Rapeseed volunteers in rapeseed crops can cause contamination in harvest through pollen and seeds. The aim of the study was to predict pollen and seed production by volunteers in winter rapeseed crops according to the genotypes of the crop and the volunteers. Firstly an experimental analysis of intraspecific competition was carried out in 2000 and 2001 on two and six genotypes, respectively. The main competition period begins with vegetation onset and affects pollen and seed production of the volunteers. Relations between density, height, yield and number of flowers were developed based on results from experiments and literature. Knowing the densities and heights of volunteers and crop on the plot, the yield of the volunteers can be predicted. For example, for volunteers 60cm lower (higher) than the crop, at a density of 1volunteer/m2 in a crop at a density of 50 plants/m2, the seed production per plant of the volunteers is 12% (112%) of the production observed on a field of pure volunteers at 50 plants/m2. We propose to include the equation predicting volunteer production in the gene flow model, GS-R, in order to manage cropping systems according to harvest purity goals.