Tall Fescue Online Monograph

Improved adaptation to a given environment has been a major goal of most tall fescue breeding programs. Increased winter hardiness would be expected to expand the adaptation of Mediterranean tall fescue. For continental types, improved tolerance of drought and grazing, or close mowing in the case of turf, often are the main factors associated with improved persistence.

Indirect selection of a trait, such as drought tolerance, may be warranted where there is a high genetic correlation between the selected trait and the primary trait of interest. Greater heritability favors indirect selection, since the cost of phenotyping and selecting desirable plants is less for indirect than for direct selection. Several traits have been proposed to improve drought tolerance in tall fescue. Carrow and Duncan (2003) outlined a breeding procedure used to select for increased drought tolerance in tall fescue that was used for turf that resulted in populations showing less leaf firing under drought than nonselected germplasm. Johnson and Yangyang (1999) were successful in selecting for decreased ¹³C isotope discrimination, which has been associated with increased water use efficiency, although increased forage yield did not accompany decreased discrimination. Selecting for root traits has been suggested as a way to improve drought tolerance of tall fescue. Torbert et al. (1990) demonstrated that tall fescue plants with large diameter roots were able to extract water from greater soil depths than plants with small diameter roots. Similarly, aluminum tolerance has been suggested to confer improved drought tolerance by allowing a large root mass in acidic soils (Foy and Murray, 1998). Bonos et al. (2004) reported progress in selecting tall fescue for increased root mass at deeper depths and suggested that this could lead to improved drought tolerance.

Grazing Tolerance

Although reliable land use statistics are not available, most likely the vast majority of tall fescue planted for forage purposes in the United States and elsewhere is used as pasture. Grazing-tolerant genotypes typically are selected following exposure to heavy, often continuous grazing pressure. Grazing can result in greater plant loss than mechanical defoliation (Bouton et al., 2001) and presumably is more effective in identifying grazing-tolerant genotypes. Competition from a perennial warm season grass such as bermudagrass [Cynodon dactylon (L.) Pers.] further increases stand loss in grazed tall fescue (Bouton et al., 2001). Screening genotypes under such competition simulates farm settings typical of the southeastern United States. Space-planted and seeded sward plots are equally effective in determining persistence of populations under heavy grazing pressure (Hopkins, 2005). To date, we are not aware of any documented cases of progress from selection for improved grazing tolerance in tall fescue.

Digestibility

In a wide range of forage grasses, improved digestibility has been shown to have a large, positive impact on animal performance (Casler and Vogel, 1999). The primary limit to animal performance in tall fescue has been endophyte toxicity. Addressing this problem with either E- or nontoxic endophyte cultivars can improve animal weight gains by as much as 60 to 100% (Bouton et al., 2002; Hopkins and Alison, 2006)-few if any other technologies or practices in forage agriculture can increase productivity so dramatically. Increased digestibility is expected to enhance these gains further and remains an important breeding objective. However, to our knowledge, there have been no documented cases where selection has led to tall fescue cultivars with increased digestibility.

Productivity Traits

As with drought tolerance, various traits have been investigated as a means to select indirectly for increased forage yield in tall fescue. Although photosynthetic rate is a primary contributor to biomass yield in plants, Nelson et al. (1975) reported little relationship between net carbon exchange and forage yield in field-grown tall fescue. Genetic variation for dark respiration rate exists in tall fescue (Volenec et al., 1984). Selection for decreased dark respiration rate has been suggested as a means to improve forage yield (Sleper, 1985), as has been demonstrated in perennial ryegrass (Wilson and Jones, 1982). The results obtained by Zarrough et al. (1983) suggest that selection for increased weight per tiller could be used to develop tall fescue populations with improved forage yield under infrequent defoliation and with greater compatibility with legumes. Yield per tiller in tall fescue appears to be a highly heritable trait (Volenec et al., 1984). Two cycles of recurrent selection for increased leaf area expansion rate resulted in improved forage yield in tall fescue (Nelson and Sleper, 1983), which was attributed to increased tiller density and weight. Selection for leaf area expansion rate was not accompanied by changes in photosynthetic rate.

Seasonal Distribution

In many environments, distribution of forage yield throughout the growing season is as important as the total amount of forage produced. In the United States, increased fall and winter forage yield is considered highly desirable because it increases options for earlier fall grazing or stockpiling for subsequent winter grazing (see Chapter 6). Increased fall and winter growth has been associated with earlier maturity in spring for tall fescue cultivars such as AU Triumph (Pedersen et al., 1983).

Increased Palatability

Increased palatability has long been a goal of forage tall fescue breeders (Buckner and Burrus, 1968). Presence of an endophyte can reduce cattle grazing preference of tall fescue (van Santen, 1992), so early efforts at selecting for palatability may have been confounded with endophyte status. Cultivars such as Lubrette, Barcel, Sopline, Adora, Quantum, and Advance were selected for leaf softness in an effort to improve intake and palatability (Allerit, 1986). Leaf softness, as quantified by leaf tensile strength, in some instances appears to have little relationship with IVDMD (Nguyen et al., 1982). Correlations between cattle preference and leaf tensile strength (r = -0.20) and leaf shear strength (r = -0.16) were minor compared to that with leaf width (r = 0.25-0.66) (MacAdam and Mayland, 2003), suggesting that palatability might be improved by selecting for wide leaf genotypes. Breeding for late maturity also might be useful in improving palatability of tall fescue (van Santen, 1992). Tall fescue normally requires vernalization to flower (Hare, 1994; Wang et al., 2003). Some tall fescue plants display aftermath heading, in which flowering tillers are produced, presumably without vernalization, long after spring reproductive growth has ceased. Selection against aftermath heading has not been reported, but if successful might improve palatability and digestibility of tall fescue summer growth.

Other Traits

Progress from selection has been documented for several traits in tall fescue. Barker et al. (2003) reported progress from two cycles of recurrent selection for resistance to stem rust and demonstrated improved seed yields for rust resistant populations when substantial disease pressure occurred (see Chapter 24). Watson and McLean (1991), as well as Pedersen et al. (1989), were successful in breeding for delayed maturity. Progress from selection in tall fescue also has been documented for reduced alkaloid concentration (Adcock et al., 1997), carbon isotope discrimination (Johnson and Yangyang, 1999), and increased concentration of Ca, P, and Mg (Sleper et al., 2002).

Improved tall fescue seed yield is evident based on reports from Oregon (available at http://cropandsoil.oregonstate.edu/seed-ext/; verified 10 Jan. 2010). Yields from commercial production fields increased from 1210 kg/ha in 1994 to 1765 kg/ha in 2004 (see Chapter 23). Presumably a portion of this increase can be attributed to genetic gain, given the strong emphasis on selecting turf types with increased seed yield. Woodfield (1999) estimated the rate of genetic gain at slightly less than 1% per year for forage yield and lamb growth rate, using two tall fescue cultivars developed in different eras in New Zealand. Further detailed studies are needed to examine genetic improvement over time for traits such as seed yield, disease resistance, and forage and turf quality in tall fescue, similar to those conducted in other forage grasses (Casler et al., 2000a, 2000b) and grain crops (Duvick, 1984; Ustun et al., 2001).