Managing Endophyte Infested Pastures
Most tall fescue pastures in the United States are comprised of the KY-31 cultivar; most of these pastures now are infested with N. coenophialum (see Chapter 14). This grass-endophyte symbiotic combination produces compounds that benefit the host tall fescue by increasing tolerance to pests and stresses. However, some of these compounds are alkaloids that cause the livestock disorder known as fescue toxicosis. Therefore, managing pastures of "common" tall fescue for optimum animal performance requires procedures that will alleviate the magnitude of the toxicosis.
A strategy for managing pastures of toxic tall fescue is suggested in Fig. 6-12. The first step is laboratory testing to determine the level of endophyte infestation (see Chapter 15). If the endophyte level is low, managers should keep their tall fescue. If it is high, managers should consider replacing toxic fescue with a nontoxic forage or managing the fescue to minimize the toxic effects. Replacement may be inappropriate when the endophyte level is low or when the grazers are dry beef cows. In addition, replacement may not be feasible when the land is rented or has a steep slope (Roberts and Andrae, 2004).
If replacing toxic tall fescue is not an option, managers must follow practices that reduce the effects of toxicosis. In so doing, they should consider two important concepts: alkaloid management and incremental alleviation.
Alkaloid management involves a series of practices that reduce the amount of ergot alkaloids produced in the growing, stored in harvested forage, or ingested by the animal. Alkaloid production can be reduced by limiting the rate of N fertilization because nitrogen application can increase alkaloid production in tall fescue. Alkaloid concentrations can be lowered within the plant by harvesting tall fescue for hay; most of the reduction during haymaking occurs while hay is being cured (Roberts et al., 2006) because ergot alkaloids are unstable and degrade in the presence of light, heat, and oxygen (Garner et al., 1993). Concentrations can be reduced further if hay is ammoniated (Roberts et al., 2002). Total alkaloid levels in a tall fescue pasture can be decreased by diluting the pasture with other forages; this usually occurs by interseeding legumes. Finally, the amount of alkaloids ingested can be reduced by avoiding grazing during periods of high alkaloid concentration, such as during reproductive maturity in late spring or summer. The amount of alkaloids ingested can also be reduced by supplementing of the diet with nontoxic forages and feeds.
Incremental alleviation is based on the multifaceted approach to management of toxic tall fescue. According to this concept, performance of animals grazing tall fescue improves in additive increments as each of the multiple practices is implemented. For example, daily gain of steers can be increased by the addition of legumes to the field. This increase in performance can be augmented by supplementing the diet, which can be improved further by providing summer pasture. As each practice is adopted, animal performance increases incrementally. However, no single practice, such as dilution with legumes, supplementation, or summer grazing, provides a complete cure for tall fescue toxicosis. Rather, they are combined to provide a management package with a cumulative impact on animal performance.
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