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Growing degree days versus corn heat units

Q & A with CPS

Q: What are growing degree days (GDD) and how do they differ from Corn Heat Units (CHU)?

A: Temperature plays a significant role in the development and maturity rate of crops. A good indicator of potential maturity is growing degree days (GDD). Corn heat units (CHU) is a measurement of cumulative heat over the growing season.

In Western Canada, GDD and CHU are daily calculations that start in mid-April to early May and are cumulative as the season goes on. GDD can estimate insect emergence in crops, extreme heat impact on crops, harvest timing, flowering of a crop, weed emergence and frost free days, and compare variety/crop suitability for the local area. For example, agronomists estimate that 10 per cent of female wheat midge emerge at 660 GDD and 90 per cent emerge by 880 GDD. This helps growers determine when to scout for midge and potential insecticide application timing.

The below maps show how GDD and CHU vary on the Canadian Prairies.

Formulas for calculating GDD and CHU appear complex but the concepts are relatively simple:

  • GDD: [(max daily temp + min daily temp)/2] – base temp = GDD
  • CHU:[1.8(daily min temp – 4.4) + 3.3(daily max temp – 10) – 0.084(daily max temp) -10)2]/2 = CHU

GDD always uses a base temperature of 5 C while CHU uses a base temperature of 10 C. Most crops will have little to no growth development with a base temperature of five degrees, while corn needs a minimum of 10 degrees. The reason the calculation for CHU is slightly different than GDD is because with corn crops, there is no plant growth if day temperatures are below 10 degrees and night temperatures fall below 4.4 degrees. It is also assumed in the heat unit models that crop development halts at temperatures above 30 degrees.

The GDD and CHU models consider temperature the greatest variable for crop production; however, soil moisture, soil type, precipitation, topography, evapotranspiration and local vegetation all impact crop growth and also need to be a considered within the specific micro climate.

Jade Delaurier is a manager of agronomic services with Crop Production Services in eastern Saskatchewan.

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