Early weed competition and high seeding densities are stresses that affect grain yield. The objectives of this study were to study the effect of early stresses on plant-to-plant variability and how this may affect grain yield. Field experiments were conducted in 2008 and 2009. Maize was exposed to different levels of early stress, created with intra- and inter-specific competition. Different levels of intra- and inter-specific competition where created with initial plant densities of maize and time of weed removal, respectively. Early stress decreased total dry matter per plant, increased plant-to-plant variability and reduced grain yield. Yield loss, however, was not attributed to an increase in plant-to-plant variability but rather to a decrease in intercepted photosynthetic active radiation. We hypothesize that an increase in plant-to-plant variability caused by early stress may reduce yield if stressful conditions occur during the critical period for kernel set.