Diet selection, referring to the food that is ingested by an animal along with its nutritional composition, is essential for animal fitness, survival and behavior. The hirola (Beatragus hunteri), with ~ 500 individuals remaining, is classified as critically endangered by IUCN. For effective hirola conservation, there has to be a clear understanding of habitat-specific forage quality, more importantly as there are planned re-introduction efforts from managed into natural habitats. We used populations in natural and managed breeding habitat in eastern Kenya to assess the dietary selection of hirola. Hirola consumed a total of 17 species with Chloris virgata being the most preferred grass, while Commelina benghalensis and Commelina diffusa were the most preferred forbs. Cyperus kilimandscharicus was the only moderately preferred sedge. We then analyzed nutrient content for 14 variables, including crude protein (CP), fiber fractions, in vitro dry matter digestibility (IVDMD), and key minerals. Forage in natural habitats was more closely associated with higher levels of phosphorus (P), magnesium (Mg), and IVDMD, while managed habitats were characterized by elevated sodium (Na), potassium (K), and CP levels. Seasonal differences were also detected, particularly in calcium (Ca) and Mg concentrations. Our results suggests that hirola must cope with changing plant chemical composition, forage quality and diversity. Future habitat restoration programs should focus on monitoring and reseeding preferred forage species and the provisioning of supplementary diets with appropriate chemicals to newly released herds or in the breeding facility.