A mixed crop-dairy system named OasYs, based on temporary grasslands and on the grazing of diversified forage resources, was implemented in 2013 (Lusignan, France) in a plain area affected by summer droughts. Conducted without irrigation and with limited inputs, this agroecological system aims to permit farmers to live from their dairy system in a context of climate change, while preserving environment and contributing to animal welfare. We present the nitrogen (N) balance and use efficiency, and the carbon (C) footprint of this system assessed by the CAP'2ER tool over 2017-2020. OasYs showed small average N surpluses (47 kg N ha-1 UAA) and good N use efficiency (49%). Greenhouse gases (GHG) gross emissions expressed in kg CO2-equivalent per litre of corrected milk decreased from 2017 (0.90) to 2020 (0.78). They were lower compared to other grassland-based systems from lowland areas. These results can be explained by the sharply reduced use of N fertiliser and concentrates counterbalanced by a large use of legumes and a new breeding strategy. The net C footprint of milk is however higher than the average of grassland-based systems from lowland areas, because of lower soil C storage resulting from the ploughing of temporary grasslands.