day length, and planting date interact to determine the number of days necessary to reach maturity. Therefore, there is no accurate chart that assigns a precise number of days from first flower to physiological maturity. Photoperiod sensitivity also means the plant can adjust to the season or “catch up” when planting is delayed. The photoperiod and temperature drives soybeans to mature more quickly as the days shorten in the fall.