Predicted and observed data for the amount of light energy intercepted by mixtures of clover and ryegrass are compared, in an additional test of a model which relates competition for light to crop dry matter production. The closeness of predicted and observed values for light interception implies that the optical properties of the component species do not change significantly when they are grown in mixtures. If this is true, then it should be possible to predict the growth of mixtures using information about their optical properties when they are grown in monocultures.