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.

Figure 1. Relative Photoperiod Sensitivity of North American Soybean Varieties. Source: Jiang et al. (2014)

Are Early Maturing Soybean Varieties Less Photoperiod Sensitive?

Soybean varieties are classified by maturity group (MG). These thirteen MG’s are expressed in Roman numerals (MG 000 being the earliest and MG X being the latest). Because of the difference in climate as you move from southern to northern Ontario soybean varieties range from MG 00 to MG III in this province. As plant breeders have developed varieties that are adapted to very short growing seasons such as northern Ontario and western Canada these varieties have become less photoperiod sensitive than later maturing varieties. One study of North American soybean varieties showed early maturing varieties demonstrate little photoperiod sensitivity compared to later maturing varieties (see Figure 1). Logically, this makes sense as ultra earlier maturing varieties simply don’t have the time to adjust to the season. They are maturing as quickly as the plant can physiologically make it through its growth stages.

When does it become necessary to switch to an earlier maturing variety?

Some years planting is delayed. But, unlike corn this does not mean switching to an earlier maturing variety is always necessary. The soybeans photoperiod sensitivity will allow for later plantings (within reason) to still reach physiological maturity before a killing frost. For this reason, agronomists have suggested that switching to an earlier maturing variety is not necessary until at least the middle of June. Since there is a limit to how much a plant can compensate June 15th has been proposed as a reasonable date to start switching. In 2019 OMAFRA conducted 4 trials to assess this strategy. The study found that planting an adapted variety in a given area yielded higher than switching to an early maturing variety when planted after June 15th. Just as importantly normal dry down was reached in the fall when seeding an adapted variety after June 15th (see Table 1).

Table 1. Yields for Late Planted Soybeans in 2019

*Varieties were considered adapted for a given site based on the Maturity Grouping Map found on Early varieties were approximately 0.5 MG shorter than adapted and late were 0.5 – 1.0 MG longer than the adapted. Trials were conducted at Elora,Bornholm, St. Thomas, and Ridgetown.


Although selecting a variety that was considered longer than adapted for a given area still yielded well, harvest was delayed, and the crop was wet (18-26% moisture). The soybean varieties tested were all MG 0 or longer. It’s clear, these varieties are still photoperiod sensitive enough to adjust to the season. So, to date there is little evidence that switching to an earlier maturing variety is necessary if planting past June 15th. It must be noted that if planting is delayed past the crop insurance deadline (end of June) other studies have shown that switching to earlier maturing varieties is necessary to finish the crop and maximize yields. A more widespread study in 2020 will aim to assess what the correct date to switch to an earlier maturing variety is in various locations. And, more importantly, it will assess the correct date to switch from corn to soybeans for maximum profitability.