the study of critical period of weed control and yield of black seed (nigella sativa l.) affected by weed free and infested periods

in order to determine the critical period of weed control (cpwc), yield and yield components of black seed (nigella sativa l.) affected by weed competition and control periods, a field experiment was conducted based on randomized complete block design with 12 treatments and three replications at the agricultural research station, ferdowsi university of mashhad, iran in 2012. two sets of weed-infested and weed-free treatments were employed in this study. at the first set of treatments, weeds were allowed to compete with black seed until 0, 14, 28, 42, 56 and 70 days after emergence (dae) and then plots were kept free of weeds till harvesting. at the second ones, plots were kept free of weeds until the mentioned stages and then weeds were allowed to grow until harvesting. in this experiment, echinochloa cruss-galli l., amaranthus retroflexus l. and chenopodium album l. were the main dominant species, indicating by the highest leaf area index and dry matter. the results showed that leaf are index, number of follicle per plant, grain and oil yields of black seed were significantly affected by weed infestation and weed free periods. the results indicated that at 5 and 10% yield loss levels, onset of cpwc was 11 and 14 dae (87 and 130 growing degree days (gdd)), respectively. also, based on these levels of grain yield loss, the end of the cpwc were 64 and 57 dae (922 and 783 gdd).