The patchy yellow colors beginning to appear in soybean fields are a sure sign that harvest is approaching.

As growers prepare for another harvest season, SDSU Extension Crop Production Associate Jonathan Kleinjan reviewed some of the basics of soybean maturity.

"An understanding of soybean relative maturity is important for growers to select the varieties best adapted to their production areas," he said.

Kleinjan explained that it is best to pick a variety with sufficient maturity to maximize vegetative growth and thus node production prior to entering reproductive stages, however, planting a variety that does not flower soon enough may result in crop losses due to late season dry weather or early frost.

Soybean is a photosensitive plant. "In other words, day length is what signals the plant to enter the reproductive growth stages (flowering and setting pods)," Kleinjan said. 

While this is commonly referred to as 'daylight sensitivity', Kleinjan explained it is actually the length of darkness during the night hours that signal responses in the plant. "Day length can vary with latitude and that is why most soybean varieties are only well adapted to a north-south range of 100 miles or so," he said. "Soybean maturity can also be affected by planting date and temperature; but day length is the main driving factor causing a shift to reproductive mode."

History on soybean maturity groups

William J. Morse, a USDA scientist and arguably one of the founding fathers of the soybean industry in the United States, was the first to classify soybean maturity into three groups in 1918, Kleinjan explained.

The number of groups was expanded to five in 1925 and nine in 1949. "Today there are thirteen major groups recognized, ranging from maturity group (MG) 000 through MG X. With the exception of MG 0s, the major group rankings are often, although not always, referred to with Roman numerals," he said.

Kleinjan explained that each of the major groups is further subdivided ten times to designate the actual relative maturity (RM) rating for a soybean variety. The RM ratings usually do not use Roman numerals and are reported in common decimal format where the first number is the major group and the number to the right of the decimal point denotes the subdivision.

"For example, a soybean variety designated RM 1.4 is in the fourth subdivision of maturity group I," he explained.

References describing the origins of the RM rating system are difficult to find, Kleinjan said, but, he explained, the common explanation for the basis of the rating system includes the latitude of the I-80 corridor (roughly 42° N latitude) and the calendar month of September.

"Each subdivision of the RM scale is said to correspond to a day in the month of September at the given latitude," he said. "For example, in a normal year, a RM 1.4 soybean should reach maturity (R7) at 42° N latitude on September 14th. A RM 2.1 soybean should be mature on September 21st, and so on."

It is important to realize that the rating system has evolved over time based on individual company needs and objectives, Kleinjan said. "It is not uncommon for soybean varieties with the same RM rating from two different seed companies to actually reach maturity a few days apart."