Free-Roaming Horse Diet and Body Condition Differences Across Seasons and Ecologically Diverse Herd Management Areas

Increasing populations of free-roaming horses (Equus caballus) residing on federal lands pose management challenges across the American West, affecting rangeland health and co-occurring wildlife and livestock species. To better understand how free-roaming horses interact with rangeland ecosystems through herbivory, we used amplicon sequencing (P6 loop of chloroplast trnL) of horse fecal material to quantify plant composition of diets across a gradient of herbaceous availability in 16 Herd Management Areas (HMAs) managed by the Bureau of Land Management. These HMAs encompassed several ecosystems, including the Colorado Plateau, Great Basin, Mojave Desert, and Wyoming Basin. We collected 1,409 visual body condition scores (BCS) (on a 1-to-9 scale) and 465 individual fecal collections in summer 2020 and winter 2020/2021 across HMAs. Because horses are considered grazers, we explored whether the dietary proportion of graminoids (i.e., grasses and grass-like plants) changed seasonally between and among HMAs. The proportion of graminoids in fecal material differed by HMA and ranged from 31.17% to 83.50% in summer and 11.00% to 82.60% in winter. Summer diets trended toward higher graminoid composition in most HMAs, and many winter diets shifted to include nongraminoid plants in the Asteraceae and Chenopodiaceae families. Despite varying dietary graminoid composition, herd average BCS indicated most free-roaming horses were in good condition. Across HMAs, herd average BCS in summer averaged 5.01 (minimum 4.59 and maximum 5.24) and averaged 4.98 in winter (minimum 4.72 and maximum 5.22). Understanding which plant groups form seasonal diets of free-roaming horses across different environments is important for managers balancing potential forage competition among free-roaming horses, wildlife, and livestock. Our results indicate that while free-roaming horses are considered grazers, they are also capable of subsisting and maintaining good body condition while consuming a variety of plants, with graminoids not always forming the majority of the diet.