Two weed competition experiments in broadcasted seeded rice were conducted during 2007 and 2008 summer seasons at EL-Serw Station, Damietta Governorate, Egypt. Ten manual weed competition and weed removal at early or late times after sowing were: - weed competition for the whole season, weed competition for 4, 6, 8 and 10 weeks from sowing, weed free for 4, 6, 8 and 10 weeks from sowing and weed free for the whole season, to determine when a natural infestation of weeds start to reduce rice yield and when stopped without yield losses in broadcasted rice. Dominant major weeds in experimental fields were barnyardgrass Echinochloa crus – galli (L.)Beauv, jungle rice (Echinochilon columum (L.) Link), small flower umbrella plant (Cyperus difformis (L.), nutsedge (Cyperus longus (L.) and hemeira Dicanthium annulatum. Results showed that the maximum yield losses were 43 to 46% due to weed infestation in field experimental with average 16 ton/fed fresh weight of total weeds in both seasons, respectively, as compared to weed free plots. Using classical approach for determination the critical periods indicated that yield losses start after 3 weeks from sowing and stopping yield losses due to weed competition after 8 weeks from sowing. Using regression approach showed that the mathematical calculated model for study the relationship between weed competition or weed removal period and grain yield (ton/fed), straw yield (ton/fed) of broadcasting seeded rice was fit quadratic function. These functions had R² value 0.983, 0.899 and 0.994, 0.986 in the first season, 0.990, 0.879 and 0.993, 0.986 in the second season for grain yield and straw yield, respectively. Application this function showed that to maintain 95% of maximum grain yield of rice, growers rice should start controlling weeds not later than two weeks and continue until 7 weeks from sowing to maintain the rice potential yields by applying early suitable recommended post emergence herbicides for controlling the abovementioned spectrum of weeds and strategies of weeds management.