Introduction

Guar (Cyamopsis tetragonoloba L.), also known as cluster bean, is an industrial, drought-tolerant legume of significant economic importance owing to the valuable galactomannan gum stored in its seed endosperm. In arid and semi-arid agroecosystems like Gonabad (Iran), guar represents a strategic crop for crop diversification and water-use optimization. However, its productivity is severely constrained by its slow initial growth rate and poor early-season competitiveness against weeds, potentially leading to yield losses exceeding 80%. Identifying the optimal sowing date to synchronize phenological stages with favorable environmental conditions, coupled with selective and sustainable chemical/integrated weed management strategies, is pivotal to maximizing guar yield potential and competitive ability in arid climates.



Materials and Methods

A two-year field experiment was conducted during the 2022 and 2023 growing seasons at the Agricultural and Natural Resources Research Station of Gonabad, Iran (57°45’ E, 34°5’ N, 1060 m a.s.l.). The experiment was laid out in a split-plot design based on a randomized complete block design (RCBD) with three replications on adjacent, distinct plots each year. The main plots comprised four sowing dates: 5 May, 22 May, 5 June, and 22 June. Subplots included seven weed management regimes: trifluralin (2 L ha⁻¹, PPI), EPTC + dichloramid (3 L ha⁻¹, PPI), imazethapyr (0.75 L ha⁻¹, PRE), clethodim (1 L ha⁻¹, POST) + supplementary broadleaf weeding, bentazon (2 L ha⁻¹, POST) + supplementary grass weeding, imazamox (60 g ha⁻¹, POST), and a season-long weed-free control. Herbicides were applied using a calibrated manual compression sprayer (5 L capacity) operating at an operating pressure of 2–2.5 bar, calibrated to deliver a spray volume of 400 L ha⁻¹. The nozzle height was maintained at approximately 40–50 cm above the canopy/soil surface. To prevent spray drift between adjacent experimental plots, transparent plastic barriers were placed along plot borders during application. Because error variances were heterogeneous across years according to Bartlett’s test (e.g., grain yield: χ² = 38/01, P < 0/01), analysis of variance (ANOVA) was performed separately for each year using SAS (v9.2), and treatment means were compared using the Protected LSD test at P < 0/05.



Results and Discussion

Weed flora analysis revealed that broadleaf weeds dominated the field community (accounting for ~95% of total dry biomass), primarily consisting of perennial species such as camelthorn (Alhagi spp.) and seashore laburnum (Sophora alopecuroides L.), alongside Russian thistle (Salsola kali L.), whereas annual grasses accounted for only ~5%. In the first year, the interaction effect (D×H) was not significant; the 5 June sowing date produced the highest biological yield (4604.03 kg ha⁻¹). In the second year, the D×H interaction significantly affected plant height (P < 0.05), branch number (P < 0.05), and pod number per plant (P < 0.01). The maximum plant height (97.45 cm), branch number (21.81 branches plant⁻¹), and pod number (207 pods plant⁻¹) were recorded under season-long hand weeding on 5 June. Among chemical treatments, imazamox on 22 May achieved the highest plant height (92.36 cm), whereas EPTC and bentazon on 5 June yielded the highest branching (17.78 branches plant⁻¹), and EPTC on 22 May produced the highest pod number (186.12 pods plant⁻¹). Across both seasons, the 5 June sowing date consistently maximized grain yield (1087.98 and 1633.91 kg ha⁻¹ in 2022 and 2023, respectively) and 1000-seed weight (41.28 and 45.70 g). Early planting (5 May) hindered initial seedling establishment due to low soil temperatures, whereas late planting (22 June) abbreviated the vegetative period, forced premature reproductive onset, and coincided seed filling with autumn temperature decline. Among weed controls, season-long weeding was superior, followed by clethodim and bentazon combined with supplementary weeding. Conversely, trifluralin exhibited poor efficacy due to soil incorporation limitations and high summer volatilization under dry soil conditions.



Conclusion

The findings of this two-year investigation demonstrate that sowing guar during the first half of June (particularly around 5 June) provides optimal thermal and environmental conditions to enhance vegetative development, optimize yield components, and maximize both seed and biological yields under the arid environment of Gonabad. In contrast, early sowing (5 May) significantly suppressed yields due to sluggish seedling establishment under lower early-season temperatures, while delayed sowing (22 June) induced premature reproductive transition and exposed the pod-filling stages to suboptimal autumn temperatures. Regarding weed interference, guar exhibited severe vulnerability to early-season competition, with season-long weed-free conditions securing the highest productivity. Among chemical tactics, although bentazon and clethodim combined with supplemental hand weeding showed high agronomic efficacy, escalating labor costs make pre-plant incorporated EPTC and post-emergence imazamox more economically viable alternatives. Overall, synchronizing early-June sowing with targeted, cost-effective weed management provides a sustainable agronomic strategy to optimize environmental resource-use efficiency and stabilize guar production across the arid agroecosystems of southern Razavi Khorasan Province.