Two experiments were carried out to evaluate the effect of different periods of control or coexistence of Euphorbia heterophylla on soybean (cv. BRS-133) grain yield at low seeding rate, at Boa Terra Farm, Quarto Centenário-PR, Brazil, during 1999-2000 growing season. The treatments for both experiments consisted of keeping the crop either with or without E. heterophylla for 5, 12, 19, 26, 33, 40, 47, 54, 61 and 68 days after emergence (DAE) and during the whole cycle. At the first weeding, for those treatments kept for increasing periods with weeds, and at harvest, for those treatments kept for increasing periods free of weed competition, E. heterophylla densities and biomass were recorded. The presence of weeds caused a yield daily loss of 5.15 kg ha-1, whereas their absence provided a yield daily gain of 7.27 kg ha-1. Weed-crop coexistence for up to 17 DAE did not cause any negative effect on crop yield, and the maximum length of time in which weeds had to be controlled to prevent crop yield losses was 44 days after emergence. The critical period of interference was, therefore, from 17 to 44 DAE.
soybean; competition; wild poinsettia
