New Look at Invasive Weeds: Leafy Spurge 

By Jeremiah Vardiman 

A recent University of Wyoming (UW) study indicates there is no synergistic effect between grazing and chemical leafy spurge control.  

Most leafy spurge research focuses on upland and arid systems. The study investigated control options for wet and seasonally flooded areas of the Yampa River in Colorado.  

Wyoming producers are too familiar with difficult-to-control leafy spurge. Leafy spurge has severely impacted rangeland ecosystems in the western U.S. It is an aggressive and invasive perennial species that produces from both seed and vegetatively, which allows it to spread rapidly, displacing native vegetation and establishing monoculture communities.  

Control is challenging with limited long-term results, especially since control options provide variable results and are limited in wet or seasonally flooded areas. Chemical control options are limited because the most effective chemicals cannot be applied near water, while the effectiveness of products that can be applied near the water is less known. 

  Establishment and impact of biological control agents on leafy spurge is variable. The local environment and conditions play a role in establishing these control agents, which takes time.  

Leafy spurge control 

A recent study from the UW Plant Sciences Department explored how targeted grazing, herbicide applications or combining these two control efforts affected leafy spurge seed production and cover within the flood plain of the Yampa Valley in Colorado.   

Half of the plots were grazed by sheep in the spring, while the other half were not. Then, late-season herbicide applications were applied two months after grazing to both grazed and ungrazed sites. Herbicide applications consisted of four different treatments, including the active ingredients quinclorac, aminopyralid, imazapic and aminopyralid plus florpyrauxifen-benzyl.  

Results 

Results indicated a one-time intensive grazing with sheep affects leafy spurge’s vigor by reducing seed production within the treatment year. However, the following year leafy spurge will respond to that stress by increasing seed production even though there was a decrease in leafy spurge cover.  

From a management perspective, the increase in seed production is counterproductive, even though there is positive reduction in cover.    

Opposite to grazing, herbicides did not have an impact on seed production within the treatment year because the late-season application occurs after seed maturity. There is a clear reduction in seed production one year after treatment.  

Alone, herbicide treatments do reduce leafy spurge populations, especially quinclorac or aminopyralid plus florpyrauxifen-benzyl. 

The study found no interaction between grazing and herbicide treatments. This is possibly due to the plants responding to the grazing stress by increasing aboveground biomass, which also pushes seed production. The study also found the native plant community was not affected by any of the treatments: grazing, herbicides or combination.  

Results indicated herbicide applications over multiple seasons is the most logical approach to managing leafy spurge along the Yampa River in Colorado. In particular, control of leafy spurge by targeted grazing is not economically effective, especially if the populations are small, not contiguous and where access is difficult. 

Management options 

As for control of leafy spurge in Wyoming, this study supports the literature recommendations for best control options. Utilizing rotational or continued season grazing within the same location provides the best results than a single grazing event.  

The only successful grazing control of leafy spurge was achieved by a continuous grazing system for four consecutive seasons, or utilizing multiple season applications of labeled herbicides. Even though there is no synergistic effect between grazing and herbicide, both control measurements should be incorporated when feasible.  

Leafy spurge is a very challenging weed to control and takes multiple years to gain significant progress. For management support of leafy spurge, contact your local weed and pest control district office or your county UW Extension office. 

Jeremiah Vardiman is a University of Wyoming Agriculture and Horticulture Extension educator. He can be reached at jvardima@uwyo.edu.