A field experiment was conducted over 2004-2006. An field experiment was established in 2003 at the Experimental Station in Tomaszkowo, province Warmia and Mazury. To compare perennial ryegrass and festulolium grown in mixtures with white clover with respect to gas exchange parameters, chlorophyll content in leaves and yielding depending on multiple nitrogen rates. It was found that perennial ryegrass was characte- rized by a faster rate of photosynthesis and transpiration as well as by higher leaf greenness values than festulolium. However, the values of water utilization coefficient were substantially lower in perennial ryegrass, as compared to festulolium. In both species nitrogen fertilization rate optimal for CO₂ assimilation was 60 kg N·ha^-1. The presence of white clover in the sward had a positive effect on the photosynthesis rate of grasses. Photosynthesis rate was higher in non-fertilized treatments than in treatments fertilized with mineral nitrogen of 120 kg N·ha^-1. Nitrogen reduced water transpiration and contributed to better water utilization by the species tested. Perennial ryegrass grown with white clover yielded significantly higher than festulolium grown with white clover. Mineral nitrogen fertilization caused a yield increase only in the years characterized by high precipitation totals. In the rain-deficient year, a significantly higher yield was recorded in non-fertilized plots. High temperatures and precipitation deficiency inhibited the intensity of photosynthesis and transpiration of grasses and decreased the yielding of mixtures, however, they promoted chlorophyll accumulation in leaves.

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