The ability to compete for resources is a main determinant of a plant?s success within a plant community. In the case of agricultural crops, competition among weeds and crop plants is a major determinant of crop yield losses. The objectives of this study are: i) to improve the understanding of the role of weeds in water use and crop water productivity, ii) to test, through field experiments and an individual based model, the effects of size inequality and plant spatial arrangements on the competitive relationships among plants, the success of individuals, the evolution of plant sizes distributions and the ability of individual plants to capture resources, and iii) to explore through a modeling exercise the effects of soil nitrogen and weather on competitive growth of Abutilon theophrasti M. (velvetleaf) and Zea mays L. (maize). For this purpose a mechanistic individual based model was developed, which estimates light interception, photosynthesis, respiration, growth, photosynthate allocation and root growth at the leaf or plant level, allowing plants to compete for solar radiation and soil resources. Each of the model components had been calibrated and cross validated with data collected on field experiments where maize and maizeA.theophrasti mixtures were established on 2005, 2006 and 2007 with A. theophrasti plants emerging before, simultaneously and after maize. In these experiments intensive measurements of plant height, plant leaf area, leaf area distribution, plant biomass, leaf stomatal conductance, soil water extraction and yield were taken, providing additional experimental evidence to this research. Despite exceptionally dry conditions in the field in some years, maize-weed mixtures and maize in monocrop had similar total soil water contents and rates of water extraction through the profile and maize was no more water stressed in the weedy treatments than in monocrop. However the partitioning of transpired water among plant species shifted according to the amount of leaf area and height of the plants, which ultimately determined the amount of solar radiation intercepted. Plant size distributions for each species cohort changed over time, becoming more positively skewed as the dominant plants in the cohort capitalized on their advantage. This effect was more obvious as competition for resources was intensified by greater availability of water and nitrogen or when the species cohort became dominated by another cohort of plants (i.e. due to timing of emergence or spatial arrangement of rows). High soil nitrogen availability enhanced maize competitiveness, suppressing weeds and reducing yield loss, especially when other environmental factors allowed high maize yield potentials. High soil nitrogen levels and in season weather also changed the relative heights of maize and weeds (height difference), which were closely related to maize yield loss and therefore were good early predictors of maize yield loss. Weather effects on yield loss were guided primarily by changes on soil nitrogen availability and changes in the rates plant growth that modified both nitrogen uptake and the dynamics of competition for other resources. Both model and field results also highlight that maize-A theophrasti competition is highly dynamic, and dependent on the initial conditions, primarily plant sizes and rates of growth. The analysis of how crop and weeds compete for resources can not only help reduce crop yield losses due to weeds, but also contribute to understanding how plant communities in natural systems respond to the environment.