Bitterbrush and cheatgrass quality on 3 southwest Idaho winter ranges

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.