What the other guys said--plus low weed content. Around here we fight yellow star thistle and fiddleneck infestations constantly in dry land hay farming. Both of these weeds are toxic to horses and cattle. Sheep and goats are less susceptable. My neighbor grows alfalfa on about 30 irrigated acres and has to control the Johnson grass that gets into the crop periodically. This year we have an outbreak of whitetop (hoary cress) that's taken over some of the hayfields that are out of production. That stuff is toxic to horses and sucks up lotsa soil nutrients.
Exactly the reason making horse hay with a profit as a business from other owners fields is a pipe dream. If you don't spend money to control the weed problems,add lime and fertilizer, as needed, and have a lease to avoid what happened to "blueriver" after you spend the time and money the crop will continually dwindle and get weedy to the point it has no value as horse hay.. Most owners expect you to replace the nutrients removed from the soil.
Most folks in this area have found it makes a lot more sense to invest time and money in their own properties rather than others.