دوره بحرانی کنترل علف‌های هرز در گیاه زنیان در شرایط آبیاری معمول و کم ‌آبیاری

The study aimed to determine the critical period of weed control in ajwain (Trachyspermum opticum L.) under normal irrigation and deficit irrigation conditions. It was conducted as a split-plot experiment within a randomized complete block design with three replications in the spring of 2021 at the research farm of the Faculty of Agricultural Technology, Aburaihan Campus, University of Tehran, in Pakdasht. The main factor included normal irrigation (depletion of up to 50% of available water) and deficit irrigation stress (depletion of up to 75% of available water). The sub-factors consisted of weed infestation periods lasting 15, 30, 45, 60, 75, 90, 105, and 120 days, and weed-free periods of 15, 30, 45, 60, 75, 90, 105, and 120 days after ajwain emergence. Traits such as plant height, plant dry weight, seed yield per hectare, essential oil percentage, and essential oil yield were measured. Non-parametric analysis was used to display and analyze the response of ajwain traits to weed interference and control periods. The dominant weeds in the field were Amaranthus and Purslane. The results indicated that by weeding ajwain from 24 to 38 days after emergence, ajwain yield did not differ significantly from complete season-long weeding. With an increase in the duration of weed interference, whether at the beginning or the end of the growing season, economic yield decreased by 40-60%. The treatments of full weed interference under deficit irrigation, 90 and 105 days of interference under deficit irrigation, without significant differences among them, resulted in the lowest essential oil yield of ajwain. Overall, early-season weeding had the most significant impact on yield, yield components, and essential oil percentage of ajwain.