Abstract Mixed swards of perennial ryegrass /white clover were grown in competition under controlled environmental conditions, at two temperatures and with different inorganic nitrogen supplies. The swards were studied after canopy closure, from 800 to 1200 °C d cumulative temperatures. Clover contents did not vary significantly during the period. A simulation model of light interception was used to calculate light partitioning coefficients and radiation use efficiencies for both components of the mixture in this controlled environment experiment. Additionally, this same radiative transfer model was applied to the field data from Woledge (1988) (Annals of Applied Biology112: 175 –186) and from Woledge, Davidson and Dennis (1992) (Grass and Forage Science47: 230 –238). The measured and simulated values of light transmission, at different depths in the mixed canopy, were highly correlated (P In the controlled environment experiment, the radiation use efficiency of the legume was lower than that of its companion grass. For both species, radiation use efficiency was negatively correlated with the mean irradiance of the leaf. The role of a compensation between light interception and light use for stabilizing the botanical composition of dense grass –clover swards is discussed.