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.
Horse pastures can be difficult to manage even under intensive grazing situations because of the horse's selective grazing habits. One way to over come this issue is to use smaller paddocks and move the horses more often. This article outlines how to follow horses with other species to clean up what the horses miss. Multi-species grazing. I thought it was going to be about forages. It turned out to be about animals.
So how do you know if the hay you are buying is really "good" horse hay? Lots of people equate alfalfa with "good" hay, but alfalfa is really too high in protein to be fed to almost all horses. Some horses that get a huge workout every day can deal with alfalfa. Horses can receive all the nutrients needed to thrive from hay. grain is needed only on rare occasion and oats is the preferred grain if used. Quality of hay required depends on the growth stage of the horse, the amount of work the horse does and whether or not the horse is pregnant. Purdue University has some very good information on hay quality and horse requirements.
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