Tall fescue has anatomical characteristics and nutritive value similar to those of other perennial C3 grasses. Its consumption by ruminants should result in daily intake of dry matter comparable to that of any perennial cool-season grass containing adequate nutrients that can be converted readily to support very acceptable daily animal responses. An assessment of the literature, however, provides a striking paradox, with daily animal responses ranging from those expected, based on laboratory estimates of the nutritive value of tall fescue, to greatly suppressed performance that does not reflect the measured nutritive value. This discrepancy has been attributed to the occurrence of the fungal endophyte N. coenophialum in tall fescue. The tall fescue-N. coenophialum symbiont produces alkaloid compounds that can have negative impacts on animal physiology and, hence, animal behavior, including daily performance, but remain separate from changes in nutritive value. In production settings this paradox is complicated by the influence of the endophyte-alkaloid association affecting plant and animal physiologies and resulting in a wide range of animal daily responses that may have little to no bearing on the measured nutritive value of tall fescue. Interacting plant factors and animal factors contribute to this paradox when animals are consuming tall fescue and are paramount in producing the ultimate in forage evaluation, the animal daily responses. Delineated below are some of the major plant and animal factors, along with associated examples of their interactions, which form the basis for the paradox.
The major plant-related factors that contribute to variation in data found in the literature and often attributed to nutritive value range from imposed grazing management strategies to plant- endophyte symbiosis to weather influences (see Chapter 4) are listed below.
1. Tall fescue has optimum daily maximum growing conditions only during the cooler portions (18-24°C) of the growing season, yet it is frequently utilized to some extent during 12 mo/year in the north-south transition zone. Expectations, although unfounded, frequently exist of continued good production and high nutritive value throughout the year. This is attributed, in part, to its tendency to remain green year-round except during periods of high summer temperatures (>32°C) and water deficit stress.
2. Within a pasture, some plants may contain the toxic endophyte that imparts plant tolerances to a wide range of stresses, thereby favoring plant survival, but has no influence on nutritive value. Thus, the proportion of infected plants may increase in older swards and, with time, dominate the forage stand.
3. Among pastures, tall fescue stands can vary widely in the proportion of E+ plants, ranging from zero infection to 100% infection, providing the grazing animal with a wide range of choices in alkaloid concentrations in its diet but with little difference in nutritive value.
4. The endophyte activity in infected plants increases as ambient air temperatures climb above 20°C in late spring and with the onset of summer. This shift is more likely to affect animal responses negatively.
5. The occurrence of the endophyte generally is greater in the lower portion of the stems than in the leaves, having important implications in grazing utilization strategies.
6. The literature often lacks documentation of the presence of the endophyte or the endophyte infestation frequency, if present, in the tall fescue stand being evaluated and the subsequent alkaloid concentrations of the associated forage. Further, the name of the cultivar being evaluated sometimes is omitted, with reference made only to tall fescue or "fescue grass".
The major animal-related aspects of tall fescue utilization that contribute to variation and contradictory information in the literature that are attributed to nutritive value, are associated with animal responses to environmental stresses and to alkaloid type and concentrations. The major animal factors are listed below.
1. Animal physiology is sensitive to certain types and concentrations of specific alkaloids produced and present in tall fescue forage.
2. As ambient air temperature rises above 27°C and relative humidity increases in late spring with the onset of summer, the animal experiences an increase in environmental stress. Environmental stress and stress that is associated with alkaloid toxicity further aggravate toxicosis signs and reduce dry matter intake and growth rate.
3. Within and among breeds and species, animals have different tolerances and exhibit varying degrees of physiological stress under similar temperature and relative humidity and have different physiological tolerances to the presence, type, and concentrations of specific alkaloids in tall fescue.
The plant and animal factors listed above can interact to modify the observed toxicosis, while in other instances they may act independently. Some examples of plant and animal interactions to generate the basis of the paradox in a production setting are noted below.
1. Animal foraging behavior varies among individual animals, but in general they select green leaves when given the opportunity; however, selectivity declines as forage "on offer" declines. As forage variability declines, animals graze deeper into the canopy, thereby increasing the likelihood of consuming more of the basal stems. Consequently, in tall fescue pastures infested with the toxic endophyte, animals shift from consuming a diet greater in nutritive value with less occurrence of endophyte (diet with greater proportion of leaf) to a diet that is of lesser nutritive value and greater occurrence of endophyte (diet with a greater proportion of stem and stem bases).
2. As ambient air temperatures rise above 27°C and relative humidity increases, animals normally experience greater physiological stress, but concurrent with this stress is an increase in endophyte concentrations and a reduction in nutritive value of tall fescue forage. This increases the alkaloids but reduces the nutrients consumed by the animal, compounding the effect. The degree of the stress response, however, remains specific to the individual animal and is complicated by genetic inheritance and hybrid crosses.
3. Dilution of E+ tall fescue by weedy species or by interseeding with other forages, such as legumes or endophyte-free grasses, provides a pasture with reduced overall endophyte infection. Animals will graze selectively, however, and selectivity will remain animal specific; thus alkaloid consumption will be highly variable. This will change, as will nutritive value, as available herbage mass shifts from the most preferred to the residual species and plant parts.
4. Animal response data frequently are reported in the literature as means over a period of time, often designated as either spring or fall grazing, and can be extremely misleading. A spring designation, for example, may represent a grazing period beginning in late March and continuing through mid-to-late July. This period encompasses the transition from a cool (<27°C) spring environment to a hot and humid (>31°C) summer environment. During this period, if a tall fescue pasture is infested with a toxic endophyte, endophyte levels, and hence alkaloid concentrations, will be highly variable and interacting with the environment. Also during this period, grazing behavior and physiological stresses in the animal vary and interact with the environment and with the diet, including its nutritive value. The many plant, animal, and environmental factors all have variable influences on animal daily responses. Consequently, animal responses during the period reported can be sufficiently confounded by the presence of the endophyte such that interpretation of the resulting data is problematic and may be of little utility.
The integration of the plant and animal factors with accurate predictions of their interactions, as well as endophyte status, frequently is lacking in the available literature. Often the reporting of data as means compresses the spring to summer transition period in forage utilization trials and presents a major interpretive challenge for the reader. Consequently, even though a substantial body of literature exists, special care must be exercised to interpret the utility of each of its components.
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