A field experiment was carried in 2008 at the ES in Lipnik belonging to Western Pomerania University of Technology in Szczecin. Mixture of these grasses ryegrass westerwolds was cultivated in different water conditions (O-control-without supplemental irrigation and W – irrigated objects), and at different nitrogen fertilization doses (1 N – 50 kg, 2 N – 100 kg, 3 N – 150 kg N ha-1). Supplementary irrigation significantly increased the intensity of photosynthesis (27.5%) and transpi-ration (63%). Nitrogen fertilization always increased the assimilation and had a differentiated effect on transpiration. The contents of pigments decreased under the influence of irrigation and increased by nitrogen fertilization. Total yield (f.m.) of mixture of grasses harvested in this experiment was 50.1 t ha-1, whereas from not irrigated plots – 46.1 t ha-1. As effect of supplemental irrigation on the average yield was 7.9 t ha-1, which was 17.1% bigger compared to control (without supplemental irrigation). Application of nitrogen fertilizer in doses of 100 and 150 kg N ha-1 resulted in increase of yield by 41.6% and 61.1%, respectively, compared to the dose 50 kg N ha-1. Productive effect measured by yield was full significantly correlated with photosynthesis, transpiration and assimilation of pigments. Assimilation was significantly (positively) correlated with transpiration and water use photosynthetic efficiency, while negatively with transpiration.