Annual legumes sown as short-term forage crops are an important non-chemical option for the control of herbicide-resistant annual ryegrass (Lolium rigidum L.). The relative ability of 5 annual forage legume species (Trifolium subterraneum L., T. michelianum Savi., T. alexandrinum L., Medicago murex Wild and Vicia benghalensis L.) to suppress annual ryegrass seedlings was examined in a glasshouse study. The experiment investigated the importance of legume species, seed size and seeding rate in influencing the relative competitiveness of the legumes. Balansa clover (T. michelianum) and berseem clover (T. alexandrinum), the smallest seeded of the legume species, were the most effective of the 5 legume species, when compared at equivalent seeding rates, at reducing ryegrass biomass. Legume leaf area and biomass were the 2 factors measured that were most closely associated with depressing ryegrass biomass and were themselves most influenced by legume species and seeding rate. Balansa clover and vetch (V. benghalensis) were the most effective at restricting the amount of light penetrating to 50% of the height of the canopy over a range of seeding rates. The study showed that the competitiveness of forage break crops can be optimised by maximising legume biomass through selecting small-seeded legume species with high relative growth rates and dense canopies and by increasing the seeding rate where seed costs are low.