PASTURE MANAGEMENT Forage Quality of Tall Fescue across an Irrigation Gradient Kay H. Asay, Kevin B. Jensen,* Blair L. Waldron, Guodong Han, Douglas A. Johnson, and Thomas A. Monaco ABSTRACT the primary limitations to digestibility in forage grasses, whereas cell contents (organic acids, proteins, lipids, Water available for irrigating pastures in the western USA is often soluble minerals, and nonstructural carbohydrates) are limited and varies widely across sites and seasons. Objectives were usually highly digestible (Buxton et al., 1996). The deter- to determine the trends in crude protein (CP), neutral detergent fiber (NDF), and in vitro true digestibility (IVTD) of 10 cultivars of tall gent fiber system is widely used to estimate cell-wall fescue (Festuca arundinaceae Schreb.) at five levels of irrigation and concentrations in forage grasses. Neutral detergent fiber three harvest dates (early, mid-, and late season). A secondary objec- provides an estimate of the primary cell-wall compo- tive was to evaluate the effect of the endophytic fungus [Neotypho- nents, such as hemicellulose, which is estimated as NDF dium coenophialum (Morgan-Jones & Gams) Glenn, Bacon & Han- minus acid detergent fiber (ADF), and cellulose, which lin] on these trends. Cultivars differed significantly for CP, NDF, and is estimated as ADF minus lignin plus ash (Goering and IVTD, and differences were generally consistent across water levels Van Soest, 1970; Buxton et al., 1996). (WLs). Concentration of CP increased in a near linear manner Progress in breeding for improved quality in forages, (154–189 g kg -1 ) from the highest to lowest WL; however, total protein including cool-season grasses, was reviewed by Casler yield decreased from 611 to 447 kg ha -1 with less water, closely follow- and Vogel (1999). They concluded that genetic gains ing the trend in forage yield. Forage quality at the late-season harvest, as indicated by lower NDF and higher IVTD values, was significantly achieved in forages for in vitro dry matter digestibility higher at the lowest compared with the highest WL. Also, NDF was (IVDMD) were comparable to those made for grain significantly lower and IVTD significantly higher at the late-season yield in many cereal crops. Heritable genetic variation harvest at all WL. Presence or absence of the endophyte was not was found in tall fescue for leaf tensile strength, NDF, associated with CP; however, there was a tendency for the endophyte- ADF, and hemicellulose; however, no genetic variation free cultivar to have lower NDF and higher IVTD than its endophyte- was observed for IVDMD (Nguyen et al., 1982). In later infected counterpart at the higher WL. The magnitude of quality studies with tall fescue, Bughrara et al. (1991) detected differences among the 10 cultivars indicates that forage quality should significant genetic variation for IVDMD, ADF, and be a major consideration in the choice of cultivars for pastures of the NDF, and their results indicated that selection for im- Intermountain region. proved IVDMD would be effective. Poor animal performance and other disorders have been reported in animals grazing tall fescue (Schmidt R estrictions on the use of public lands for livestock and Osborn, 1993), and the presence of an endophytic production has stimulated interest in improving the fungus has been associated with these maladies (Bacon, performance of animals grazing irrigated pastures in the 1995). Holstein dairy heifers (Bos taurus) produced 10% Intermountain West. Perennial cool-season grasses are less average daily gain on endophyte-infected tall fescue widely used in irrigated pastures of the Intermountain hay compared with the endophyte-free forage of the region; however, most cultivars of these species were same cultivar in trials conducted in northern Europe developed for more humid areas (Alderson and Sharp, (Emile et al., 2000). However, no relationship was found 1994). The USDA-ARS has initiated a breeding pro- between IVDMD or chemical composition and the pres- gram to develop improved perennial grasses that are ence of the endophyte in these trials. Bush and Burrus both productive and possess high quality for semiarid (1988) concluded in their review that CP, ADF, NDF, pasturelands, which are characterized by extreme tem- and IVDMD were not affected by the presence or ab- perature fluctuations and limited water resources. sence of the endophyte. The endophyte did not consis- Forage quality has been defined as the relative perfor- tently affect forage yield, IVDMD, or concentrations of mance of animals when the forage is fed ad libitum N or NDF in tall fescue in Kentucky trials (Collins, (Buxton et al., 1996). It is influenced largely by nutrient 1991). concentration, intake potential, and digestibility of the A line-source sprinkler system was designed (Hanks forage. Constituents of cell walls (polysaccharides, lig- et al., 1976) and used to study the responses of several nin and phenolics, proteins, cutin, silica, and water) are crop species, including cool-season grasses, to controlled irrigation levels (Johnson et al., 1982; Rumbaugh et al., USDA-ARS, Forage and Range Res. Lab., Utah State Univ., Logan, 1984; Asay and Johnson, 1990; Asay et al., 2001; Jensen UT 84322-6300. Joint contrib. of the USDA-ARS and the Utah Agric. et al., 2001). Some constraints must be considered in Exp. Stn. Journal Paper no. 7441. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of Abbreviations: ADF, acid detergent fiber; CP, crude protein; the product by the USDA or Utah State University. Received 7 Jan. IVDMD, in vitro dry matter digestibility; IVTD, in vitro true digest- 2002. *Corresponding author (kevin@cc.usu.edu). ibility; NDF, neutral detergent fiber; NIRS, near infrared reflectance spectroscopy; WL, water level. Published in Agron. J. 94:1337–1343 (2002). 1337