Most everyone associated with livestock production in the areas where tall fescue is grown knows fescue toxicity is caused by a fungus (endophyte) growing inside fescue plants. When fescue contains a toxin-producing fungus (and most fescue in Alabama does), gains, reproduction, and the appearance and behavior of cattle are often adversely affected. Late spring is the time when the effects of this disorder are most evident and dramatic.
Environmental chamber studies, as well as livestock producer experience, have shown, as the temperature rises, fescue toxicity symptoms become more evident. This is because toxins produced by the fungus interfere with heat regulation within the bodies of livestock. The inability of affected animals to deal with heat accounts for their desire to stay in shade or near water an abnormally-high percentage of the time, their tendency to pant and/or salivate excessively, and the fact they typically have an elevated body temperature.
Why is late spring, rather than summer, the time when fescue toxicity is most evident? Studies where the quantity of toxins produced by endophyte-infected fescue has been monitored over time suggest one reason may be the greatest quantity is present in late spring. Weather conditions promoting rapid plant growth apparently are also conducive to toxin production by the endophyte.
A second reason is associated with fescue seed head production. It is known the fungus and its products tend to be concentrated in the seedheads of fescue plants which, of course, are present in late spring. Despite the fact endophyte-infected fescue seed heads are in essence "loaded with toxins," grazing animals often selectively consume them, thus ensuring they ingest substantial quantities of toxins.
Third, in spring, fescue is truly the overwhelmingly dominant species in most Alabama fescue pastures and, unless there are legumes present with the fescue, a particularly high percentage of the diets of animals grazing such pastures comes from fescue at this time of year. Later, warm-season species like crabgrass and common Bermudagrass are usually present in substantial quantities thus providing a dilution effect in the diets of grazing animals.
Finally, while summer temperatures are usually higher, it often gets rather warm by late May or June. The combination of a high percentage of fescue forage in the diet, plus high levels of toxin production, plus relatively high temperatures inevitably result in readily-evident fescue toxicity symptoms in cattle grazing many endophyte-infected fescue pastures.
There are two reasons why livestock producers who use infected fescue should be aware of this late-spring toxic fescue situation. First, if one is ever going to recognize the visible symptoms of fescue toxicity, late spring is the time they are most likely to do so. A person who is unaware a rough hair coat and the behavior traits described earlier are indicative of fescue toxicity may not realize there is a problem. (Actually, research has shown that even when no outward symptoms of fescue toxicity are present, animal gains can be significantly lower on infected fescue. Therefore, failure to recognize visible fescue toxicity symptoms is particularly unfortunate.)
A second reason for being alert for fescue toxicity symptoms at this time of year is cattle with severe symptoms need gentle treatment. Causing these animals to become excited, and especially causing them to physically exert themselves, can actually result in death due to overheating! Therefore, this is not a good time to force grazing animals to move around unnecessarily.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.