The way that I base the feed and supplements is:
1- How hard my horse works,
2- If my horse has any muscle issues or other known problems, (example) My horse has stifle problems so she gets a joint supplement (which is basically Glucosamine)
I hope this helps you, if you have any other questions just contact me
Thank you very much for your answer!
The estimate of each horses needs be defined from scientific publications and our observation on the ground, that allow us, year after year to become more to refine these estimates. The knowledge of the nutritional composition of fodder allows us to calculate necessary supplements to cover the specific needs of each horse.