Biomass yield has been widely accepted as the major trait for improvement in breeding switchgrass as a bioenergy crop since the beginning of the US Department of Energy sponsored Bioenergy Feedstock Development Program (McLaughlin and Kszos 2005). Currently more than 10 switchgrass breeding programs have breeding efforts mainly focusing on improving biomass yield as the principal trait and developing superior cultivars with improvement in biomass yield and other selected traits (Casler et al. 2011). Biomass yield is considered to be the most important factor contributing to the development of an economically viable biofuel industry if switchgrass is selected as the major feedstock crop. Genetically improving biomass yield in switchgrass through the delivery of higher-yield cultivars will increase the profit margins of producers, reduce the cost per unit biomass yield, and decrease transport delivery distance of feedstock from farmer’s fields to a target biorefinery, consequently benefitting the whole chain of biofuel production. It is recommended for one-cut system by the end of a growing season to maximize yields or after the first frost to allow a longer harvest window and to increase retranslocation of nutrients to root systems or soil (Sanderson et al. 1999; McLaughlin and Kszos 2005; Makaju et al. 2012).

Biomass yield is a highly complex trait inherited in a quantitative manner and regulated by a large number of unknown genes, and heavily affected by environmental factors and genotype by environment interaction. Selection for biomass component traits may be feasible to indirectly improve biomass yield. Biomass yield is positively correlated with plant height with a significant coefficient of 0.69 and negatively correlated with plant maturity (r= -0.45) (Talbert et al. 1983). Lemus et al. (2002) also reported a high correlation between biomass yield and plant height. Using 11 lowland populations tested in two locations, Das et al. (2004) reported biomass yield was positively correlated with tiller number per plant with a coefficient of r=0.60 to 0.68, but not with plant height. The latter study suggested selection for more tillers per plant would be the most effective method for indirectly increasing biomass yield. Leaf blade length and width, stem diameter and node number per tiller have effects on biomass yield but are not consistent across locations (Das et al. 2004). Bhandari et al. (2010) reported positive correlations between biomass yield and tillering ability (r=0.73), and plant height (0.52), and stem thickness (0.38). Boe (2007) and Boe and Beck (2008) reported significant correlations between biomass yield and tiller weight, suggesting tiller weight can be used as a trait in indirect selection for biomass yield improvement. Overall, it is promising that use of tiller number, plant height and tiller diameter size as indirect selection traits for switchgrass biomass improvement.

In addition to biomass yield, many other traits, individually or in combinational forms have been included in breeding and selection activities. Seed dormancy is a long existing issue to the successful establishment of new stands using seed in switchgrass. Efforts to select for low post harvest seed dormancy have been made and led to a substantial increase in germination rates in ‘Alamo’ germplasm and a release of ‘TEM-LoDorm’ germplasm (Burson et al. 2009). Adaptation to target environments, cell — wall recalcitrance, abiotic stress (drought, cold) tolerance, and biotic stress resistance are some additional important traits in switchgrass breeding and selection process (Casler et al. 2011). Drought tolerance is critical as switchgrass is targeted to grow on marginal lands without supplementary irrigation. Leaf rust caused by Puccinia emaculata and smut by Tilletia maclagani are important diseases for switchgrass if grown in large areas (Parrish and Fike 2005).

Several experiments were conducted to estimate heritabilities for biomass yield and related traits, and forage quality traits. Forage use of switchgrass has taken place much before the identification of the species’ potential use as a bioenergy crop (Vogel 2004). One important trait for forage use is to improve forage quality, including dry matter digestibility. Godshalk et al. (1986) and Hopkins et al. (1993) reported moderate to medium narrow- sense heritability for in vitro dry matter digestibility (IVDMD). Phenotypic recurrent selection was effective in improving populations for increased IVDMD, which led to the development and release of ‘Trailblazer’ (Hopkins et al. 1993) and ‘Performer’ (Burns et al. 2008a). Among the traits related to biomass yield, heritabilities for days to heading and days to flowering are relatively high (Bhandari et al. 2010). For plant height, Talbert et al. (1983) reported high narrow-sense heritability, while estimates were low based on variance component analysis of half-sib families, and were medium based on parent-half-sib progeny regression by Bhandari et al. (2010). Similarly, narrow-sense heritability estimates for stem thickness, tillering ability, plant spread and spring regrowth are inconsistent due to differences in genetic structures (half-sib versus full — sib families) and data sources (individual plant data versus plot mean) in the experiments.

Being the major trait as targeted for developing the species as a bioenergy feedstock crop, biomass yield has narrow-sense heritability estimates ranging from 0.12 to 0.25 (Talbert et al. 1983; Godshalk et al. 1986; Rose et al. 2008; Bhandari et al. 2010 and 2011). The low values were estimated on the basis of variance components from data of individual plants among families. But in some of the same experiments, higher heritabilities were also obtained on plot yield mean data, suggesting heritability estimates from midparent-progeny regression tend to be biased upwards (Bhandari et al.

2011) . Collectively, the tested low heritability for biomass yield indicates that direct phenotypic selection for increased biomass yield may not be effective. Breeding and selection procedures capturing additive variation and increasing frequency of additive genes responsible for the trait need rigorous progeny evaluation in the improvement of breeding populations. Improved populations can serve as germplasm pools for making synthetics and even be released as cultivars per se.