This is especially a problem with first cutting hay because it is harvested during a period of time when it rains frequently and the weather conditions are less than ideal for hay drying. Always use clean, unmoldy forages when feeding horses. In addition to molds and fungi, some forage species contain chemical compounds that can have negative health effects on horses. Sudangrass and sorghum-sudangrass hybrids contain compounds which can cause muscle weakness, urinary problems and death in severe cases. Older varieties of tall fescue contained an endophyte fungus that could cause severe health problems if horses have only tall fescue to eat during the summer months.
Mares are especially sensitive to the health problems associated with tall fescue.
On pastures that contain endophyte infected tall fescue, remove mares from the pastures during the last three months of gestation. When consumed, the beetle causes irritation to the lining of the digestive tract which usually results in death. Alfalfa hay that has been produced in southern areas of the U.S. is most generally associated with the potential to contain blister beetles. Poisonous plants in pastures or hay can be fatal to horses. Ornamental shrubs and nightshade are the most common poisonous plants in Pennsylvania.