Free-roaming horse study reveals diverse diets through genetic testing

A groundbreaking study on America's free-roaming horses demonstrates how genetic tools provide an effective and noninvasive way to understand diets in herbivores, offering new insights for conservation and management. A valuable academic partnership, led by University of Wyoming's Jeff Beck and Courtney Buchanan with Boise State University's Jennifer Forbey and Stephanie Galla, the collective research identifies horses as adaptable feeders, capable of thriving on diverse plant species.

This research was part of a $6 million award funded by the National Science Foundation's Established Program to Stimulate Competitive Research (EPSCoR).

Entitled Genomes Underlying Toxin Tolerance (GUTT), the projects explore diversity of toxin tolerances in vertebrate herbivores while building research and education strengths across Idaho, Nevada, Wyoming and Alaska.

Forbey was the principal investigator with co-principal investigators at College of Idaho, Caldwell; University of Nevada, Reno; University Wyoming, Laramie; and University of Alaska, Fairbanks.

Free-roaming horse populations are thriving and can increase in size by 20% each year.
(photo courtesy of Courtney Buchanan)

Background: America's free-roaming horses

Free-roaming horses are often seen as symbols of the American West: majestic creatures that mirror the spirit, strength and resilience of a vast and untamed frontier. However, America's horses are not a native species; they are descendants of domesticated horses brought to North America by colonists starting in the 1500s. For centuries, they've inhabited the West, increasing in number and adapting to changing environments.

Now numbering 73,130, America's horse population is largely protected and monitored by the Bureau of Land Management (BLM) - though the National Forest Service (NFS) coordinates with BLM to manage herds on NFS lands. Under The Wild Free-Roaming Horses and Burros Act of 1971, these agencies are responsible for maintaining the health of most herds and the ecosystems they share with native species and livestock.

Key findings: not just surviving, but thriving

Contrary to the common belief that horse diet consists largely of grasses, this study reveals many herds consumed substantial amounts of shrubs and forbs - nonwoody, flowering plants. Diets varied by season and location, but the majority include those in the plant family Asteraceae (the sunflower family, which includes sagebrush) and Chenopodiaceae (the goosefoot family, including winterfat).

Between summer and winter samples, nongrass vegetation sometimes comprised nearly 40% to 50% of diets, indicating these horses have adaptable digestive systems capable of processing a wide range of forage.

Combined with visual observations during both seasons, the data indicates these animals aren't just surviving - they're thriving. Moreover, this research demonstrates horses interact with a much wider network of plant species than previously understood.

Immersive fieldwork

In May 2020, graduate student Courtney Buchanan set out on the first series of expeditions across seven states and 16 herd management areas (HMAs). Guided by University of Wyoming ecology professor Beck and mentored by Boise State biologists Forbey and Galla, Buchanan's largely solo fieldwork took her across hundreds of miles of rugged countryside.

Armed with binoculars and occasionally relying on advice from BLM staff, Buchanan tracked herds by truck and on foot, often hiking many miles or waiting at watering holes to find the wide-ranging creatures.

Upon sighting horses, Buchanan operated at a safe distance, collecting fecal samples and visually examining animal body conditioning using the Henneke Body Conditioning Scoring System. Site expeditions lasted one to three days.

Courtney Buchanan conducting fieldwork during winter 2020-21.
(photo courtesy of Courtney Buchanan)

In the winter months, Buchanan repeated the process, securing a second data set to compare diets during times of food abundance and scarcity. Despite the extraordinary circumstances of pandemic fieldwork, Buchanan's research offered her a uniquely rewarding academic experience.

"While a lot of people were saying, 'I spent months not being able to leave my house,' I could say, 'I drove all across the country,'" Buchanan said. "I loved getting to work in a variety of landscapes from the Mojave Desert to the Wyoming Basin. Not only did I get to spend time studying the horses which was enjoyable in itself, but I also was able to observe native wildlife species and explore interesting environments."

Innovative lab work

Back at the University of Wyoming Genome Technologies Lab, Buchanan preserved, processed and sequenced fecal samples with the aid of University of Colorado. Then, mentor and fellow researcher Galla assisted with analysis.

Galla, then a postdoctoral researcher at Boise State, analyzed the genetic data using bioinformatics, an interdisciplinary approach combining science, computer programming and mathematics to identify what the horses were eating during those winter and summer periods.

Courtney Buchanan coring fecal sample at the University of Wyoming.
(photo courtesy of Courtney Buchanan)

While dietary analysis traditionally employed a method called microhistology, the team used metabarcoding, a genetic sequencing procedure that breaks down a complex sample and identifies the individual DNA markers present in the fecal samples. This allows researchers to identify fully digested, otherwise unobservable, plant matter using their DNA.

GUTT's impact: cultivating collaboration and capacity

GUTT gave Buchanan and Galla outstanding research opportunities and a supportive community during their professional journeys. This is evident through other aligned research, including studies on other species such as pronghorn.

Buchanan has since completed an internship with the United States Forest Service and earned her doctoral degree in Rangeland Ecology and Watershed Management from the University of Wyoming. She is currently teaching at a community college.

Galla is now a tenure-track assistant professor of avian biology at Boise State. Her research focuses on conservation genomics work with native birds including Columbian sharp-tailed grouse, gyrfalcons and golden eagles. The GUTT initiative also helped Galla launch Boise State's Conservation Genetics Lab, a center for genetic and genomic research that enables studies to assist in conservation efforts for primarily avian species.

"I wouldn't be where I am today without that EPSCoR grant, which, along with Jen [Forbey's] visionary leadership, allowed us to build new capacity in dietary metabarcoding," Galla said.

This publication was made possible by the NSF Track 2 EPSCoR Program under award number OIA-1826801.

Research was conducted with support from the Bureau of Land Management, Little Book Cliffs Friends of the Mustangs and Pine Nut Wild Horse Advocates.

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