In June we've done two releases that will improve the offering you can build into your application.
Our science and modeling team is hard at work vetting and building out a variety of models that you can use to give your users more insight into their fields. The latest of these APIs is the new Growth Stage Models API. These APIs will allow you to build in plant stage information into your applications, supporting in-season decision, scouting scheduling, etc. With a location and a planting date, the API will return the likely current growth stage for the selected model + crop.
There are 19 models for barley, canola, cotton, oat, sugarbeet, sunflower, wheat, and corn. The models come from established sources (generally university extension offices) and have been vetted by our science team.
Related to this release is a new GDD equation available via the Observations API: the Modified Standard equation. This equation is pretty common, and essentially ensures that when the temperature is outside a plant's usable range, that the excess heat is excluded from the GDD equation.