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2015 RESEARCHS witchgrass (Panicum virgatum L.) is a predominant component of the North American prairie. It is adapted to a wide range of landscapes and ecosystems in North America (Casler et al., 2007a). Its traditional uses include pasture or hay production, prairie renovation, and erosion control (Vogel, 2004). After the 1991 decision by the USDOE to develop switchgrass as a dedicated lignocellulosic feedstock for bioenergy production, there has been overwhelming motivation for research on this species to understand its agronomy and biology, as well as to develop genomics tools to improve its biomass yield potential and conversion efficiency. The species needs significant improvement in biomass yield potential for its competitive use as a bioenergy feedstock. Cultivar breeding to improve biomass yield, targeting bioenergy feedstock use, is underway at several universities and USDA Agriculture Research Centers.Conventional breeding to improve yield relies on capturing both additive and dominant genetic variation. Additive genetic variance, as well as part of the dominant genetic variance, can be captured through recurrent selection using phenotypic or genotypic selection on the basis of half-sib, full-sib, or other types of progeny performance. The dominant genetic variation is responsible for hybrid vigor or heterosis, which can be better exploited through hybrid cultivar development. Heterosis in plant vigor, ABSTRACT Biomass yield improvement of switchgrass (Panicum virgatum L.) is critical for its use as a viable bioenergy feedstock. Population improvement through accumulation of favorable genes and exploitation of heterosis are two ways to improve biomass yield. The objective of this study was to assess biomass yield heterosis in biparental crosses between genotypes of lowland switchgrass. Forty-four genotypic crosses between two predominant lowland populations 'Alamo' and 'Kanlow', along with nine crosses among genotypes of other lowland germplasm, were evaluated in Knoxville and Crossville, TN. The experiment was established in summer 2014 using a randomized complete block design with three replications. Each plot within each replicate was a single row of 10 plants planted on 30-cm spacing, and rows were spaced 90 cm apart. Parental clones of 22 crosses were evaluated only in Crossville sites. Biomass yield was recorded at the end of fall 2015 and 2016. Biparental crosses were different in biomass yield (p < 0.001), and significant genotype ´ environment interaction for biomass yield was observed. Nineteen crosses produced 18 to 53% higher biomass compared with the average of the Alamo and Kanlow population bulks. Average yields of a matured stand ranged from 11.16 to 23.3 t ha −1 . Four crosses demonstrated 25 to 41% midparent heterosis (MPH), and one cross showed 23% high-parent heterosis. Two of the crosses showed negative MPH. Heterosis in both directions suggests presence of both favorable and unfavorable complementary dominant genes in switchgrass. These findings could help improve cultivar breedi...

253 RESEARCHS witchgrass is a C4 perennial grass native to North American prairie lands (Newell and Eberhart, 1961). Like most grasses, switchgrass is produced in a sward, which is a densely planted expanse of grass, rather than in rows or as single plants. In switchgrass, acceptable stand density in a sward is ~0.3 m between plants (Launchbaugh and Owensby, 1970;Schmer et al., 2006;Vogel, 1987;Vogel and Mitchell, 2008;Weaver, 1968). For breeding purposes, switchgrass evaluations are usually performed on single plants at a lower planting density, allowing a breeder to walk through the nursery and evaluate each individual plant. The discrepancy in evaluation and selection conditions vs. production conditions may lead to less effective or ineffective selection because of differences in plant performance under varying degrees of competition. Studies in a large number of grass species have examined some of the issues arising from these disparate evaluation and production conditions. Differences between spaced and sward evaluations have been noted in many grass species that are produced under sward conditions. Waldron et al. (2008) found that in an evaluation of 22 tall fescue (Festuca arundinacea Schreb.) half-sib families, values of ABSTRACT Switchgrass (Panicum virgatum L.) is an important emerging biofuel crop. In breeding nurseries, plants are typically widely spaced; however, production is in densely planted swards. This disconnect may hinder cultivar improvement. This study measured the efficiency of lowdensity, spaced-plant selection on improving biomass and ethanol yield in a high-density, simulated sward. Fifty-six full-sib families were clonally replicated in two adjacent nurseries in Knoxville, TN. The spaced-plant nursery consisted of single plants on 1-m centers. The simulated-sward nursery was created by planting four by seven plants on 0.33-m centers with 1-m alleys. In 2013 and 2014, biomass yield, ethanol yield, and morphological traits were evaluated. Trait means, correlations, and efficiency of indirect selection (E) were calculated. Significant interaction (p < 0.05) between year and nursery was observed for all traits except ethanol yield. The identified high-yielding ideotype differed between biomass and ethanol yield and between spaced-plant and simulated-sward nurseries. Selection under spaced-plant conditions for simulated-sward performance was efficient for ethanol yield (E = 0.96) but highly inefficient for biomass yield (E = −0.31). Several morphological traits evaluated under spaced-plant conditions were identified as efficient indirect selectors for simulated-sward biomass or ethanol yield. Results suggest selections for sward biomass yield may be more appropriate under sward-like conditions, but that spaced-plant nurseries are efficient for selection of ethanol yield performance under sward-like conditions and for indirect selection of sward yield traits using morphological traits.

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