Planting wheat into high populations of P. thornei reduced the extraction rate (kl) in the intolerant cultivars; this in turn caused a delay in the root extraction velocity and effectively changed the crop lower limit. The early season stress applied by the reduced extraction rate impeded canopy development that consequently reduced demand for water and nutrients, growth and yield potential. The resulting yield loss was consistent across both a wet and a dry wheat growing season with the intolerant cultivar yielding 34% less than the tolerant cultivar. Understanding the mechanism with which P. thornei reduces yield allows the impact of P. thornei to be included in simulation models to better understand how to manage this constraint to the Australian grain industry.