Nutritional stress is an important mortality factor for wintering mule deer (Odocoileus hemionus hemionus Rafinesque), particularly fawns. The rate at which fawns utilize existing fat stores is at least partially dependent upon the quality of available forage during winter. Although numerous studies have determined the nutritive value of various forage species, more research is needed to determine whether individual forage species vary in quality across the landscape. We determined whether differences existed in the nutritional quality of antelope bitterbrush (Purshia tridentata [Pursh] DC.) and cheatgrass brome (Bromus tectorum L.) among 3 winter ranges and 6 habitats within the winter ranges. In vitro dry matter digestibility (IVDMD) of bitterbrush varied among winter ranges in 1996 and 1997 (P < 0.001). The highest mean IVDMD measured on a winter range was 29.8% (n = 36, SD = 3.87) in 1997 while the lowest was 15.2% (n = 38, SD = 4.42) in 1996. Bitterbrush crude protein (CP) was different among habitats in 1997 (P = 0.005), with mean CP values ranging from 7.0% (n = 19, SD = 0.73) to 8.0% (n = 13, SD = 0.70). The length and diameter of available bitterbrush leaders varied within and among winter ranges because of differential utilization. Bitterbrush IVDMD and CP varied in relation to the mean diameter of leaders obtained from each random sampling site (P < 0.001). The quality of bitterbrush decreased as browse intensity increased. Cheatgrass IVDMD was different between winter ranges (P < 0.001) in 1996, with mean values ranging from 65.8% (n = 36, SD = 4.34) to 69.6% (n = 36, SD = 3.83). Site-specific variation should be considered when evaluating the nutritional quality of mule deer habitat, at least during winter when species diversity in deer diets is limited. Key Words: nutritional quality, mule deer, in vitro dry matter digestibility, crude protein, Purshia tridentata, Bromus tectorumWinter malnutrition is a common cause of mortality for mule deer (Odocoileus hemionus hemionus Rafinesque) fawns, particularly during severe winters. Nutritionally stressed fawns are also more susceptible to other proximal causes of mortality. Since mule deer largely depend on pre-winter fat stores to meet energy requirements during winter, over-winter fawn survival is often determined by the duration and severity of winter (Wallmo et al. 1977, Torbit et al. 1985. Nutritional stress arises from a lack of necessary quality in forage to sustain mule deer through winter (Wallmo et al. 1977). Although nutritional maintenance requirements are rarely met, differences in the nutritive value of the forage may be very important. Deer selecting higher quality diets should deplete existing fat stores more slowly, thereby increasing the probability of survival. ResumenEl estrés nutricional es un factor importante de mortalidad invernal en los venados (Odocoileus hemionus hemionus Rafinesque), particularmente para los cervatos. La tasa a la cual los cervatos utilizan las reservas existentes de grasa es parcialmente dependiente ...
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