Nine tests were made from 1973 up to 1976 on different soil types in five localities (three in the Triângulo Mineiro and two in the Northern Region of Minas Gerais State, Brazil) to Study the critical periods of weed competition with cotton (Gossypium hirsutum L.). The treatments consisted of: weed free during the first 2, 4, 6, 8 weeks and all the cycle; and competition during the first 2, 4, 6, 8 weeks and all the cycle. The results showed yield losses of 90,22% in the Triângulo Mineiro and 70,73% in the Northern Region when weed were always present. Compared to the check free of weeds during all the cycle, the best yield was obtained when cotton was kept free of weeds during the first six weeks after emergence in the Triângulo Mineiro as well as during the first eight weeks in the Northern Region. However, there was no difference among the treatments consisting of 4, 6, 8 weeks, and all the cycle without weed competition for both regions studied.
Weeds; weed competition; Cotton