The effect of weed interference duration on yield and yield components of cumin Cuminum cyminum L.) in irrigated and rainfed condition

Background and objectives
Cumin is one of the most important medicinal plants, due to its specific characteristics and its cultivation area of 18,000 hectares, it has the first rank among cultivated medicinal plants in the country. Khorasan Razavi is one of the most important producers of this crop. Annually. Cumin cultivated on 8100 ha as irrigated and on 4000 ha as rainfed. One of the main problems of producing this valuable herb is the less competitive ability with weeds, so that weed competition can reduce the grain yield by up to 92%.
Materials and methods:
To investigate the effect of weed interference duration on yield and yield components of Cumin (Cuminum cyminum L.) in irrigated and rainfed condition, a field experiment was conducted in Sabzevar in 2013-2014. Experiment was conducted as split plot in a randomized complete block design with three replications. Experimental factors were cultivation method (irrigated and rainfed) as the main plots and duration of weed interference (0, 15, 30, 45, 60 and 75 (weedy) days after emergence)) as sub plots. In irrigated conditions, four irrigation stages were carried out during planting, branching, flowering and seeding, while in rainfed conditions only one irrigation time was carried out at planting. Other agricultural operations were similar in both conditions. A three parameter logistic equation was used to determine the beginning of the onset of reduced yield based on the duration of the weed interference period.
Results:
The results of analysis of variance showed that irrigated cultivation had more weed dry weights, number of umbrellas per plant, number of seeds per umbrella, biological yield and economic yield than rain-fed, which had a significant difference with rainfed cultivation method, while weed density and number of lateral branches were not affected by cultivation method. Means comparison showed that irrigated condition had 1.92% and 26.22% more weed density and weed dry matter compared with rainfed condition, respectively. In irrigated condition, Cumin had more plant height, number of branches, number of umbels per plant, number of seeds per umbels, biological and economic yield than rainfed condition. Increasing of interference duration was decreased plant height (24.28%), lateral branches (33.37%), the number of umbels per plant (30.99%), the number of seeds per umbels (43.88%), biological yield (45.25%) and economic yield (44.81). In both irrigated and rainfed condition. Increasing of weed interference was decrease yield and yield components of cumin which decline was higher in rainfed conditions. Fitted logistic regression function coefficients showed that the onset of yield loss was earlier (23.15 days after emergence) in rainfed condition while in irrigated condition the onset of yield loss was postpone to 28 days after emergence. The critical timing of weed removal to prevent 5% grain yield loss was 14 and 16 days after emergence and 10% grain yield loss was 18 and 25 in irrigated and rainfed condition, respectively.
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Conclusion:
In conclusion, results showed in both irrigated and rainfed condition, weed interference reduces economic yield. In rainfed condition weed interference until 23 days after emergence and in irrigated condition weed interference until 28 days after cumin emergence had not significant reduction on yield.