The Penn State College of Agricultural Sciences notes that:
"Hay is the most important part of the horse's diet and makes up 50-100% of [it]."
Known as "Florida's alfalfa," peanut hay is a magnificent substitute for alfalfa due to its remarkably similar nutritive qualities. Like alfalfa, it is a legume, but it is restricted to the warm, nearly-tropical climate of Florida and south Georgia due to its intolerance of cold. Thus, it is a great choice for Floridian equines when it comes into season as the summer rains develop. Peanut hay is typically only available during the summer, and is often one of the first hays to suffer from winter shortages. Due to this, it is a hot commodity during the sultry summer months!
Alfalfa, a legume, is an extremely nutritious and high quality hay suitable for horses of any activity level. It is frequently mixed with other grasses in fields to achieve other desired characteristics and most horses will eat it with gusto, mixed or not.
Here in the Southeast, coastal hay is the most prevalent and is readily available nearly anywhere. "Coastal" is a variety of Bermuda grass (Cynodon dactylon L. Pers.) and is not the only variety used in baling. Other strains in pastures include "Alicia," "Brazos," and "Jiggs." Its prevalence and affordability make it one of the most popular hays around. It is often used as the main forage for horses and can also be fed to goats and other animals, used as bedding, or even used for ground cover in a garden.
(For in-depth analysis on specific varieties, click here.)