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Completing the circle – Growing degree days and crop growth

By Mitchell Japp, MSc, PAg, Provincial Specialist, Cereal Crops

Cool weather at the start of May and at various points
through the month have slowed down crop growth. GDD
based on Environment Canada weather data for Regina,
Sask., starting May 1, base 0 C.
After a dry summer in 2018, and little rainfall so far this spring, there is a lot of concern about moisture. But, there may also be some concern about the slow crop development.

Based on the Crop Report, very few areas of Saskatchewan have adequate soil moisture. That, coupled with cool weather, has resulted in slow crop growth. The recent heat will be advancing crops more rapidly and moisture will be needed soon.

Growing degree days (GDD) are used to calculate the accumulated thermal time to estimate crop growth. Crop growth stages are much more consistent when compared with thermal time than calendar days because crops need heat to grow. It is important to calculate the GDD for your area, with your seeding date, to provide an indication of crop advancement.

So far, 2019 has accumulated fewer GDD than
any year since 2010. Data for Regina, Sask.,
based on Environment Canada weather data,
May 1 to June 3, 2010 through 2019, base 0 C.
For example, it has been estimated that the fifth leaf stage in wheat occurs at an average of 21 days after germination, or 350 GDD after germination. The error on calendar days is +/- nine days, while the error on GDD is +/- two to three days. While thermal time using GDD is a much better measure than calendar days for crop development, crop development can still vary with fertility, genetics and available moisture.

Each crop has different tolerances for the temperature at which growth stops. Incorporating a base temperature allows the calculation for GDD to be more accurate for each crop. Research conducted at Swift Current and Scott Agriculture and Agri-Food Canada stations for wheat use a base temperature of 0 C. Other sources indicate a base temperature of 5 C, so it is important to ensure that any comparisons made are using the same base temperature.

Crop staging will be important to ensure crop protection products are applied at the right stage of growth. Crops will be advancing, but, with limited moisture, they may be stunted. GDD can give an estimate of crop growth stages. A crop growth model developed by Perry Miller and Stu Brandt suggests barley could be into tillering if it was planted on May 1. But it’s more likely the development is not as far along as it might be, due to moisture limitations.

While a GDD model can be a useful tool, and help remind us that crop development shouldn’t be as far along as recent years, it doesn’t help alleviate all concerns about lack of rain.

For more information, see Montana State University’s GDD Plant Stages document or visit Farmzone.


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