Biomass Yield Evaluation for Switchgrass Breeding: Seeded Swards vs. Transplanted Plots Yield Different Results

Switchgrass (Panicum virgatum L.) is undergoing intensive development as a biomass crop to support the production of energy from perennial grasslands. Plant breeding and selection represent a significant component of this development effort, requiring breeders to evaluate large numbers of genotypes and families for performance under real-world field conditions. Experimental unit size, shape, and interplant spacing can vary widely both within and across breeding programs, and there is some evidence that experimental units established from seedling transplants are poorly predictive of biomass yield in seeded swards. The purpose of this study was to determine the relationships among five types of experimental units, three based on transplanted seedlings (spaced plants, row plots, and simulated sward plots) and two based on seeded rows (bordered and unbordered sward plots). Nine switchgrass populations were evaluated with or without nitrogen fertilizer under all five plot types. Simulated sward plots had the lowest predictive value for biomass yield of sward plots. Spaced plants and row plots had greater predictive ability than simulated swards, but they accounted for less than 50% of the variability, suggesting that biomass measurements on these types of plots should not be considered as the same trait measured on seeded sward plots, especially considering the fact that the conditions of interplant competition are drastically different between these groups of plot types. Whereas biomass yield on a per-hectare basis would be an appropriate trait name for seeded swards, plot-specific terms such as “plant biomass” or “row biomass” are more appropriate for biomass measured on spaced plant plots or row plots. These terms would clearly indicate that measurements on seeded plots where there is intensive interplant competition are not the same trait as those made on plants without competition.