Effects of tebuthiuron and imazapic on sagebrush and herbaceous understory

Background/Question/Methods

Herbicide application is frequently used to control undesired plants in arid and semi-arid ecosystems.  However, the detailed responses of various types of non-target vegetation to herbicide application are not thoroughly understood, and the possible additive effects of using multiple herbicides are relatively unexplored.  The Sagebrush Steppe Treatment Evaluation Project (SageSTEP) is a regional experiment that evaluates the response of sagebrush and woodland ecosystems to different management techniques, including the use of tebuthiuron and imazapic.  Results from this subset of SageSTEP treatments are presented here.  Tebuthiuron was applied at 1.5 pounds per acre to thin sagebrush (Artemisia tridentata ssp. wyomingensis), while imazapic was applied at 5-6 ounces per acre to control invasive annual species, especially cheatgrass (Bromus tectorum).  Cover determined from line-point intercept measurements was monitored for three years following treatment for sagebrush, cheatgrass, Sandberg’s bluegrass (Poa secunda), all other perennial grasses, and perennial forbs.

Results/Conclusions

Compared to untreated plots, tebuthiuron significantly reduced sagebrush cover, but only in the second (20%, p<0.01) and third (37%, p<0.01) years after treatment.  Imazapic did not significantly affect sagebrush cover.  During the first year post-treatment, cheatgrass cover declined to nearly zero under any combination of tebuthiuron and imazapic, with values of 0 to 5%.   In contrast, cheatgrass cover persisted in areas receiving neither herbicide (10% versus 9% pre-treatment).  This interaction was highly significant (p=0.01), but disappeared during the second and third years post-treatment, when previous imazapic application alone explained the significantly lower values for cheatgrass cover (2% with versus 11% without imazapic in the second year, p<0.01; 5% with versus 12% without imazapic in the third year, p<0.01).  Similarly, Sandberg’s bluegrass cover was significantly lower with than without imazapic (31% to 45%, p<0.01).  Imazapic had no effect on other perennial grasses as a group, but under tebuthiuron, this group had 19% (p=0.01) more cover three years after treatment than did areas not receiving tebuthiuron.  Perennial forbs were not significantly affected by either herbicide treatment.