Wild horses and cattle have the greatest potential for grazing conflicts across the western rangelands of the US, a major review has found.
The systematic review carried out at the University of Wyoming took more than a year. It looked at the findings of 33 largely unrelated studies on grazing habits going as far back as the 1970s.
The work was led by Derek Scasta, an assistant professor and rangeland specialist within the Department of Ecosystem Science and Management at the university.
Scasta says he hopes the findings of the review will help inform the debate around the management of grazing species on the western rangelands to promote the well-being of the animals and the ecosystem.
The review drills down into what types of plants are eaten by which animals, and even identifies seasonal variations in diet.
Scasta has published the findings online, with work now progressing on a paper that covers more potential variables and more extensive statistics. It will be targeted at a rangeland journal.
He says the issue of livestock and wildlife interaction on the rangeland concerns not only ranchers and wildlife managers, but the general public.
There is, he says, public interest in the ecological impact and well-being of wild horses and other species on the rangelands, centering on competition for food resources.
His review provides a summary of data from western North America that compares the diet composition of wild horses, beef cattle, domestic sheep, elk, pronghorn, and mule deer across spring, summer, fall, and winter.
Scasta found that the rangeland diet of wild horses comprised mostly grasses, at 82 percent, followed by forbs (herbaceous flowering plants) at 10 percent and shrubs at 8 percent.
By comparison, the diet of beef cattle comprises 74 percent grass, 14 percent shrubs and 12 percent forbs.
This represented the greatest potential conflict between species he identified in the review of the studies, all of which were based on the analysis of collected dung samples to determine the diet breakdown of each species.
Even the seasonal variations in diet identified in the review showed similar trends between wild horses and beef cattle.
Scasta says the management of wild horses on the rangeland is a major issue.
US Bureau of Land Management figures from last November reveal an estimated 41,000 wild horses were free on the range, well above the mandated appropriate management level across all herd areas of 28,000. Another 47,000 wild horses are being held in long-term or short-term grazing facilities.
He said his aim was to provide unbiased information from the scientific literature on the eating habits of the species in his review, saying management decisions needed to be based on solid data.
However, Scasta says the equation is not altogether simple, with dietary selection and seasonal variations forming only part of the picture.
For example, the review did not look look at the physical overlap of habitat use by species on the rangeland and the competition this may create, nor the distances covered by some animals. The range, he said, was so big, and the species so different.
Nor did it take into account potential increases in conflict and behavioral changes arising from the likes of drought. He notes, for example, that feral horses in Australia have been shown to wander up to 55 kilometres from water sources. Research had shown that some only drank every fourth day.
Water and its availability have been shown to increase habitat-use conflicts around water sources between wild horses, cattle and pronghorn antelope, he says.
He believes seasonal variations in diet identified in his review were a result of feed availability, feed quality, and biological preference.
Surprisingly, horses on average did not appear to shift their diet much, but there were exceptions.
Dietary changes for horses tended to be most pronounced in the winter, as they were for cattle, when both species increased the use of shrubs during severe winters. Other seasonal changes may result, for example, when nutritious seed pods become available on some plants in the southwestern US.
Agencies had to make decisions on rangeland management that affected all species, including the seasonal management of beef cattle on allotments.
“Ultimately, society has to make the decision on where they are going to place their value,” he says. The report presents the data and others must decide how it is applied.
The Government has been trying to manage several potentially competing rangeland interests and conflicts, and that process has not been helped by the wide variety and types of studies and data, Scasta says.
“My goal was to compile the wide variety of studies into a practical format. It is not an opinion; not what I think. [The information] is summarized in a meaningful way.”
He says the project began in January 2014 during discussions with Dr Jeff Beck, an associate professor in wildlife habitat restoration ecology at the University of Wyoming, giving rise to the current report, with work now continuing on the scientific paper.
The pulling together of 33 papers, some dating back to the 1970 and 1980s, was a major undertaking and one which he admits got tedious at times. He said he was surprised that studies in the field went back so far, saying some of the early work was very good.
The initial thought had been to undertake a diet study, but in the end Scasta said the review of existing literature showed that many of those types of studies already existed.
For now, many questions remain about how to move the research forward beyond simply what the animals eat. What, for example, are the potential conflict possibilities that arise among grazing animals should there be more droughts and higher temperatures?
There were issues, too, in counting animals, especially horses, with their greater capacity to move distances, often across state lines.
Scasta says that no matter what opinions were held on wild horses and other species, it was important that policies and decisions were guided by good data.
The review, titled “Dietary composition and conflicts of livestock and wildlife on rangeland”, can be read here.