Crop Heat Units (CHU), Grower Degree Days (GDD), or Thermal Time are all measures of heat units we use to measure physiological events in the life of plants and animals. These units can predict the leaf stages of corn, flowering of Cucurbits, and even when an insect will emerge from dormancy.  

The formula is simple. It is the Daily Temperature Average minus a Base Temperature. The Base Temperature is the temperature high or low extreme where the plant quits growing. This temperature is crop specific. As a general rule, 50 is used.

Here are base (threshold) temperatures of a few crops:                                                                             Crop                              Temp (ºF)

• Wheat, Barley                        32
• Potato, Oats                           40
• Sorghum, Maize                    50

We grow oats here on the farm. To calculate our GDD yesterday, here is the equation:

(76+55)/2= 65.5 – 40 = 25.5 GDD’s for 3/23/17 on oats.

We add up the daily GDD’s to come up with the Accumulated Heat Units for a specific crop during a specified time.

GDD was first recognized by  French physiologist Renee Reaumur in the 1700’s. He noticed that the same grapes grown in different regions matured at different times. He conducted studies and introduced the idea of Grower Degree Days and Accumulated Heat Units.

So basically GDD is a generalized relationship between temperature and the length of time required for a plant to complete development.

Here is CHU chart for Barley:

Name	GDD Required	Accumulated GDD Req
Planting Date	0	0
Emergence Date	176	176
Leaf 1 (Fully Extended)	69	245
Leaf 2 (Fully Extended)	139	384
Leaf 3 (Tillers Begin To Emerge)	139	523
Leaf 4 (Fully Extended)	139	662
Leaf 5 (Tillering Ends)	139	801
Leaf 6 (Fully Extended)	139	940
Leaf 7 (Fully Extended)	139	1079
Flag Leaf Emerged (Collar Visible)	139	1218
Boot Swelling Begins (Flag leaf stem elongated) (Awns just peeking out of boot)	139	1357
Mid-Boot (Head half emerged)	69	1426
Head Emerged (Boot Complete) (Flowering Complete)	70	1496
Heading Complete	139	1635

Source: https://ndawn.ndsu.nodak.edu/help-barley-growing-degree-days.html (3/27/17)

Germination:

As you can see from the chart above, even seed germination requires a certain amount of Accumulated Heat Units. An increase in temperature can speed up or slow down germination. This is in lieu of a percentage of hard seed, or specific seed dormancies that need to be addressed prior to planting.

Leaf Emergence

The GDD required for one leaf to emerge is called a Phyllochron. The interval of time between leaf tip appearance is known as Phint (phyllochron interval). This time varies from plant to plant. As you can see from the chart above on Barley, There takes about 70 Phyllochron (dayºF) for leaf 2.

Apical Development

The leaf collar (start of reproductive development) on Barley shows at around 1218 GDD, and 1496 GDD’s for flowering to be complete. The excess in accumulated heat units may cause early flowering. The deficit is accumulated heat units may cause a delay in flowering. Each of these may be detrimental to the seed yield. It may cause a yield loss in seed quantity, quality, or cause a crop failure.

Plant Date

As you can see from above, the accumulated heat units start from plant date. In a study on sorghum, planting two weeks late casued many heat units to come too rapidly. This  caused the GDD to flowering to be higher by 100 GDD’s while the actual days to flowering

Conclusion

While there are other factors that effect on the growth of crops (soil types, fertilizer availability, water shortage or surplus, etc), GDD is an important tool for predicting crop growth.  It is also a good tool agronomists use for other aspects of farming. It can be used to determine the best time to apply pesticides and fertilizers. The developmental stages of certain crops initiates certain insects to come out of a period of diapause. An example is Alfalfa Weevils. These weevil eggs need 300 accumulated GDD units to hatch, and their first Instar period is 71 accumulated GDD units. So if you take the  minimum threshold temperature and wait 350 GDD units, you can catch most of the infestation before the first generation is able to reproduce.

 

Sources:

Click to access 10–reitz-degree-day-models.pdf

https://ndawn.ndsu.nodak.edu/help-barley-growing-degree-days.html

http://www.aganytime.com/Corn/Pages/Article.aspx?article=739

http://plantsinaction.science.uq.edu.au/content/1422-thermal-time-and-crop-development

https://www.researchgate.net/publication/254862120_Impact_of_Crop_Heat_Units_on_Growth_and_Developmental_Physiology_of_Future_Crop_Production_A_Review