The Upland Goose Chloephaga picta is a migratory species, with breeding grounds in the grassy wetlands of Patagonia and most of whose wintering grounds are located in cultivated areas of the Argentine Pampas region. Knowing geese food preferences is key, not only for ecological implications but also for their conservation, since the use of cultivated resources has been the cause of the conflict that led to their population decline in the last century. The aim of this study was to describe and compare the botanical composition of the diet of the Upland Geese between breeding and non-breeding seasons, and to evaluate plant group dietary preferences based on environmental availability. Microhistological analysis of faeces was carried out and the contribution of three plant groups to the diet (i.e. grasses, eudicots, and graminoids) was quantified. Eighteen plant species were identified. The diet was dominated by native grasses (Distichlis spp., Hordeum spp., and Bromus setifolius) and graminoids (Eleocharis spp.), with minor contributions of eudicots such as Nitrophila australis and Trifolium repens. A high degree of overlap was found between the breeding and non-breeding seasons (98%), indicating similar diets. Regarding the selection of plant groups, a significant selection of graminoids was observed in both seasons, followed by grasses, indifference to eudicots, and rejection for shrubs. The high proportion of native species used compared with cultivated species (60% vs 10%) underscores the importance of natural habitats with native vegetation. This area appears to be beneficial, as it provides valuable and strategic year-round trophic resources, potentially helping to avoid long migrations. It also underscores the need for further studies on population fluctuations, habitat use, and the detection of potential threats.