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Wild Horse Island, Flathead Lake Montana


Location:  Northwest Montana


Public Wildlife Management Area


Wildhorse Island is renown for its iconic wildlife include wild horses and bighorn sheep.  The populations are protected in part by their island refuge which is managed for wildlife.  The largest bighorn sheep skull in the world was found on the island in the year 2016 kindling excitement for the conservation of bighorn sheep in this natural environment.


Annual grasses have begun to encroach on the south-facing grasslands used by bighorn sheep for forage leading to concern for habitat quality and parallel bighorn sheep fitness. The Rangeland Analysis Platform ( was used to estimate the cover of both perennial and annual grasses on the south-facing slopes and suggests that perennial grasses have declined from 50% to 40% since 1984 while annual grass have replaced them and increased from 15 to 20% during the same period.


Three application rates and three habitat types were selected to demonstrate the effectiveness of micronutrient fertilization in promoting the growth of perennial grasses. The sites selected were a disturbed site where the soil was previously tilled by homesteading, a south-facing natural grassland (Eagle Cove Site) and a ponderosa pine understory community dominated by rough fescue.


Demonstrate that innovative fertilizer-based treatments can be used to promote the growth of perennial grasses allowing them to outcompete annual invasive grasses.


Granular  350 pounds fertilizer/acre

Granular  Low Rate (175 pounds/acre), Medium Rate (350 pounds/acre), High Rate (525 pounds/acre) 


Granular fertilizer was applied using a hand broadcast spreader to 1600 square foot plots in September 2018.


Overall:  Perennial grasses have generally improved following treatment at Wild Horse Island and increased in cover on the treated plots.  The highest fertilizer rate had some negative effect on perennial vegetation (rough fescue) at the Ponderosa Site where treatments were applied to healthy grasslands with low annual grass cover.  Complicated vegetation trends were observed in the data set associated with variation in vegetation cover observed between years on the untreated control plots at the Disturbed and Ponderosa sites.  The best increases in perennial grass cover were associated with the highest rate at the Eagle Cove site and the lowest rate at the Disturbed and Ponderosa sites.


Commentary:  Field discussion occurred about what would constitute ‘success’ of the applied treatments.  No universally accepted threshold was obvious such as perennial grasses above a certain numeric target or as a percentage of the untreated control.  The three sites evaluated were all sufficiently unique the field team felt that it was better to think of them as different independent sites rather than a continuum of low, medium, and high functioning of similar sites.  The team felt like the best measure of success at a given site was the trajectory of vegetation condition either improved, worsened, or unaffected compared to the pretreatment condition.  Many of the plots were distinctly improved, several were worsened and comparisons to untreated control plots were complicated by seasonal variation in the vegetation cover of the untreated plots.  The Eagle Cove plots presented in this case history were the most clear-cut comparisons available during the second growing season (2020).  Apparent increases or appearance of non-native Poa pratensis (Kentucky bluegrass) to both treated and untreated plots at all sites may be an important future storyline since it was common in 2020 and rarely observed in 2019.  Three species of annual grasses were observed on these sites:  Bromus hordeaceous (soft brome), Bromus tectorum (Downy brome), and Bromus Japonicus (Japanese brome) in decreasing order of prevalence. 

Acknowledgments:  Thanks to Montana Fish Wildlife and Parks for their engagement in this project including providing the field site, assisting in plot installation, boat transport to the island, and vegetation monitoring support.  Thanks also to the Montana Chapter of the Wild Sheep Foundation for providing seed funding for this project and for wild sheep habitat advocacy.


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