Predicting the effects of land management practices and habitat disturbance on wild herbivore populations requires understanding how these activities influence the quality and quantity of forage, how forage metrics influence the amount of digestible energy and nutrients animals can acquire, and how ingested nutrients are converted to body stores and reproduction. Of these, quantifying links between forage characteristics and animal intake has remained the most challenging. Therefore, we evaluated the relationships between 8 potential forage metrics and daily digestible energy intake (DEI, kJ/day) and dietary digestible energy content (DDE, kJ/g) of tractable White-tailed, Black-tailed and Mule deer (Odocoileus spp.) foraging within 130 field sites spanning the diverse ecoregions of Washington. DEI of deer across study areas increased with all measures of forage biomass (total, acceptable, selected, suitable, and top species), but the best model for predicting DEI across all sites was suitable biomass (biomass of forage that meets minimum nutritional requirements), followed by top species biomass (biomass of forage that composes at least 1% of a deer’s diet in the region). DDE increased with suitable biomass and weighted DE, but decreased with increasing total biomass, acceptable biomass, and selected biomass. However, the explanatory power of top forage metrics for DEI and DDE ranged between only 19-32%. Our results suggest the best metrics for predicting energy that can be obtained by deer within a habitat require measuring plant biomass and either nutritional quality by taxa and plant part or diet composition of deer within the region and season of interest.
