Harvest frequency and burning effects on vigor of native grasses

Burning and harvest frequency can affect the vigor of switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), and indiangrass [Sorghastrum nutans (L.) Nash]. A field study was established in 1986 and from 1988 to 1991 treatments were applied with burning in March, April, or May with unburned controls. Forage was harvested from plots 1 (June), 2 (June and July), or 3 (June, July, and August) times with unharvested control plots included. Treatments were applied to the same plots annually and were arranged in a split-split plot, randomized complete block design. The main plot was species, the subplot was burning, and the sub-subplot was harvest frequency. Big bluestem produced 147 and 122% more etiolated biomass in spring than did switchgrass or indiangrass, respectively. Effects of harvest management on plant vigor occurred after 1 growing-season, but changed little during the remainder of the study. Etiolated biomass declined more as harvest frequency increased from 2 to 3 harvests than from 1 to 2 harvests (213, 205, and 162 g m^-2 for 1, 2, and 3 harvests per summer, respectively). Big bluestem produced 95 and 33% more tillers than switchgrass and indiangrass, respectively, and burning stimulated tillering an average of 32% across all species and harvest treatments. Harvest frequency increased tiller density. However, plant vigor as measured by etiolated growth decreased as harvest frequency increased. This suggests that with these species tillering may occur at the expense of energy storage with frequent defoliation. Vigorous spring etiolated growth and high tillering potential may partially explain the dominance of big bluestem in the tallgrass prairie.