Plants Toxic to Horses

into hay are toxic. Like most toxic plants horses will avoid milkweed unless they have no other food source. Signs of milkweed poisoning are1
weakness
seizures
respiratory difficulties
coma
death
Milkweed is the host plant for the important pollinator the monarch butterfly. So although it should not be present in hay fields safely planting this flower in beds away from grazing livestock is acceptable.
Pigweed

Pigweed can be very toxic if eaten in large quantities. Horses are unlikely to eat this plant unless there is no other food available. This weed seems to grow everywhere from pastures to vegetable gardens roadsides to barnyards. It is still toxic if dried and baled into hay. Pigweed and its relative lambs quarters can cause kidney failure. Other symptoms of pigweed ingestion may include3
weakness muscle tremors
lack of coordination
kidney failure