The first goal of this study was to evaluate the chemical composition of vegetation consumed by goats on semiarid rangelands. Secondly, antioxidant activity and identified bioactive compounds of this vegetation were estimated. Twenty-five samples were analysed, including leaves, stems, fruits, or a combination of them. Chemical composition demonstrated large differences among species and smaller differences within species. Some species had high protein contents, e.g. Acacia schaffneri, Celtis pallida and Prosopis laevigata. Methanol:water was the best solution to extract bioactive compounds, which we found in larger amounts in complete plants and stems than in fruits and leaves. A. farnesiana pods had the largest polyphenol contents, whereas A. farnesiana, A. schaffneri, Leptochloa dubia and others showed the best antioxidant activity. A positive correlation was observed between antioxidant activity and polyphenol-flavonoid concentration. Based on these results, it seems necessary to study in more detail some species, e.g. the fruits of Acacia farnesiana and A. schaffneri, to better understand their implications for ruminant feeding and nutrition.