Comparison of methods to examine diet of feral horses from non-invasively collected fecal samples

Feral horses (Equus ferus caballus) have become abundant on public lands in the American West, particularly over the past 10 yr. In areas where they are overabundant, there is risk of habitat degradation. Most previous studies on diet and habitat use of feral horses were conducted more than 20 yr ago; rangelands have changed considerably in that time, so it is useful to revisit horse diets. We conducted a study to examine the diet of feral horses using noninvasive methods and subjectively compare diet analysis techniques. We collected feral horse fecal samples from a sagebrush/pinyon-juniper ecosystem in Colorado in May, August, and October 2014. We analyzed 30 fecal samples from each collection session by both microhistology and plant DNA barcoding. Both microhistology and plant DNA barcoding results indicated horse diet consisted primarily of graminoids (78.5% and 68.8%, respectively, both of which are in greater proportion than availability based on ecological site descriptions); however, the two methods differed in species composition of grasses. Similar to other studies, microhistological analyses underestimated the proportion of forbs in the diet compared with plant DNA barcoding analyses, which showed a surprisingly high contribution of forbs to the diet compared with previous studies. Our results suggest plant DNA barcoding analyses have great potential, although both methods have inherent biases.