Metadata for Sorghum Growth Stage Model

The sorghum growth stage model is a heat-unit or growing degree-day (GDD) model in which sorghum’s growth and development is based on the plant’s physiological response to temperature. The sorghum stage model is initiated by assuming an average planting date, and accumulated GDD heat units are then calculated and scaled according to sorghum growth stages as defined by Vanderlip and Reeves (1972). In addition, GDD accumulations for the entire growing season are variety specific, with short season varieties requiring less accumulated GDD units than long season varieties.

Vanderlip and Reeves (1972) defined the growth stages of sorghum as:

Stage Number Phenological State
0 Emergence
1 Collar of 3rd leaf visible
2 Collar of 5th leaf visible
3 Growing point differentiation
4 Final leaf visible
5 Boot
6 Half bloom
7 Soft dough
8 Hard dough
9 Physiological Maturity

The above stages were modified by grouping into similar stress threshold periods and including the time from planting to emergence stage. The stress periods are defined in the table below, and two generic sorghum varieties were then developed with the following respective GDD accumulations in Celsius:

Stress Period Phenological Growth Stage Var 11. var 21.
0-1 Planting - Emergence 70 70
1-2 Emergence – Leaf 7 500 441
2-2.9 Leaf 8 – Boot 1050 917
2.9-3.4    
3.4-3.9 Half bloom - Soft dough 1500 1306
3.9-4.9 Soft dough - Hard dough 1774 1543
4.9-5.8 Hard dough - Maturity    
5.8-6 Physiological Maturity 1944 1690

1. The GDD accumulations in the above table are based on Celsius degree-days, which are not the same as Fahrenheit degree-days because a Fahrenheit degree is smaller than a Celsius degree. It therefore takes nine Fahrenheit degree-days to make five Celsius degree-days, or GDDc = 5/9 (GDDf) and correspondingly GDDf = 9/5 (GDDc).

The above generic seed varieties were developed from actual seed varieties but later used as generic varieties ranging from short- (1690 GDD) to medium- (1944 GDD) season varieties. The corresponding generic seed variety was then matched to the average seasonal GDD potential for a region, where GDD each day is calculated as follows:

GDD = (Tadj-max + Tadj-min)/2 – Tbase

where, Tbase = 50 F (10 C) for sorghum

      Tadj-max is set to 86 F (30 C) when Tmax is above 86 F (30 C)
      Tadj-min is set to 50 F (10 C) when Tmin is below 50 F (10 C).

Properly matching the generic seed variety to the average GDD potential for the region allows the growth stage model to approximate the actual phenological stages for different regions.

Additional GDD information and calculations are described in the metadata section for the corn growth stage model.

References:

Vanderlip, R. L. and H. E. Reeves, 1972. Growth stages of sorghum. Agronomy Journal, 64:13-17.