African savannas are experiencing an array of human-induced changes, including reductions and extirpations of large mammalian herbivores. The outcomes of these changes will be at least partially dependent on the functional responses of species that persist, including changes to plant-herbivore interaction networks. However, experimental tests of how ecological context, season, and competition jointly regulate foraging behaviors of large mammals are sparse. Therefore, we combined dietary DNA metabarcoding and a long-term herbivore-exclusion experiment in semi-arid Kenyan savanna to understand how a small browsing antelope (Guenther's Dik-dik, Madoqua guentheri, similar to 5 kg) responds to the absence of larger herbivores across seasons (e.g., wet vs dry) and sites (e.g., mesic vs xeric). We hypothesized that: (i) dik-dik diets would differ from those of co-occurring large browsers due to size-based differences in forage selection and accessibility; (ii) exclusion of larger herbivores would increase dik-dik activity and enable them to select high-quality browse, consistent with competitive release; and (iii) this selective foraging by dik-dik would suppress the growth and recruitment of their preferred woody food species. Consistent with our hypotheses, dik-dik diets differed significantly from co-occurring-but much larger-mixed-feeders, including the African Savanna Elephant (Loxodonta africana) and Impala (Aepyceros melampus). Dik-dik foraged selectively according to plant nutritional properties, especially protein content, and they avoided long-spined Acacia species, particularly during dry seasons and at the xeric site. In the absence of larger herbivores, dik-dik activity increased >2-fold, and they increased selectivity for the most nutritious, least-defended Acacia species. Shifts in selectivity due to competitive release were strongest under resource-scarce conditions, including dry seasons and xeric environments, amplifying local impacts of large-herbivore losses on savanna plant communities. Finally, in the absence of larger browsers, dik-dik suppressed the growth of their preferred species (Acacia mellifera), but only at the xeric site. Together, these results provide strong evidence that dik-dik exhibit flexible foraging behavior in response to larger herbivores, especially in resource-scarce conditions. If large herbivores are extirpated, subsequent diet shifts by dik-dik may suppress long-term plant diversity in this savanna.