By: Mitchell Japp, MSc, PAg, Provincial Specialist, Cereal Crops
Over the past few weeks I’ve heard from individuals wondering if crop development is behind other years. Rather than the calendar date, the tool best used to estimate crop development is growing degree days (GDDs).
GDDs are used to calculate the accumulated thermal time in order 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.
GDDs have to be very specific to be used correctly, and need to be used in accordance with seeding date to be predictive. They are regionally specific because the weather varies.
I thought I would start by comparing GDDs so far this year with previous years. In this example, I’ve calculated GDDs for Regina, starting May 1 and ending July 7, using base 0 C.
As it turns out, GDDs are running very similar to last year, at least through to late June. Some cooler days in the last 10 days of June caused the GDDs to fall slightly behind GDDs accumulation in 2016. Both 2016 and 2017 are ahead of GDDs accumulation in 2015. The hot weather experienced since July 1 is leading to increased GDDs relative to 2016, although perhaps not as much as expected due to some cooler nights.
Despite similar GDDs accumulation, there have been setbacks this year that may have limited crop development:
- Later seeding this year relative to 2016. In 2016, 15 per cent of the crop was reported seeded as of May 2, while in 2017 only one per cent was reported as of May 1.
- In 2017, we experienced some strong winds that may have led to sand blasting. This damaged early leaves, causing regrowth or reseeding. Other biotic and abiotic stresses may have led to similar delays in development, including cold stress or freezing temperatures and insect feeding.
- Some fields may have noticeably less canopy fill compared with other years, leading to moisture stress. Physiological plant development is fairly normal when this occurs, though plants are smaller.
In comparison to individual comments, the June 27 to July 3 Crop report indicates that most crops are at normal crop development, with some being delayed due to moisture stress. Although a number of factors may have led to delays in 2017 relative to 2016, crop development in 2016 appears to have been relatively rapid.
Government of Canada Historical Weather data
Using Growing Degree Days to predict plant stages