Authors as Published

David L Holshouser, Extension Agronomist

Some plants sense changes in night length and initiate flowering only after the night is longer (and days grow shorter) than a critical length. Although technically incorrect, such plants are termed "short-day" plants. The length of the required dark period depends on the species and variety of a species. 

Soybean is a short-day crop and its development is largely determined by variety-specific daylength requirements that initiate floral development. In other words, as the days grow shorter soybean will flower and enter into reproductive development stages. Due to this photoperiod requirement, days from planting until maturity cannot be accurately estimated for soybean due to variation in planting date and other environmental variations.

After flowering, temperature drives development and the days until maturity can be estimated. The number of days from floral initiation (R1) until physiological maturity (R7) is usually independent of variety, but will vary slightly from year to year due to temperature differences between years.

Although most sensitive to daylength, soybean flowering will be delayed to some extent with later planting dates. However, later planted soybean initiates flowering during a warmer time of the year; therefore, post-flower development speeds up. The precise number of days from full flower (R2) until R7 cannot be predicted, but Table 1 gives fairly reliable estimates. In Virginia, most full season soybean planted in May will enter into floral development in late-July to early-August, depending on the variety. For soybean planted after small grain, flower initiation can be delayed two to three weeks. Soybean maturity may also be delayed by stresses (biotic or abiotic) that cause flower or pod abortion. Drought is the most common cause of flower and pod abortion, but insect feeding may do the same. If pods are not set, soybean will continue to produce flowers and remain in the R2 development stage until the stress is relieved. Although the crop will be delayed, the number of days between R2 and R7 will likely decrease as it does for later-planted soybean.

Table 1. Days to physiological maturity of soybean after floral initiation.
NumberDescriptionFull SeasonDouble CropExtremely Late Planting
R3Small Pods534739
R4Full Sized Pods443832
R5Small Beans343026
R6Full Sized Beans191715
R7Physiological Maturity000

Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

September 9, 2010