by CottonInfo Program Manager, Janelle Montgomery.
Recently there have been some issues raised about Day Degrees (DD), particularly when growers and consultants are looking at their petiole testing results. There remains some legacy where some tools and understanding still utilize the DD12 approach to accumulating day degrees. The DD12 approach still works well in seasons that have average conditions. The DD1532 approach provided by CSD (or can be calculated) is more accurate and simpler to calculate than the DD12 approach. The CSD Day Degree calculator reports day degrees as DD1532 and includes the crop developmental stages utilising DD1532. You can still access the DD Base 12 data from the CSD DD Calculator via the Season Comparison tab or in the downloaded data.
Day degree accumulation using the DD1532 will also be lower than DD12 approach because the base temperature is higher; meaning that average daily temperatures need to be higher to accumulate day degrees. Fortunately, there is a relatively simple conversion to take DD12 estimates to equivalent DD1523 day degrees (and vice versa), and this can be used to update tools and information provided by industry. It is important to recognize that you cannot convert the day degree targets using this conversion. For timing of first square and first flower the DD12 target is modified by cold shocks and thus cannot be compared to the DD1532 targets (see table 1 below).
The figure below is off the Back Paddock website, the Day Degrees after sowing is DD Base 12. Back Paddock is now ensuring that the figure will have DD12 marked on the x-axis of the graph with the plan to move to DD1532 next season.

We are still in a transition period, you will come across factsheets, research papers that are all in DD12, however the Australian Cotton Industry has changed over to DD 1532 for good reason and it will become the norm (bit like the days of converting from imperial to metric – for those that can remember!).
What you need to know: the differences in key crop developmental stages.
Table 1. Source: Australian Cotton Production Manual 2024, CSD DD Calculator
If you want a simple conversion: In terms of a simple conversion that is quite accurate you can convert DD12 to DD1532 with the following equation:
DD1532 = DD12*0.6559-4.6256, which is visually presented in the figure below. NB: You cannot convert the day degree targets (Table 1) using this conversion.
Day Degree (DD) = A unit combining temperature and time, useful for monitoring and comparing crop development.
Cotton development can be predicted using DD. Note that day degrees are generally calculated using a nearby weather station and represent average growing conditions. The conditions in the field and the temperatures that a crop is exposed to can be very different.
CottonInfo's Janelle Montgomery together the figure below so you can see the differences in DD12 and DD15 for this season along with their 10-year average across the entire cotton season (based on Forbes Airport automatic weather station, data sourced from CSD Day Degree Calculator).
Why has the Australian Cotton Industry changed the Day Degree Calculation? Read more here.