Cultivar mixtures of perennial ryegrass (Lolium perenne L.), which mostly contain both diploids and tetraploids, are widely used in agricultural grassland systems, and yet limited knowledge exists on their competitive interactions after sowing. Two diploid cultivars (AberMagic, Twystar) and a tetraploid (Greengold), sown in different binary mixture ratios, were monitored over a 2-year period under frequent cutting to simulate cattle grazing and conservation managements. Composition was determined from fluctuations in phosphoglucoisomerase allozyme frequencies relative to the frequencies of the component cultivars.

An interaction occurred between cultivar sown ratio and seasonal period whereby greater changes in proportions occurred early in the first year. Cultivar proportions were significantly influenced by sown ratio, seasonal period and management. The diploid mixtures remained similar to their sown ratios, whereas the diploid–tetraploid mixtures, sown at 15% and 30%, increased to 33–61% under simulated grazing and 40–59% under conservation use. No changes were observed in the 50 : 50 diploid–tetraploid mixtures. A competitive equilibrium was attained after 1 year and compositions remained stable throughout the second year. The differences in large canopy structure between ploidies are a likely factor affecting competitive ability, and mixture construction should account for these hierarchies to achieve the desired sward composition.