Forage quality affects physiological condition, population dynamics, habitat use, and distribution of ungulates. We studied how rainfall, temperature, forage biomass, and forage growth are related to water content, crude protein (CP), and in vitro dry-matter digestibility (IVDMD) of some common forage species of desert mule deer (Odocoileus hemionus eremicus Mearns) in the Sonoran Desert, California. We established vegetation transects in desert washes to collect forage samples and to measure forage biomass, growth, rainfall, and temperature on a quarterly basis. Percent water and CP were positively associated with forage growth (P < 0.001) and with rainfall (P ≤ 0.025). There were positive relationships between IVDMD and forage growth (P < 0.001), forage biomass (P < 0.001), and the combination of temperature and rainfall (P < 0.001). These findings suggest that the highest quality landscapes for deer are those with rapidly growing forage where forage water, CP, and IVDMD are greatest. With the quantified relationships between rainfall, temperature, and forage characteristics presented here, the nutritional constituents for deer forage can be predicted.