Genetic improvement of biomass yield in upland switchgrass (Panicum virgatum L.) using secondary plant morphological traits

Switchgrass (Panicum virgatum L.) is currently undergoing intensive breeding efforts to improve biomass yield. Direct selection for biomass yield in switchgrass has proven difficult due to the many factors influencing biomass yield. In developing breeding schemes for increasing biomass yield, consideration must be made to the relative importance of spaced plantings to sward plots for evaluation and selection. It has previously been suggested that selection schemes using secondary plant morphological traits as selection criteria within spaced plantings may be an efficient method of making genetic gain. This research sought to identify secondary morphological traits in parental plants that are predictive of biomass yield in progeny swards, estimate heritability of secondary morphological traits and empirically test the effects of direct selection for secondary morphological traits on biomass yield. Limited predictive ability was observed for sward biomass yield using individual and combinations of plant morphological traits. A comparison of models using a Bayesian model averaging approach revealed common traits among the best predictive models including plant height, single-plant dry biomass, and second leaf width. Predictions of single-plant biomass, using the same set of morphological traits, revealed a large effect for tillering related traits. Moderate heritability was estimated for plant height and was greater for selection of increased height. Heritability for tiller count was low overall, with greater values observed for reduced tillering selections. Flowering date was estimated to have high heritability overall in both selection directions. Divergently selected populations for each trait were developed from the WS4U upland tetraploid germplasm and evaluated for biomass yield at five locations in Wisconsin during two growing seasons. Significant variation was observed between maternal parents of the selected populations for both selected and non-selected traits. Despite substantial differences between parent plant populations for plant morphology, significant differences were not observed for sward-plot biomass yield or sward-plot morphology relative to the base population. Results of this research demonstrate the challenges of selecting for increased biomass yield in switchgrass within spaced-plant nurseries. Based on these results it is recommended that greater emphasis be placed on evaluation biomass yield within sward plots for improving biomass yield.