We monitored five-year changes in the vegetation of 31 hay meadows under an agri-environment scheme in Wallonia, Southern Belgium. Management included delayed mowing (in July) and fertilizer prohibition. It resulted in increasing cover of characteristic forbs (such as Leucanthemum vulgare, Lotus corniculatus, Centaurea jacea) and oligotrophic grasses (Avenula pubescens, Festuca rubra), while the competitive grasses, such as Holcus lanatus, Phleum pratense and Alopecurus pratensis, tended to decrease. We interpreted this as a vegetation shift from typical hay meadow to oligotrophic grasslands due to soil impoverishment following the current management. Both habitats are of conservation value. Despite these changes in the meadow plant communities, only one of the four criteria used by the Walloon administration to indicate hay meadow conservation status changed significantly over the six-year period. This was a decrease in the cover of species indicating high grazing intensity. The number and cover of characteristic plant species, and the cover of nitrophilous species, did not change significantly.