By: Mitchell Japp, PAg, Provincial Specialist, Cereals
Timely application of crop protection products (herbicides, fungicides, foliar fertilizer, growth regulators, etc.) is important for many reasons. Crops may be more sensitive to the application outside of the proper staging, the product may be ineffective, or it may make the crop unsuitable for marketing (see previous articles on maximum residue limits for oilseeds and cereals).
Based on last week’s Crop Report, most fields across Saskatchewan have good moisture conditions. Adequate moisture for crop growth and warm weather make excellent growing conditions. Crops will be advancing rapidly. Rapid growth in combination with early seeding will lead to crops reaching target growth stages at earlier times than recent years. Field operations must be timely in order to be effective.
Growing degree days (GDD) are used to calculate the accumulated thermal time in order to estimate crop growth. Crop growth stages are much more consistent with thermal time than calendar days because crops need heat to grow.
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.
Growing degree days are accumulated from a specific starting point, such as seeding date or the first weed emergence date. This is called a biofix, and acts as a reference point.
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 base 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.
Either metric or imperial temperature scales will work to calculate GDD. Fahrenheit based GDD can be simply multiplied by 5/9 to get GDD in C, or GDD in C multiplied by 9/5 to get GDD in F.
Using GDD effectively relies on individual information, such as seeding or emergence date and GDD will vary with the weather for each area. It is challenging to map GDD to be relevant for an individual, but there is some potential to use a GDD calculator.
Field scouting cannot be replaced by GDD in combination with crop growth models, but GDD are useful for estimating when to visit the field for key growth stages. Models that use GDD for crop staging should be validated locally and adjusted for the conditions the crop is experiencing.
For example, based on the GDD accumulated for Regina so far in 2016 and the crop growth model developed by Stu Brandt and Perry Miller, wheat in the Regina area could be at stem elongation if it was seeded May 1. Various stresses to the crop, such as frost, may delay crop development in relation to the growth model. This example helps define the need for local verification and adaptability when using GDD and a crop growth model.