Enhancing total yield and improving seed quality is a dynamic area of research. Both breeding and biotechnology have been successful in creating soybean cultivars with increased oil or protein content with much research directed at total yield. We conducted a 2-year field experiment to investigate the potential impact of interspecific competition on seed quality and seed development of five soybean cultivars (Forrest, Essex, Asgrow 4603, Asgrow 4403 and Asgrow 3903). Three levels of weed competition were established using different application rates of pre-emergence herbicides s-metolachlor and cloransulam. Several seed related parameters were measured and many showed similar trends of significance for cultivar and weed competition treatments across both years. Higher levels of weed competition resulted in soybean seeds containing higher protein content (increase between 0.7 and 1%). For protein in 2005, there was a significant interaction term for competition level and cultivar (F_8,44 = 2.38, P < 0.05). Seed oil tended to decrease with higher levels of competition, but by less than 1% each year. Seed yield showed a significant decrease in response to competition in 2005 (F_2,44 = 56.82, P < 0.0001), but not in 2004. Seed per plant decreased in response to competition both years (2004: F_2,44 = 8.23, P < 0.05; 2005: F_2,44 = 8.71, P < 0.05). Seed per plant was impacted both years by competition. The number of aborted pods per plant was greater for plants under low and medium competition; however, significance was seen only in 2004 (F_2,44 = 3.33, P < 0.05) but the trend was evident in 2005. Seed quality changes are responsive to the presence of neighbouring plants even if their interference does not reduce total yield. Future studies should be designed to investigate how competition and interference alter soybean metabolism such that seed quality is maximised.