Perennial ryegrass varieties are evaluated in monoculture plots, but in practice are typically sown as combinations of three or more varieties. Monoculture performance is used a predictor of mixture performance, but is questionable, particularly for grazing efficiency traits that interact with the grazing animal. Two field experiments were carried out to examine the production, grazing efficiency and nutritive quality traits of varieties sown in monoculture, and their binary, ternary and quaternary mixtures. Both studies contained two diploid and two tetraploid varieties comprising of only Late heading varieties in Study 1 and combinations of Late and Intermediate heading varieties in Study 2, all managed under cattle grazing with 16 grazing events taking place over two years. Synergistic effects, defined as a higher performance above that of the component variety mean (based on their respective component monoculture performance), were observed within some mixtures for certain traits although these rarely exceeded that of the best component monocultures. Performance differences attributable to ploidy in monoculture were expressed in the mixtures but differences in the spread of heading date within a mixture had few effects. Differences in grazing efficiency between varieties were also expressed within mixtures but with no consistent evidence of a synergistic enhancement of the cattle’s willingness or ability to graze sward mixtures more effectively. Mixture performance values were within the range of the component varieties for most traits, indicating that monoculture evaluation can be used as a reliable indicator of the performance of perennial ryegrass mixtures.