A series of studies were conducted under glasshouse and field conditions with the general objectives of understanding the mechanisms of interference between wheat and two annual grass weed species viz. Phalaris minor and Phalaris paradoxa by adopting replacement and additive series designs and the divided box technique to partition competition effects. Replacement series experiments showed that wheat exhibited intraspecific competition in monoculture. In general, wheat outcompeted both Phalaris spp. in mixtures and was more competitive than both weeds at 75:25% and 50:50% mixture ratios. In additive series experiments, it was observed that yield components of spring wheat were significantly reduced at density of 200 and 50 Phalaris spp plants m⁻² under glasshouse and field conditions respectively. Wheat yield reduction was 22. % and 20.9 % following competition during spring against P. paradoxa and P. minor respectively; however, it was 60.8 and 34.9% during winter at a weed density of 400 plants m⁻².