Kitty,
Would you share your resource on plants poisonous to horses? I think you might want to confirm from a few different sources before making a final decision. For example, clover is fine for horses they eat it all the time in the field. However, as a
hay, if not dried properly mold can grow and that is bad for horses. Also the rye grass itself is not the problem, but a toxin that is produced by a fungi that can grow in the grass. Too much can cause the staggers. In your temperatures, I am not sure if fungus would be such a problem. But there are also strains that are free of the problem.
Ryegrass can be infected with endophyte fungus. The toxins the fungus produces causes nerve and muscles disorders. Symptoms of poisoning are staggering, stiffness, swaying back and forth when standing still, muscle twitching, excessive salivation, teeth grinding and convulsions.
Ryegrass varieties that are used in lawns must not be used in pastures – these seeds are not endophyte free. Ryegrass varieties developed for pasture and hay should be labeled endophyte free.
Alfalfa
should work. As would cow peas (both legumes).
There is a lot of misinformation and half truths in the horse world. Verify your sources carefully before a final decision.