A field experiment was conducted over 2004-2006. The experiment was established at the Experimental Station in Tomaszkowo, the Warmia and Mazury Province (53^o42' N; 20^o26' E). The aim of this study was to compare Festulolium and Festuca pratensis L. grown in mixtures with Lotus corniculatus L. with respect to gas exchange parameters, chlorophyll content of leaves and yield, depending on multiple nitrogen rates. It was found that under conditions of temporary water stress the rate of photosynthesis was higher for Festulolium, while under adequate moisture conditions – for Festuca pratensis L. Over the three-year experimental period Festuca pratensis L. was characterized by a significantly higher rate of transpiration and substantially higher leaf greenness values than Festulolium. Under optimum growth conditions water use efficiency was higher in Festuca pratensis L., whereas under water deficit conditions – in Festulolium. Mixtures composed of the tested grass species did not differ significantly with respect to dry matter yield. Mineral nitrogen fertilization increased the rate of photosynthesis and the yield of the investigated grass mixtures only under favorable weather conditions; under rainfall deficiency conditions the above parameters were considerably affected by the presence of Lotus corniculatus L. in the sward. Nitrogen fertilizers enabled to limit transpiration as well as to increase water use efficiency in grasses and the chlorophyll content of leaves.

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