Determination of Critical Period of Weeds Control in Sesame (sesames indicum) in Mashhad Condition

In order to improve the efficiency of weed management and reducing herbicide application, determining the most sensitive weed-crop competition period is very important. There is a special period in plant life cycle, that if the crop is kept free of weeds, yield reduction has not significant economically and also weed control after that will not affect crop yield improvement. This period is known as the critical period competition affected by climate, weed species dominance, field management methods and crop type and their cultivars. Sesame is one of the important crops due to its high oil quality. This low input crop is very considerable in terms of economic in arid and semi-arid subsistence farming area. Despite the importance and antiquity of sesame cultivation in Iran, there is low information about the effects of weed on its growth and yield and also critical period of weed competition. So, the present study was carried out to determine the critical period of weed in two Common genotypes of sesame (Oltan cultivar and kalat Landrace) in Khorasan Razavi province.
Material and This study was conducted at Research Farm of College of Agriculture, Ferdowsi University of Mashhad with 36º 15' N latitude and 59º 28' E longitude and 985 m above the sea level, during 2009 growing season, Mashhad, Iran. The experiment was carried out as split plot based on the randomized complete block design with three replications. Treatments were sesame genotypes (Olten cultivar and Kalat landrace) as main plot and weed-free periods (20, 30, 40, 55 and 70 days after emergence (DAE)) and weed interference -periods (20, 30, 40, 55 and 70 DAE) with two full season weed control and full season weed competition, were considered as sub plots. Sampling from weeds was conducted before weeding and before the end of sesame growing season in interference and weed free treatments, respectively. Sesame plants harvested at physiological maturity stage, after eliminating the border effect area, from six m-2 and after drying in natural condition, sesame seed yield was reordered. To determine the critical period of weed, non-linear regression was used by fitting the data to Logistic and Compertz models. MSTAT-C software also was applied for analysis of variance and means comparison.
Results showed that, the highest weed density was
recorded in weed interference conditions 30 days after sesame emergence and reduced after that. The dominant weeds were Portulaca oleraceae, Convolvolus arvensis, Amaranthus retroflexus, Chenopodium album and Echinochloa crus-galli. Weed dry weight was decreased on average by 85 percent in weed free treatments up to 44 days after emergence compared to full season weed interference control treatment. Sesame yield was decreased with increasing the weed interference periods, but it was increased with increasing weed free period treatments. Weed dry weight decreased significantly by increasing weeding duration. It was estimated that the critical period of weed control in Kalat landrace were 27-48 and 30-40 days after emergence with considering 5 % and 10 % acceptable yield losses, respectively, while in Oltan cultivar were 22-72 and 34-58 days after emergence, respectively. So the critical period of weed control in Kalat landrace was shorter than Oltan cultivar. It seems that in kalat genotype once weeding during its critical period of weed control is enough, while it is necessary at least twice in oltane cultivar. Short critical period of weed control in kalat landrace sesame may be due to its shorter growing season and its faster canopy closure than Oltan cultivar.