Abstract Prosopis caldenia and P. flexuosa are native woody plants in central Argentina. Circumstantial observations have shown that goats consistently select the first species and avoid the latter. The present study was designed to compare preference of goats for old and new branches of both species under controlled conditions. We ran choice tests that paired old and new branches of P. caldenia and P. flexuosa in all possible binary combinations. For each option a preference index was calculated by dividing the number of bites on the particular option by the total number of bites on both options. Goats showed the following preference order: old branches of P. caldenia >new branches of P. caldenia >old branches of P. flexuosa =new branches of P. flexuosa . Goat preference was positively correlated with total phenols and condensed tannins, whereas it was negatively correlated with neutral detergent fibre.