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NDSU Develops Soybean Growing Degree Day Model

The model predicts the maturity date of soybeans for eastern North Dakota and northwestern Minnesota.

North Dakota State University researchers have developed a new model to predict the maturity date of soybeans for growers in eastern North Dakota and northwestern Minnesota.

“The new model is just one of the many North Dakota Agricultural Weather Network (NDAWN) online tools available to growers,” says Adnan Akyuz, North Dakota state climatologist and professor of climatological practice at NDSU. “This model will help growers judge their risks associated with a projected soybean maturity and frost dates.”

In North Dakota, soybean varieties with a relative maturity (RM) rating of 00 are adapted for production in the most northern parts of the state, while group 0 and the early group 1 are adapted for the southern part of the state.

Each variety is classified with a 0 to 9 decimal number following the group number. A variety designated as 0.1 could be grown in the northern part of the 0 RM zone, while a 0.9 RM variety can be grown in the southern part of the 0 maturity zone.

Soybean growth stages are divided into vegetative and reproductive.

“The soybean development stage with one pod on the main stem with a mature color is called ‘beginning maturity,’ or R7,” says Hans Kandel, NDSU Extension Service agronomist. “The R8 stage is defined as 95 percent of the pods on the plant having reached the mature color and is called full maturity.”

The new model predicts the maturity at the R8 growth stage and is based on growing degree days (GDD), also called growing degree units. This model was developed to predict the soybean maturity date (R8) based on accumulated GDD from the planting date.

Based on 1,816 data points, a regression analysis was performed that predicted 1,694, 1,890 and 2,058 average GDD to maturity for 00.8, 0.5 and 1.1 RM soybeans, respectively.

Northern soybean cultivars are limited in growth duration due to the short growing season. Soybean plants may show freeze damage when the air temperature falls below 32 F, and in North Dakota, a killing frost is possible in the second half of September. Therefore, selecting varieties that will be mature before the fall freeze is important for growers.

For model predictions, agriculturists can select the RM for a variety, date of planting and nearest NDAWN station to estimate the maturity date. The output provides the predicted maturity date, as well as average first day of various frost risks.

Wet or dry conditions may influence how quickly the crop matures in the fall, and therefore, the predicted maturity date for this model may be slightly different, depending on the late-season weather conditions.

The new GDD model can be found at

NDSU Agriculture Communication - June 13, 2016

Source:Adnan Akyuz, 701-231-6577,
Source:Hans Kandel, 701-231-8135,
Editor:Kelli Armbruster, 701-231-6136,
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