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Soybeans in the Spotlight: How sensitive are they to day length?

Nathaniel Ort, Dr. Yvonne Lawley, University of Manitoba and Dr. Malcolm Morrison, Agriculture and Agri-Food Canada 

MANITOBA IS A part of the northern frontier of soybean production in North America. Photoperiods here are longer than those experienced in traditional soybean growing regions. Understanding the physiological mechanisms and genes controlling photoperiodism in soybeans has been crucial for breeders developing suitable varieties for Manitoba.

Some plants are sensitive to the amount of light they are exposed to in a single day. This period of light is called photoperiod, but is more commonly called day length. Soybeans can start sensing photoperiod as early as the unifoliate stage. This phenomenon in plants is called photoperiodism. Long photoperiods have a delaying effect on the time to first flower in soybeans. However, soybean varieties vary in their sensitivity to photoperiod. Most varieties with an earlier relative maturity rating have been selected to have decreased sensitivity to photoperiod and may even be termed “photoperiod insensitive.”

A study led by Dr. Malcolm Morrison (Agriculture and Agri-Food Canada, Ottawa) and Dr. Yvonne Lawley (University of Manitoba, Winnipeg) was designed to evaluate soybean phenology in Manitoba relative to Ontario. Field experiments were conducted by graduate student Nathaniel Ort to compare the same ten soybean varieties with relative maturity groups ranging from 000.9 to 1.3 over five years in these two provinces.

From 2008 to 2010, the experiment was conducted in Morden, Manitoba (49.2°N) and Ottawa, Ontario (45.4°N), while in 2017 and 2018, the experiment was repeated in Carman, Manitoba (49.5°N) and Ottawa. The two study locations in Manitoba were about 4° north of Ottawa, which translates into a photoperiod that is 49 minutes longer in Manitoba during the summer solstice (the longest day of the year). We recorded soybean growth stage for each variety three times a week, from emergence to maturity.

In Manitoba, differences in days to maturity between varieties were observed, which of course, was expected given the range of maturity groups tested. Despite these differences in maturity group, the days to R1 were the same on average for the range of varieties tested. That suggests the varieties we tested had a similar response to Manitoba’s photoperiod and temperature during the vegetative part of their life cycle. It was later on during the reproductive stages that varietal differences became prominent. For example, there was a three-week difference in time to maturity between a soybean variety with a relative maturity of 1.3 planted right beside a variety rated 00.0 (Figure 1) in our 2018 field experiment in Carman, MB.

When the same ten soybean varieties were grown in Ontario, flowering occurred 14 to 19 days earlier than in Manitoba. Because flowering occurred much earlier in Ontario, more time was spent in the reproductive stages compared to Manitoba (Figure 2).

This difference resulted in fewer crop heat units (CHU) being recorded from planting to R1 in Ontario than in Manitoba (Figure 3). Soybeans grown in Ontario accumulated more CHU to R8 than in Manitoba. Soybeans in Ontario also matured in fewer days than when they were grown in Manitoba. That was because the daily accumulation of CHU in Ontario was greater than in Manitoba. This difference in CHU accumulation is partly due to nighttime temperatures that are lower in Manitoba than in Ontario, especially at the tail end of the growing season.

The differences observed in time to flowering and duration of flowering may affect final yield. Seed yield is dependent on the number of flowers a plant has produced and the length of time it spends in the flowering stage. Because soybeans grown in Manitoba spend more time in the vegetative stage and less time in the flowering stage, it is likely that they will yield less than when grown in Ontario. In our experiment, soybean yields in Ontario were consistently higher, though only significantly more in the later maturing varieties (Figure 4). A shorter flowering window may also put the crop at greater risk of yield loss from extreme weather events, such as heat or drought stress, which leads to increased floral damage and ultimately reduced yield.


In our field study, it was nearly impossible to isolate photoperiod effects from differences in temperature and moisture when comparing the two growing environments. Growing conditions varied between the two environments every year the field experiment was conducted. That sparked an idea for a controlled environment experiment in a growth chamber to isolate potential photoperiod effects on the same soybean varieties that were grown in the field. Varieties were grown in 14-, 15-, 16- and 17-hour days with a constant temperature (25°C, for both day and night).

All varieties grown in the field experiment were significantly affected by photoperiod in the growth chamber. The longest photoperiod treatment (17h) extended the time from planting to the first flower by two to three days compared to when it was grown in the shortest (14h) photoperiod treatment. The rate of development from planting to R1 was slower for varieties with a later rated maturity group than the earlier maturity groups tested.


Photoperiodism in soybeans depends on two things: variety and photoperiod. Soybean varieties differ in their photoperiod sensitivity and will respond differently to short vs. long photoperiods or latitudes. That continues to be important for soybean breeders that are selecting varieties for northern growing environments that will yield similarly to soybeans grown in Ontario. The next step in this project is to develop a growth stage model specific to Manitoba, based on temperature and photoperiod. This growth stage model, coupled with local variety trials, will be helpful to farmers when selecting varieties suited to their respective growing regions. Until then, local replicated variety trials, or variety trials conducted at similar latitudes, remain the best sources of information to consult before selecting the right varieties for your farm.