Burning and defoliation effects on the vigor and productivity of three warm season grasses

Burning and harvest frequency affect the vigor and productivity of warm-season grasses. This study was initiated on 2-year old monoculture stands of switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), and indiangrass (Sorghastrum nutans (L.) Nash) near Mead, Nebraska. Monocultures were burned annually in mid-March, mid-April, or mid-May or left unburned. These grasses then either were not harvested or harvested one, two, or three times each summer. An end-of-season biomass harvest was taken from each plot after plants were dormant. The same burning and harvest treatments were applied to the same plots annually. Big bluestem produced more etiolated biomass during spring in all 3 years (375, 333, and 322 g/m$\sp2$) than switchgrass (135, 122, and 164 g/m$\sp2$) or indiangrass (138, 114, and 217 g/m$\sp2$), partially explaining its dominance in the U.S. tallgrass regions. Grasses that were not harvested until the end of the growing season produced the most etiolated biomass each year, one and two harvests intermediate, and three harvests least (274, 213, 205, and 162 g/m$\sp2$, respectively, averaged over the 3 years). Grasses that were not harvested or harvested one, two, or three times during summer produced 4970, 3270, 2800, and 2120 kg/ha of total biomass, respectively. Unburned plots produced the most summer yield and total biomass for all species and productivity declined as burning occurred later in spring. Plant vigor (etiolated biomass) and stand biomass were affected after only one year of treatment. Etiolated biomass production then remained constant through 2 more years of treatment, but summer yields declined over the study with increasing harvest frequency. As a result, 1991 summer yields were 105, 92, and 72% of 1989 summer yields for the one, two, and three harvest treatments, indicating that warm-season grasses need rest to maintain high levels of productivity