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The performances of grass and grass/legume swards at three levels of nitrogen were compared under irrigation over a three?year period. The swards were grazed by dairy cows and the influence of nitrogen on dry matter yield, protein content, protein yield and botanical composition was measured. The addition of legumes to grass swards resulted in highly significant increases in yields of dry matter and protein. Festuca growing in association with clover contained 2?3% inore protein than when grown in glass swards. The increase in protein yield resulting from the addition of legumes to the swards, diminished with increasing nitrogen level. The protein yield and protein content of grass swards increased as a result of nitrogen application. A narrowing of the grass/legume ratio also resulted from nitrogen application to mixed swards, but the botanical composition of grass swards was not influenced by nitrogen level. A total of 80 inches of irrigation water was applied annually to all swards. The experiment demonstrated the superiority of grass/legume swards over grass swards in terms of nutrient production and indicated the importance of high nitrogen level for satisfactory grass yields.