Soybeans and day length

Soybeans are a short-day crop whose reproductive onset (flowering) is influenced primarily by day length but also by temperature and planting date. The short-day designation means that increasing day length favors vegetative growth until the summer solstice (June 20 or 21) occurs and day length begins to decline, at which point flowering is favored.

Flower initiation is also affected by accumulated temperature or heat units, requiring some minimum growth before flowering is initiated, regardless of day length. This feature protects soybeans planted after the summer solstice from flowering with virtually no nodes or plant size. Accumulation of short days also hastens the period between flowering and pod formation. All of these photosensitive triggers in soybean favor reproduction as fall approaches.

📸: @ossyugioh via Canva

Soybeans were cultivated in Asia centuries ago, where scientists noticed and categorized the varieties into early, mid-, and late-maturities. In the US, University of Illinois agronomists Scott and Aldrich delineated proposed zones for the various maturity groups (Modern Soybean Production, 1970). Maturity groups range from 000 to 9, with the lower numbers being suited for the far northern reaches of soybean production. The MG zones run roughly east and west, parallel to latitudes, with each belt about 100–150 miles from north to south. Each full number change in MG results in a 5–10 day difference in maturity. There is adequate support for many approaches, provided there is less than a 0.5 change from the suggested maturity group.

For example, if your region calls for a 3.5 MG, you can have some comfort in trying group 3.0–4.0 soybeans. The central Corn Belt is dominated by MG 2–4. Most of the varieties used in the northern US (MG 0-4) are indeterminate, meaning that some vegetative growth continues after the first blossom appears. A general rule of thumb is that under good growth conditions, the number of nodes on the soybean plant will double precision after R1 (appearance of first blossom). Many of the MG 5–9 varieties are determinants in growth habits, so once the terminal node forms, vegetative growth stops. Determinate varieties have a more predictable harvest date. This attribute fits well in the South, where other harvest operations and the threat of hurricanes necessitate a predictable early harvest.

Day length is not changing, but temperature is. USDA has recently altered long-standing hardiness zones for horticultural crops to reflect this change. Since temperature is a secondary but important determinant for soybean flowering, it makes sense that the proposed zones would have changed since 1970. For the most recent mapping of soybean maturity group, see: http://www.coolbean.info/library/documents/SoybeanMG_2016_FINAL.pdf

What do changes to the suggested maturity group do?

Pushing an MG soybean variety farther north extends the period of vegetative growth and delays flowering and soybean maturation due to longer day length and likely fewer heat units. This can create a larger plant with more nodes, but it also risks running out of time for pod fill before a killing frost. A northern-adapted variety planted further south than its recommended range will do the opposite, shortening the vegetative growth period through earlier flowering and signaling to the plant to stop flowering due to shorter day length and more heat units, thus ending grain biomass accumulation where good growing conditions may still exist. The picture below shows four maturity groups planted on the same day, illustrating the differences in growth stage under the same light and climatic conditions.

📸: BASF Contributing Writer/Featuring soybean maturity groups 4 to 1 (upper left to bottom right) pictured Sept 18, 2023, Champaign IL. Note maturity based on leaves and plant height.

The phenomenon shown below is a classic example of the effect of increased day length on soybean maturation. The majority of the soybean field is fully mature and drying down for harvest, while a small semi-circle in the same field that senses light from a street light is continuing to grow (likely ~ MG 3.5 indeterminate variety). A gradation from fully green and growing to delayed maturity to maturity is present as distance from the nightlight source increases.

📸: BASF Contributing Writer/Featuring a soybean field next to street light, exhibiting effects of night lighting on soybean maturity. Sept 20, 2023 Champaign, IL.

Should you change maturity group selection for early or late planting?

There is some wiggle room here and some slight room for different opinions, depending on who you ask. Keep in mind the approach for getting more mature beans at harvest time, which requires the optimal number of pods and soybean size but also adequate maturation to allow for combining mature soybeans not terminated too early by frost.

Recent trends to plant soybeans earlier than traditionally have raised the question of planting a later maturity group. Most research suggests using your normal adapted variety or perhaps adding 0.2 to 0.3 to take advantage of the longer growing season. The double precision-cropped soybeans shown below just barely made it to maturity ahead of a killing frost and were likely slow to fill pods and mature as the weather cooled significantly ahead of harvest. In that case, it is sometimes prudent to use a slightly earlier-maturing variety.

📸: BASF Contributing Writer/Featuring group 3.5 soybeans planted July 15 shown here Sept 15 and Oct 15, Champaign, IL.

Spreading your maturities across a range of 0.5 around your suggested maturity group zone will help spread out risk and harvest times. Yields within this tight range of maturity groups will often be based on the vagaries of weather and the timing of rainfall. Selection of soybean varieties within your chosen maturity group is the subject of my next article.

Coming next: Selecting next year's soybean varieties

This content is being brought to you in partnership with Grow Smart Live and guest contributing authors. BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.