The alkaloid ergovaline causes the constriction of blood vessels in animals. When cattle are in a warm environment, their main way of losing heat is to increase the blood flow through capillaries under the skin. This carries heat from the inner part of the body to the surface where the heat is lost to the air. However, if an animal consumes enough alkaloid to constrict these capillaries, the volume of blood flow and the amount of heat moved to the body's surface is reduced. The animal then becomes more subject to heat stress during warm weather.
Heat stress can cause reduced feed intake and decreased animal performance.
Early embryonic death in cattle during hot weather also may be related to heat stress. Knowing the life history of tall fescue and its endophyte helps us understand how to manage tall fescue to reduce fescue toxicosis. In cool, spring weather, the tall fescue plant is leafy and palatable. The leaf's growing point is at or below the soil surface. In May, reproductive tillers develop growing points that rise above the soil surface, pushing the seed head up through the leaf sheath.