By Don Ball
Fescue toxicosis is a term used to encompass three disorders of grazing animals that are associated with tall fescue: fescue foot (a gangrenous condition of the body extremities of cattle), bovine fat necrosis (a condition in which masses of hard fat form in the abdomens of cattle that can cause digestive or calving problems) and fescue toxicity. Fescue foot and bovine fat necrosis can be a serious problem for an individual producer, but occur relatively infrequently; fescue toxicity is a widespread problem.
Fescue toxicity symptoms include reduced feed intake, decreased weight gain, lower milk production, higher respiration rate, elevated body temperature, rough hair coat, more time spent in water and/or shade, less time spent grazing, low blood serum prolactin level, excessive salivation, and lower reproductive performance. Some or all of these responses have been observed with various species of animals consuming pasture, green chop, hay and/or seed of fescue. In addition, horses consuming fescue forage often have reproductive problems including abortions, prolonged pregnancy, foaling problems and agalactia (little or no milk production).
About 30 years ago, an endophyte (internal fungus) was linked with fescue toxicosis. The fungus is exclusively seed-transmitted and its presence can only be detected via laboratory analysis. Ergot alkaloids it produces are believed to be responsible for the disorders previously mentioned. Unfortunately, most ‘Kentucky 31’ fescue (the variety in most fescue pastures in the USA) has a high level of fungus infection. Thus, animal production on most fescue pastures is substantially lower than it could be. Several ways to reduce economic losses caused by the fescue endophyte have been identified.
Avoidance of the Endophyte:
In some situations a livestock producer can avoid some or all of the problems that would otherwise result from animals grazing toxic endophyte fescue. The classic example occurs with horses. The foaling and other reproductive problems of horses result mainly from mares grazing toxic fescue during late pregnancy. Thus, simply preventing a pregnant mare from grazing toxic fescue during the last three months of pregnancy avoids the problem.
Avoidance has some application with other animal species. For example, since consumption of even low levels of endophyte toxins sharply reduces milk production of cattle, a dairyman might allow only non-lactating animals to graze a toxic fescue pasture. Another example could be a beef producer who pastures beef cows on toxic infected fescue but excludes yearlings because of greater economic impact of ingesting endophyte toxins. Also, the adverse reaction of grazing animals to the endophyte is greatest during warm weather. Hence, in some situations the timing of grazing of toxic fescue may be an important consideration.
Use of Endophyte-free Tall Fescue:
Since the early 1980s, endophyte-free seed of a number of fescue varieties has been commercially available. Endophyte-free fescue does not contain endophyte toxins. Therefore, there are no endophyte-related livestock disorders and animal gains and reproductive performance is thus strikingly better than on toxic, endophyte-infected fescue.
It is now known that endophyte-free fescue also does not contain certain endophyte-produced compounds (believed to be non-ergot alkaloids) that are important in stress tolerance and pest resistance in fescue plants. Consequently, endophyte-free fescue stands tend to be much less persistent than endophyte-infected stands, especially in climates such as Alabama where heat and drought are stressful on tall fescue. As a result, little endophyte-free fescue seed is planted in the South, even though persistence can be favored by planting endophyte-free fescue on soils with good moisture-holding capacity, use of rotational stocking, and avoiding grazing during summer.
Dilution of Toxins:
The quantity of endophyte toxins consumed by animals is directly correlated with the amount of toxic fescue consumed. Therefore, any management technique that reduces the quantity of toxic fescue in an animal’s diet will reduce fescue toxicosis. Management of toxic fescue pastures to favor other forage species can have the result of diluting the fescue toxins in animal diets. For example, close grazing during spring will reduce shade competition by fescue for lower-growing plants, and summer application of nitrogen will encourage bermudagrass and crabgrass.
Planting legumes such as clovers, alfalfa or annual lespedeza in toxic fescue pastures is often a particularly feasible, effective, and relatively inexpensive way to dilute fungus toxins in animal diets. For most producers this is not a long term solution to the problem, but it can significantly improve animal performance in the short term, especially in beef cow-calf operations.
Close Grazing or Clipping:
Keeping toxic endophyte-infected pastures grazed closely (3 to 4 inches) have been shown to improve animal performance as compared to allowing forage to accumulate. Furthermore, mowing toxic fescue pastures during spring and early summer may reduce subsequent toxin intake by reducing the ability of animals to selectively graze seed heads in which endophyte growth (and toxin levels) tends to be greater than in other plant parts.
Increasing the level of nitrogen fertilization of fescue has been shown to increase ergot alkaloid production. Therefore, a strategy for reducing toxin intake is to avoid applying high rates of nitrogen fertilizer to fescue pastures. Lower levels of nitrogen also make fescue less competitive, thus making it more likely that volunteer species (including legumes) may be present that help dilute toxin intake by grazing animals.
Research has led to identification of endophyte strains that do not produce the toxins that cause animal disorders but do impart pest resistance and stress tolerance to fescue plants. These fungus strains, referred to by scientists as "novel endophytes," produce the desirable alkaloids but not the undesirable ones. At present only one novel endophyte, marketed under the name MaxQ, is commercially available.
Grazing trials with lambs, beef steers, beef cows and horses have shown excellent performance on novel endophyte fescue pastures, similar to that on endophyte-free fescue. A novel endophyte can also give the tall fescue plant enhanced vigor, pest resistance, and tolerance to drought and grazing similar to that of toxic endophyte fescue. Thus, novel endophyte fescue offers the potential for long-lasting pastures and high animal productivity.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.