The improvement of forage production and nutrition quality on native grasslands through plant species oversowing and fertilization (legumes in particular, coupled with phosphorus fertilization) is known to have been widely adopted worldwide. Less is known about this practice on the wet grasslands of the French Atlantic littoral marshes. The purpose of this study, conducted over a 3-year period (2012-2014) on the Saint Laurent de la Prée research farm, was to investigate the effects on the yield and nutritional quality of forage hay on a permanent wet meadow, of oversowing with different plant species and fertilization. We found that the success of oversowing was influenced by species or mixtures, and depended on their ability to develop and persist in the cover. In general, oversowing tended to provide benefits in terms of the total annual forage yield in 2013, with a slight increase in forage quality in 2012 and 2013. Fertilization provided no real benefit in terms of forage quality. There was no persistence of introduced species in the sward, as in 2014 almost all of them disappeared. In the conditions of this study, the benefits of oversowing and fertilizer applications were limited and short-lived. These results are discussed in relation to the conservation value of these wet grasslands and the need to pursue research on agroecology for their biodiversity-oriented management.