Root and shoot responses of sand bluestem to defoliation

Abstract

Knowledge of root response, as well as shoot response, to defoliation is needed to manage grasslands in environments where water and/or nutrients are limiting. The objective of this study was to document the response of sand bluestem (Andropogon hallii Hack.) roots and shoots to different times and frequencies of defoliation. Individual sand bluestem plants were grown in 15 X 100-cm polyvinyl chloride (PVC) containers which were placed in the plants' natural setting. Twelve plants (replications) were clipped to a 7-cm stubble height during mid-month for each of the following defoliation schedules: 1) June, July, and August; 2) June and August; 3) June; 4) July; 5) August; and 6) October. The October defoliation, after shoot senescence, served as the control. Multiple defoliations reduced (P < 0.05) root weight, root area, root length, and weight of total nonstructural carbohydrates (TNC) in roots by an average of 33, 42, 43, and 34%, respectively, compared to control plants. A single defoliation in June only reduced root weight, root area, root length, and weight of TNC in roots by 14, 19, 16, and 13%, respectively, compared to control plants. Defoliating plants during the growing season did not affect (P > 0.05) number of tillers, weight per tiller, above-ground weight, number of buds, weight of rhizomes, or weight of TNC in rhizomes. Grazing sand bluestem more than once during the growing season may reduce root growth and diminish its ability to compete for water and nutrients. Grazing during the dormant season or once during the early part of the growing season should be least detrimental to sand bluestem.