Spring defoliation effects on bluebunch wheatgrass: II. Basal area

Abstract

Spring livestock grazing has been suggested as a tool to improve winter forage quality of bluebunch wheatgrass (Agropyron spicatum [Pursh] Scribn. & Smith). Impacts on plant vigor and survival are important concerns associated with spring grazing. We report basal area change and mortality responses of bluebunch wheatgrass to 3 spring, 1 winter, and 3 spring + winter defoliation treatments. The study was conducted in l993 and 1994 at 2 sites in the Blue Mountains of northeastern Oregon. Basal area of individual plants was measured shortly after application of the spring treatments and again approximately 1 year after treatment. Clipping the entire basal area of bluebunch wheatgrass plants to a 7.6-cm stubble height during the mid-boot phenological stage and during the inflorescence emergence stage produced 7.0 and 7.8% declines in live basal area, respectively. Unclipped control plants and plants having only one-half their basal area clipped to a 7.6-cm stubble height during the mid-boot stage exhibited 5.2 and 18.6% increases in live basal area, respectively. Combining the mid-boot/half-plant treatment with an early winter clipping to a 2.5-cm stubble height reduced the positive live basal area response to 6.0%. No additional declines in live basal area relative to the spring-only treatments were detected for combinations of the early winter treatment with the mid-boot/whole plant treatment and the inflorescence emergence treatment. Experiment-wide plant mortality was only 0.2%. If managed for a moderate level of defoliation where a portion of the basal and of each bunchgrass plant is left undefoliated, livestock grazing during the boot stage should have little negative impact on the vigor and survival of bluebunch wheatgrass under environmental conditions similar to northeastern Oregon.