The objective of this study was to assess yield loss of weed-infested cassava and the degree of interference of weeds on the crop productivity in response to the fertilization of NPK. The study was conducted at the Universidade Estadual do Sudoeste da Bahia, in the municipality of Vitória da Conquista, BA, with two different treatment groups that were evaluated in parcels with and without fertilizer. The treatments of the first group consisted of initial coexistence periods in which the cassava plants and weeds were put to live together: 35, 70, 105, 140 and 175 days after planting (DAP); in the second group, the cassava plants, initially, remain free from weeds during the same periods. Weeds were assessed every 35 days, from the 35 to 525 days after planting of cassava, determining the fresh mass of the identified species in the evaluated treatments with and without fertilizers. The characteristics of root yield, shoot weight, harvest index, dry mass of roots, starch content and flour production were evaluated 18 months after plantation. The predominant weeds in the experimental area were: Panicum maximum, Brachiaria plantaginea, Sida rhombifolia, Pavonia cancellata, Portulaca oleracea, Cynodon dactylon and Setaria parviflora. The interference of the weeds in the cassava yield was bigger when the crop was subjected to fertilization, in coexistence periods from the 35 days after planting; however, when cassava plants were kept in the absence or in coexistence with weeds up to 35 DAP, fertilization provided increase in crop productivity factors. The competition with weeds resulted in high losses in root yield of the cassava plant, thus, being necessary the control of the invading plants in the period between 35 and 175 DAP.
