Chapter 11. Nutritive Value. Tall Fescue for the Twenty First Century

Authors: Burns, Joseph

Submitted to: American Society of Agronomy Monograph Series
Publication Type: Monograph
Publication Acceptance Date: 1/15/2008
Publication Date: 1/20/2008
Citation: Burns, J.C. 2008. Chapter 11. Nutritive Value. Tall Fescue for the Twenty First Century. American Society of Agronomy Monograph Series. pp. 159-201

Interpretive Summary: Tall fescue is the only reliable cool-season grass that perenniates across the north-south transition zone. It’s persistence as a perennial is, in part, associated with the occurrence of an endophyte that causes the production of alkaloids that provides a degree of tolerance to certain pathogens as well as tolerance to environmental ( heat and water stresses) conditions. Consumption of these alkaloids can alter animal health including loss of milk production, reduced weight gain and even abortions of the fetus. The nutritive value of tall fescue, as measured by crude protein, neutral detergent fiber and constituent fiber fractions of acid detergent fiber, cellulose and lignin, are essentially not altered by the present of endophytes regardless if they are considered toxic, nontoxic, or novel. This chapter addresses the nutritive value, including estimates of dry matter digestion via in vitro dry matter disappearance, relative to the presence of endophytes on the cultivar effect and management influences. Management aspects include grazing vs. haying, defoliation frequency, continuous defoliation vs. stockpiling, fertility levels, especially the application of nitrogen rates and water stress. Findings reported in the literature throughout the U.S. are summarized and a synthesis is directed toward future research where the novel, nontoxic endophytes can be used that retain plant persistence without the adverse animal responses.

Technical Abstract: The nutritive value of a forage is associated with subsequent animal daily performance. Generally, forages with greater concentration of cell solubles, including Crude Protein (CP), will have reduced concentration of cell walls and will be positively associated with animal daily performance. The objective of this chapter is to examine the chemical composition of tall fescue with comparison to other C3, perennial, cool-season grasses when managed as a pasture, cut and preserved as hay or silage, or accumulated in the summer as a stockpile for subsequent winter grazing. Unlike most cool-season grasses, the relationship between the nutritive value of tall fescue and animal daily response is variable. This is attributed to the presence of an endemic endophyte which produces alkaloids and can negatively influence animal responses. The chemical composition of tall fescue, however, is not altered by the presence of the endophyte. In general, the nutritive value of tall fescue is similar to that of other C3, perennial, cool-season grasses when managed similarly. Under grazing conditions the CP concentration, the most comprehensive index of nutritive value in the literature, of tall fescue and other cool-season grasses is greater in spring growth, declines to a low in the summer (July) and increases with the onset of fall (October) growth. Tall fescue is responsive to nitrogen fertilization, as are other cool-season grasses, with CP concentration increasing to over 300 g kg-1 when topdressed with > 500 kg N ha-1. Further, CP concentrations are greatest for most cool-season grasses maintained under frequent vs. infrequent defoliation regimes. When harvested and preserved as hay or silage CP concentrations of tall fescue are similar to other cool-season grasses, and all forages showed reduced CP concentration as plants matured from vegetative through to the heading stage. Forage accumulated in the summer for grazing during the fall and winter had greater CP concentrations when N application was deferred from June or July until September. Frequently, CP concentrations of tall fescue were somewhat less (10 to 30 g kg-1) than those in other cool-season grasses when accumulated and sampled in early fall. Clipping tall fescue and other cool-season grasses in the fall, compared with stockpiled, maintained greater CP concentrations, regardless of accumulation dates. When utilizing stockpiled tall fescue or other cool-season grasses in December or January, CP concentrations were reduced compared with October, but increased in February or March with the onset of new growth. Consideration is given to estimates of dry matter disappearance, fiber fractions and soluble carbohydrates. The paradox that exists between the nutritive value of tall fescue and animal daily responses is addressed and consideration is given to the direction of future research.