Prescribed Fire, Herbicide, and Whitebrush

Whitebrush occurs naturally throughout the southwestern United States and northern Mexico and is one of many woody plants associated with degradation of grasslands. We employed prescribed burning and herbicide application at sites on two different Brewster County ranches in order to investigate the efficacy of these methods for whitebrush control.

Whitebrush top-killed by prescribed fire.

We monitored whitebrush density and canopy cover on four transect sites burned in February of 2007 and on two unburned control sites. Additionally, three concentrations of each of two different herbicides – GrazonNext™ (aminopyralid+2,4-D) and Spike™ 20P (tebuthiuron) – were applied to whitebrush one year after burning and to unburned whitebrush.

Neither density nor canopy cover appeared to be reduced by burning alone and one year later canopy cover actually increased. The exception to this trend occurred at one prescribed burn site where a crown fire had occurred during the prescribed burn resulting in a reduction of canopy cover one-year post burn.

Burning did appear to increase the susceptibility of whitebrush to GrazonNext™, which resulted in 98-100% mortality at all three concentrations (0.5%, 1%, and 2%). For unburned whitebrush, near total mortality was achieved only at the 2% concentration. Complete mortality was achieved for burned and unburned whitebrush for all three formulations of Spike™ (7 grams, 15 grams, and 21 grams per plant).

Our results indicate that high intensity fire may be effective for top-killing whitebrush, but does not result in mortality. Spike™ continues to be an effect herbicide for whitebrush control, but can be costly (approximately $110/acre in herbicide cost).

Prescribed burning (approximate cost of $15.00/acre) followed by application of GrazonNext™ at 0.5% concentration (approximately $4.00/acre in herbicide cost) may be a cost effective alternative to mechanical removal or other herbicide treatments.