Does Supplemental Feed Increase Selective Foraging in a Browsing Ungulate?

Provision of supplemental feed to large herbivores is a common management practice that may motivate selective foraging, thereby influencing plant community composition. Our objective was to assess the effect of a high-quality supplement on diet composition and nutritional quality for white-tailed deer (Odocoileus virginianus). We permanently released hand-reared deer into 4 81-ha enclosures; in 2 enclosures we provided a pelleted supplement. We conducted bite-count studies seasonally to assess diet composition and quality. Supplemented deer reduced mast (fruits and pods of woody plants and cacti) in their diets (P < 0.019) during spring and autumn compared to unsupplemented deer. Diets of deer in supplemented enclosures had 2 times greater proportion of browse during spring (P = 0.065) and 5 times greater proportion of forbs during autumn (P = 0.007). Quality of the forage portion of the diet did not vary by treatment during winter or summer. Metabolizable energy concentration was 13% greater (P = 0.054) in spring and digestible protein content was 3 times greater (P = 0.006) during autumn in diets of supplemented compared to unsupplemented deer. Our results support the selective foraging hypothesis during autumn but not during winter, spring, or summer. Furthermore, white-tailed deer did not reduce the proportion of their diet composed of browse, but did reduce consumption of mast. Supplemented deer continued to eat poor-quality, chemically defended forage, perhaps to alleviate ruminal acidosis induced by the supplement or because nutrients in the supplement increased the deer's ability to detoxify chemically defended browses. A decline in mast consumption by supplemented deer could influence plant communities, depending on the role of deer in seed dispersal and seed predation. Impacts of supplemental feed on selective foraging of white-tailed deer in shrub-dominated rangelands are more complex than suggested by previous research. Long-term studies of vegetation communities are needed before wildlife managers will be able to fully incorporate effects of supplemental feed into management decisions.