Glyphosate effects on floristic composition and species diversity in the Flooding Pampa grassland (Argentina)

Temperate grasslands of Argentina are extensively grazed by domestic livestock. Primary production follows a seasonal pattern, with maximum vegetation growth in late spring and minimum in winter. During the last decade, winter forage productivity has been encouraged by promoting the growth of cool-season annual grasses through late summer applications of glyphosate. The aim of this study was to describe structural changes in grassland vegetation associated with glyphosate treatments, as applied in a commercial livestock farm in the Flooding Pampa. Vegetation composition was assessed from spring 2006 to late summer 2008, in three paddocks that had never been exposed to any kind of herbicide, and three other paddocks treated with glyphosate in late summer in the previous 5 years. In the paddocks treated with glyphosate, basal cover of cool-season annual grasses and forage value in spring were greater but basal cover of cool-season perennial grasses, warm-season tussock grasses, warm-season legumes, total basal vegetation cover and forage value in summer were much lower respect to untreated paddocks. The shift in floristic composition resulted in less rich and even assemblages, dominated by an annual species and impoverished in native and perennial species. These structural changes may alter ecosystem processes through the increase of soil salinization and water losses in summer, may affect the seasonal pattern of productivity and may threaten biodiversity conservation and sustainability of wild life.