This article from TheHorse.com goes into a good bit of detail about how and when to sample your horse pastures to determine nutritional value. It also suggests soil sampling and fertilizing to assure high quality forage.
CCA, CPSS, CPSEC
Reared on a farm in Southwestern Illinois
Educated at the University of Illinois in Urbana-Champaign (1977)
25 years with USDA-SCS (NRCS since 1994)
Soil-Right Consulting Services, Inc. 2005 - 2015
RPM Soils LLC 2016 to present