Noxious Weed Monitoring and Control on Rocky Mountain Bighorn Sheep Winter Range at Wigwam Flats


Within British Columbia, Rocky Mountain bighorn sheep are blue listed (vulnerable) because of predators, disease and habitat loss. In order to develop the quality of bighorn sheep winter ranges, an intensive noxious weed control and monitoring program was launched at Wigwam Flats in southeastern BC. The target species that was chosen for this research project was the spotted knapweed (Centaurea maculosa).

The aims of the project’s monitoring component include assessing knapweed-infested sites, selecting sites for future monitoring and installing monitoring plots. Further, both pre- and post-treatment vegetation species compositions were documented and compared in order to determine the efficiency of herbicide treatment.


Spotted knapweed are identified to be a major component of bighorn sheep diet throughout British Columbia. In winter, seeds are easily accessible and contain a high nutritive value. However, sheep rumen microbial populations are adversely impacted by the plant.

Due to expansion of weedy species on bighorn sheep winter ranges reducing available winter forage, a noxious weed management plan (2001) was written for the Mount Broadwood – Wigwam Flats area, which included starting a weed control program.

Study Area, Methods and Materials

The study area near Mt. Broadwood was split into three separate locations, which were chosen due to similar features of vegetation cover, landform, aspect, slope and elevation, which was between 1010 and 1050m above sea level. The areas were migration routes and winter/spring ranges for ungulates such as bighorn sheep, elk, mule deer and white-tailed deer.

Within the study area, plant species cover and frequency were described and permanent photopoints were established.

Between 2002 and 2006, study areas were sprayed with the herbicide Transline (active component: clopyralid), which was spread by a motor vehicle if accessible, or by backpack sprayer. The chosen herbicide operated selectively for knapweed; no other local species were harmed by Transline in this project.


In 2009, grass and shrub cover remained similar or decreased slightly compared to 2002. However, forb cover was between 7 and 23% lower, which was attributed to the significant reduction in spotted knapweed in that area. The Transline application has reduced the target species cover from between 8 and 10% in 2002 to nearly 1 to 3%.

However, the decrease of spotted knapweed enabled other weed species such as St. John’s-wort to fill the niche.

Even though herbicide effects were visible on several non-target plants, none of them was seriously damaged when Transline was applied at proper rates. Due to spray-overlaps, balsamroot and alfalfa still reacted sensitively to the raised application rates.

Discussion and Recommendations

The herbicide application has shown to be an effective treatment in controlling spotted knapweed populations without significantly damaging other species. As herbicide treatment is the only available short-term option to reduce weed species cover, it should be applied continuingly because spotted knapweed regrows from seedbanks in the soil.


To ensure further development of bighorn sheep winter ranges it is important to continue the treatment in order to enhance conditions for other feeding plants.

  • Source T.J. Ross. December 2009. Noxious weed monitoring and control on Rocky Mountain Bighorn sheep winter range at Wigwam flats. Prepared for the Ministry of Environment. Available at : [Accessed [03/2021].