Current understanding of the effects of condensed tannins (CTs) on productivity of mixed-feeding ruminants is largely based on simple laboratory and feeding experiments. These experiments do not allow mixed feeders such as goats to adequately employ their behavioural and physiological responses to plant secondary metabolites. In a field experiment, we investigated the effects of CTs on growth performance of goats. We hypothesized that CTs reduce blood circulatory nutrient and increase nitrogen in faeces. We divided 45 yearling females into three groups of 15 animals that were orally dosed daily with either CTs, polyethylene glycol 6000 (PEG, a polymer that neutralizes dietary tannins), or water (control). We measured the average daily gains, live weights, faecal nitrogen and four blood metabolites from each goat during dry and wet seasons. Live weights increased over time in both dry (P < 0.001) and wet seasons (P < 0.001). The average daily gain was consistently greatest for animals dosed with PEG and least for those dosed with CTs. Goats dosed with CTs had the greatest faecal nitrogen and the least blood protein concentrations, while the opposite was true for PEG goats in both seasons. Blood urea and non-esterified fatty acids indicated a negative influence of CTs on energy and protein metabolism. We concluded that CTs limit growth and PEG mitigates the negative effects of CTs on growth performance of free-ranging mixed feeding ruminants.