The deal with prairie grass is that is a catch all name for a variety of grasses that are typical in areas with unimproved pasture lands or spaces that are allowed to be hayed to save municpalities/governments having to pay to have them cared for. Unless you know where the grass is coming from and what location it is actually hayed then you are at the mercy of the person that hayed it. There are some really great mixes out there that horses love and are in line with what a horse has evolved to live off of. Not a monoculture so you can have a better profile that provides more than a hay of only one species. So, not knocking prairie hays - just a buyer beware.
Prairie hay that is truly Prairie hay is mostly going to consist of warm-season grasses like the bluestems and gramas, indiangrass, switchgrass, lovegrass, or prairie sandreed. There might be cool-season species present. Again all is dependent on location.