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Invasive grasses (especially Setaria pumila (Poir.) Roem. et Schult) increasingly threaten meadows and pastures as a consequence of human impact and climate change. We conducted a study in 2012 and in 2013 to better understand the growing cycle and the influence of S. pumila on forage quality of lowland meadows. We observed a rapid increase of S. pumila presence and phytomass on the southern side of the Alps across the growing season. We measured (i) above-ground phytomass, with a modified Corral-Fenlon method; (ii) botanical composition using the linear point quadrat method; and (iii) grassland chemical composition by NIRs (near-infrared spectroscopy) analyses. To test the hypothesis that S. pumila summer growth is related to specific climatic conditions, meteorological data (temperature, precipitation and evapotranspiration) were collected from meteorological stations near the study sites. Total phytomass was sorted into S. pumila and other species. We used a generalized linear mixed models ( GLMM) and found the abundance of S. pumila to be inversely correlated with rainfall and the presence of other species, but positively correlated with temperature increase. The increase of S. pumila above-ground phytomass production was linked to a reduction of forage quality. [ABSTRACT FROM AUTHOR]