Switchgrass - Bioenergy Crop Plants - page 580

best plants are selected from the best rows based on individual-plant observations, i.e., among-and-
within-family selection (Vogel and Pedersen 1993; Casler and Brummer 2008). Alternatively, family
or genotypic selection may utilize seeded sward plots if family matings allow sufficient seed for plot
testing. This breeding method has been rarely utilized in switchgrass breeding at the time of this
writing. Implementation of among-and-within-family selection is more challenging when families
are seeded into sward plots, because it is impossible to visually distinguish individual genotypes
for the second stage of selection. Furthermore, it is not clear that selection of surviving plants from
sward plots is advantageous, as it may be relatively inefficient for some species (Casler 2008). As
switchgrass swards age, genotypes are lost to interplant competition, stress susceptibilities, and
perhaps random mortality. The relative importance of these factors, the rate of genotype loss in
switchgrass swards, and the relative fitness of surviving plants (relative to mortal plants) are all
important determinants of the efficiency of among-and-within-family selection on sward plots
(Casler and Brummer 2008) and all are current unknowns for switchgrass.
Spaced plants are an extremely effective method of collecting data on individual plants and
ensuring that the best genotypes can be found and saved for intercrossing once they are identified
in the data analysis phase of selection. Spaced plantings are an extremely effective tool for selection
based on traits that have moderate to high heritability and are relatively insensitive to interplant
competition. Examples include in vitro dry matter digestibility (IVDMD) (Vogel et al. 1981; Casler
et al. 2002) and heading date (Van Esbroeck et al. 1998). Conversely, more complex traits that
may be highly sensitive to interplant competition, such as biomass yield, may have less predictable
selection responses from spaced plants. Selection for increased biomass yield of spaced plants
was effective when progeny populations were evaluated as spaced plants (the same trait) (Rose
et al. 2007) or as sward plots (a different trait) (Missaoui et al. 2005b). In the case of Rose et al.
(2007), selection was more effective under low-input conditions (no supplemental fertilizer or water)
compared to high-input conditions. Missaoui et al. (2005b) attributed their gains in biomass yield to
a fairly sophisticated approach to controlling interplant competition and spatial variation—use of a
honeycomb planting design combined with statistical adjustment for neighbor effects. Selection for
increased spaced-plant vigor was also effective for increasing sward-plot biomass yield in WS4U
switchgrass, largely by the elimination of unadapted and low-yielding genotypes (Casler 2010).
Alternatively, selection for increased biomass yield was ineffective in the EY × FF switchgrass
population, due either to lack of genetic variation for forage yield or to a low genetic correlation
between forage yield of spaced plants and sward plots (Hopkins et al. 1993).
22.7.3 B rEEding o BJEctivES
Breeding for improved forage digestibility as measured by IVDMD (Tilley and Terry, 1963) is
an effective way to increase switchgrass productivity as measured by beef cattle ( Bos taurus )
production per unit land area (Vogel et al. 1993). Divergent selection was used to develop strains
differing in IVDMD from the same base populations. These strains were evaluated in both small
plot and grazing trials (Vogel et al. 1981, 1984; Anderson et al. 1988; Ward et al. 1989). On the basis
of these trials the high IVDMD strain was released as the cultivar “Trailblazer” (Vogel et al. 1991).
In comparison to the control cultivar “Pathfinder” which had similar forage yield and maturity,
the single breeding cycle for high IVDMD resulted in the following genetic increases: IVDMD
concentration of 40 g/kg, daily live weight gains by beef cattle of 0.15 kg, beef cattle production
of 67 kg/ha, and profit of U.S.$59/ha (Casler and Vogel 1999). On the basis of certified seed
production, the area seeded to Trailblazer from 1986 to 1997 was over 63,000 ha. The principal area
of adaptation for Trailblazer is the central Great Plains of the United States and similar ecoregions.
“Shawnee” switchgrass was developed by a single cycle of selection for high IVDMD and high yield
from “Cave-in-Rock” (Vogel et al. 1996). It was higher in IVDMD than the parent cultivar with
similar biomass yield. Trailblazer and Shawnee are the only switchgrass cultivars developed with
improved forage quality.