Introduction: Cumin (Cuminum cyminum L.) is an important crop for medicinal purposes in Razavi Khorasan province, especially in Sabzevar. With 80% of the country's cultivated land, Khorasan Razavi province is the leading producing province in Iran. In the world's arid and semiarid regions, access to water is a greater constraint on crop production than access to land. Using cultivars or plant species that are adapted to the climatic conditions of these regions is one of the easiest ways to combat drought stress. Despite the fact that cumin is considered a drought-tolerant plant, drought stress, particularly in years with low precipitation, significantly reduces cumin yield. The management of water during cumin's growth and maturation can affect crop yield and quality. Cumin's low productivity is largely attributable to weed infestations. Cumin is especially susceptible to weed competition during its early growth stages. Several characteristics of cumin, including slow plant emergence, short plant height, and cumin morphology, enable weeds to effectively compete. Yield losses ranging from 40 to 90% have been sustained in cumin due to weed competition.
Materials and Methods: To determine the effect of different irrigation cut-off times on cumin yield and yield components under weed interference conditions, a field experiment was conducted in 2018 and 2019 at the Hakmabad Joven Agricultural Service Center (Khorasan Razavi) using a randomized complete block design with three replications. Irrigation cut-off time (full irrigation, irrigation cut-off in flowering stage, irrigation cut-off in seed-filling stage, and rainfed cultivation) was the main plot, while weed interference  was the sub-plot. Land preparation was done with moldboard in the fall of the previous year, the tillage operation included shallow plowing, double disc plow, and full leveling. Each subplot was 5 m long and 1 m wide, with five planting rows, 20 cm between rows, and 5 cm between plants. To prevent water from spreading laterally, plots were surrounded by dikes and separated by 2 m. The local landrace (Sabzevar), which is widespread and suitable for the region, was utilized. The height, number of sub-branches, number of umbrellas per plant, number of umbels per umbrella, and number of seeds per umbel were measured for five plants randomly selected from each plot at the end of the growing season. Before performing combined analysis across years, homogeneity was determined using Bartlett's Chi-square test. Due to the significance of the year effect, separate analyses of variance were conducted for each year using SAS (version 9.4) statistical software, and treatment means were compared using Fisher's least significant difference (FLSD) tests (p < 0.05).
Results and Discussion: The results showed that in the rainy year, the yield and yield components of cumin did not respond to irrigation cut-off times. However, in the dry year, the highest seed and essential oil yield was obtained with full irrigation, and a delay in irrigation cessation had fewer negative effects on yield and yield components.
In the first and second years, full irrigation increased the number of lateral branches (45.7% and 38.4%), the number of umbels per plant (46.1% and 12.9%), the number of seeds per umbel (36.4% and 44.7%), and 1000-seed weight (6.4 and 36.4%). All irrigation levels resulted in a lower grain yield under weed interference conditions compared to non-interference. The most detrimental effects of weed interference were observed when irrigation was stopped at the time of flowering (34.8%). In full irrigation conditions, grain yield decreased by 12.8%, but the yield reduction percentage was lower compared to other irrigation levels. Dryland cultivation yielded the highest amount of essential oil in both the first and second year. In the first year, complete irrigation produced the highest yield of essential oil, whereas in the second year, cessation of irrigation at the seed stage produced the highest yield. In the first and second year, weed competition significantly reduced the number of lateral branches (14.7 and 25.6%), the number of umbels per plant (33.1 and 21.2%), the number of seeds per umbel (17.2 and 37.2%), and the weight of 1000 seeds (9.22 and 17.2%). The percentage of essential oil was unaffected by weed interference. Delays in irrigation cessation increased grain and essential oil yield under both interference and non-interference conditions.
Conclusion: In conclusion, this experiment revealed that the response of yield and yield components to the irrigation regime depended on weather conditions and precipitation during the growing season. In conditions where the amount and distribution of rainfall was adequate during the growing period, yield and yield components did not react significantly to the irrigation regime, whereas supplementary irrigation produced the optimal grain and essential oil yield in the absence of rainfall. Weed control increased yield in both irrigated and rainfed environments. Weed competition reduces cumin yield, yield components, and the number of umbrellas per plant, but has no effect on the percentage of essential oil produced. Consequently, in wet years, full irrigation is recommended in terms of both interference and non-interference of weeds, and in terms of appropriate irrigation, cessation of irrigation after seeding is recommended to produce optimal economic performance.