Abstract
Cheatgrass (Bromus tectorum L.) is one of the most problematic weeds in western United States rangelands and sagebrush steppe. It responds positively to different forms of disturbance, and its management has proven difficult. Herbicide or targeted grazing alone often fail to provide adequate long-term control. Integrating both may afford better control by providing multiple stressors to the weed. We assessed herbicide application, targeted sheep grazing and integrated herbicide and grazing on B. tectorum and the plant community in rangeland in southwestern Montana from 2015 until 2017. Herbicide treatments included spring-applied (May 2015 and 2016) glyphosate, fall-applied (October 2015) glyphosate, imazapic and rimsulfuron, and spring-applied glyphosate plus fall-applied imazapic. Targeted grazing, consisting of four sheep/0.01 ha for a day in 5 m × 20 m plots (all vegetation removed to the ground surface), occurred twice (May 2015 and 2016). While no treatments reduced B. tectorum biomass or seed production, grazing integrated with fall-applied imazapic or rimsulfuron reduced B. tectorum cover from approximately 26% to 14% in 2016 and from 33% to 16% in 2017, compared to ungrazed control plots, and by an even greater amount compared to these herbicides applied without grazing. By 2017, all treatments except spring-applied glyphosate increased total plant cover (excluding B. tectorum) by 8%–12% compared to the control plots, and forbs were generally responsible for this increase. Bromus tectorum management is difficult and our results point to a potential management paradox: Integrating grazing and fall-applied herbicide decreased B. tectorum cover but did not increase native grass cover, while some herbicides without grazing increased native grass cover, but failed to control B. tectorum. Additional research is necessary to determine grazing strategies that will complement herbicide control of B. tectorum while also stimulating native grass recovery, but this initial study demonstrates the potential of integrated management of B. tectorum compared to grazing or herbicide alone.
Highlights
Disturbance alters plant communities by removing existing vegetation or changing abiotic conditions, facilitating plant invasions [1,2]
(11.2 g) plots treated with fall imazapic compared to ungrazed (18.5 g) fall imazapic plots (Table 4, Figure 3). These results indicate that integrated management of B. tectorum with herbicide and grazing could provide a level of control greater than herbicide or grazing alone
Two combinations of integrated grazing and herbicide, grazing with fall-applied imazapic or rimsulfuron, provided better results than herbicide alone; there is only weak support for our hypothesis that integrated management would be superior to individual treatment methods
Summary
Disturbance alters plant communities by removing existing vegetation or changing abiotic conditions, facilitating plant invasions [1,2]. The most common method to alter the trajectory of plant communities and remove undesired weeds is via herbicide application [5,6,7]. Herbicides can harm desired plants growing in the same community, especially closely related plants with the same life form as the weeds, such as rangeland grasses [8]. The herbicide imazapic can control some weedy annual grass species with minimal impact to desired grass species [9,10]. Another approach is timing herbicide application to take advantage of different phenological stages of the weedy and desired species; ideally herbicides are applied when the desired species are dormant reducing off-target damage. Managing weed populations to restore desired species is extremely difficult, and many populations persist despite repeated control efforts [12]
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