Guajillo (Acacia berlandieri Benth.) is considered a medium- to high-quality forage for both wild and domestic ruminants. However, studies have shown that guajillo contains phenolic amines and alkaloids, and condensed tannins, which may cause toxicosis and reduced fertility, intake, and nutrient digestibility. To examine the nutritional value of guajillo to white-tailed deer (Odocoileus virginianus Zimmermann) more thoroughly, we present a comparison of mixed diets of 0%, 25%, 50%, and 75% guajillo in male white-tailed deer. Four in vivo metabolism trials were completed with each diet. Dry matter intake and change in body mass did not differ among diets. Gross and digestible energy intakes did not differ among diets, whereas metabolizable energy intake decreased with increased dietary guajillo concentration. Nitrogen balance and digestibility decreased with increased dietary guajillo concentration. Urinary glucuronic acid excretion increased linearly with increased dietary guajillo concentration. Nitrogen requirements for body growth and antler development were met by diets containing, 60% guajillo, whereas energy requirements for maintenance and antler growth were met with diets containing, 20% guajillo. Therefore, concentrations of dietary guajillo, 20% will support the maintenance of white-tailed deer. The primary function of guajillo may be to facilitate maintenance of adult deer, which have fewer obligatory productive processes than young deer, during periods of drought.