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Heat Units and Development Stages of Soybean (7/7/11)

The days are getting shorter since June 21. The lengthening of the nights is a trigger point for the soybean plant to change from the vegetative to the reproductive phase of plant growth. In the first week of July we can start to see the first flowers on early maturity group soybean plants.

Crop plants require heat to develop, grow, and mature. The effect of this heat is cumulative as the growing plant progresses through its life cycle. In ND, Growing Degree Day (GDD) Models have been developed for barley, canola, corn, sugarbeet, sunflower and wheat (

A model to predict the growth and development for soybean is not yet available. Soybean varieties are adapted within a narrow north-south geographical zone mainly because the plant is photoperiod sensitive. The dates of flowering and maturity in soybean varieties are important to determine their geographical adaptation.

We analyzed growth stage data recorded during 2004, 2005, 2006 and 2007 growing season at Carrington Research Center (Endres et al.). The data contained soybean growth stages starting from VE to R8 for two different maturity groups: 0.0 and 0.4. We calculated the accumulated GDDs corresponding to the end of each growth stage using the NDAWN GDD (based 50°F) application.

Figure 1 shows the relationship between soybean growth stages and GDD for the maturity group 0.0. In Figure 1, the vertical axis is the accumulated GDD and the horizontal axis is the growth stage, 1 representing VE and 8 representing the R8 stage.

Soybean growth stage chart

Maturity group 0.4 displayed similar results. Based on these preliminary results we concluded that:

Growth stage is reliably predictable up to R1 stage. From R1 to R8, range of GDD estimating soybean growth stage widens. For example the lowest and highest GDDs predicting R2 was 891 (in 2006) and 1052 (in 2005), respectively. The difference in GDDs was 161°. This range was even higher for R8. We will continue to investigate the relationship between GGD and developmental stages of the soybean plant but as can be seen from the graph we cannot yet accurately predict how many growing degrees it will take for the plant to reach R8.

Hans Kandel - Extension Agronomist Broadleaf Crops

F. Adnan Akyüz - State Climatologist

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