Fecal glucocorticoid metabolites associated with nutrition in American plains bison

American plains bison ( Bison bison ) take advantage of naturally-available nutrients, especially protein, by shifting grazing among cool-season grasses, forbs, legumes, and warm-season grasses as plant species transition from growing to dormant states across the year. Bison also transition to a lower metabolic rate and critical temperature in winter to conserve energy. Seasonal patterns in fecal glucocorticoid metabolite (FGM) concentrations in bison have previously been described, and this study sought to investigate the role of seasonal nutritional variation on FGM. Fecal samples (n=30) were randomly collected monthly for ≥ 1 year from five herds of female bison maintained on grassland pastures at four ranches for FGM analysis. Grass clippings also were collected monthly and sent to a commercial lab for pasture nutrient analysis. Subsets of bison from two herds were removed from pasture and provided with concentrated grain- and hay-based diets, and fecal samples (n=17-30) were collected weekly or monthly for three to seven months. FGM were measured using two commercially available assays: corticosterone (CJM006) and 11-oxoetiocholanolone (72T), and an adrenocorticotropic hormone challenge was administered to a single male bison. An increase in FGM following the challenge was detected by the 11-oxoetiocholanolone but not the corticosterone assay, yet monthly FGM concentrations measured by the two assays were correlated (p ≤ 0.02, r ≥ 0.7). Both assays demonstrated a seasonal pattern in FGM concentrations in bison on pasture, which was correlated (p ≤ 0.001) with protein (r ≥ 0.7), total digestible nutrients (r ≥ 0.7), and acid detergent fiber (r ≥ -0.7) across two years. Removing bison from pasture and transitioning to concentrated grain and hay diets eliminated seasonal patterns in FGM concentrations. Changes in FGM concentrations associated with dietary quality should be considered in studies seeking to investigate bison nutrition, health, and adrenal responses to other stimuli.