Weed dynamics and crop productivity under different establishment methods in rice-wheat system

Introduction

Can the way farmers establish their rice and wheat crops really alter the weed flora and final grain harvested? This question matters because the rice-wheat cropping system spans more than 13.5 million hectares across South and West Asia [1] . In Iran, the same rotation dominates irrigated lowlands, yet average yields remain below genetic potential [2] . Weeds are often the single largest biotic constraint. Uncontrolled weed growth can reduce rice grain output by 30-50% [3] . Conventional transplanted rice relies on standing water to suppress weeds, but labour shortages and rising water costs have pushed growers toward direct seeding [4] . Direct seeded rice loses the weed-suppressive effect of puddling, allowing grasses and sedges a head start [5] . The System of Rice Intensification (SRI) uses younger seedlings, wider spacing, and alternate wetting-drying water management. Some trials report yield gains of 15-20% over conventional transplanting, partly because mechanical weeding removes weeds at a critical stage [6,7] . On the wheat side, zero-tillage reduces turnaround time after rice harvest and preserves soil structure but can shift the weed spectrum toward perennial species [9,10] . Most comparisons focus on either rice or wheat independently. Fewer investigations have tracked weed dynamics across both crops within the same rotation over multiple seasons [11] . The present research was designed to: 1) Compare weed density, dry biomass, and diversity under five establishment methods; 2) Measure grain yields across four seasons; and 3) Quantify the relationship between weed pressure and yield loss in Tehran province, Iran [13] .

Material and Methods

Research area description

The experiment was carried out at the Agronomy Research Station of the University of Tehran, Karaj (35°44′N, 51°01′E, elevation 1321 m). The area has a semi-arid continental climate with mean annual rainfall of 243 mm and mean annual temperature of 14.6°C [14] . Soil at the site was a silty clay loam (pH 7.6, organic carbon 0.81%, available N 187 kg ha⁻¹, available P 14.3 kg ha⁻¹, available K 238 kg ha⁻¹).

Material

Rice variety Hashemi (medium-duration, ~135 days) and wheat cultivar Sivand (semi-dwarf, ~170 days) were used. Fertiliser sources were urea (46% N), triple superphosphate (46% P₂O₅), and muriate of potash (60% K₂O). Pre-emergence pendimethalin (1.0 kg a.i. ha⁻¹) was applied to DSR plots, and post-emergence bispyribac-sodium (25 g a.i. ha⁻¹) to transplanted and SRI plots as needed.

Field experimental design

A randomized complete block design with four replications was used (20 plots per season, each 5 m × 4 m). Five treatments were: (T1) conventional transplanting at 20 × 15 cm; (T2) SRI with 12-day-old seedlings at 25 × 25 cm; (T3) dry-seeded DSR at 20 cm row spacing; (T4) wet-seeded DSR on puddled soil; and (T5) zero-till wheat into anchored rice stubble. A 1.5 m buffer separated plots.

Methods

The trial ran from June 2021 through April 2023 covering two rice and two wheat seasons. Weed sampling was done at 30, 60, and 90 DAS/DAT using a 0.25 m² quadrat at four spots per plot. Weeds were identified, counted, oven-dried at 70°C for 48 hours, and weighed. A net harvest area of 6 m² per plot was used for grain and straw yield. Data were analysed by ANOVA in R (v4.2.3) with Tukey's HSD test at p≤0.05 [15] . Seasonal trends (Figure 2) showed stable performance within each treatment. SRI remained the top rice treatment in both kharif seasons. Zero-till wheat showed a slight upward trajectory, possibly reflecting improved soil conditions with continued residue retention. Regression of weed density on grain yield gave R² = 0.74 (p<0.001), with each additional 10 weeds m⁻² reducing yield by ~0.18 t ha⁻¹.

Results

Discussion

The higher weed density in DSR plots aligns with earlier reports where the absence of puddling and standing water allowed weed flushes within the first two weeks after sowing [3,5] . SRI's combination of wider spacing and mechanical weeding at 10day intervals effectively disrupted weed establishment. The lower weed biomass under SRI may be linked to the vigorous root system promoted by single-seedling transplanting [7] . A 9.7% yield advantage for SRI across four seasons matches meta-analyses covering tropical Asia, where SRI gains averaged 8-12% [6] . The poor performance of DSR-dry underscores the trade-off in direct seeding: it saves labour and water at establishment but demands more precise weed management, which wasn't always achievable under semi-arid conditions [4] . Zero-till wheat's steady yield despite a distinct weed spectrum is notable. Retention of rice residues on the surface seems to have provided a physical barrier against early weed emergence [9,10] . But a subtle shift toward Convolvulus arvensis and Chenopodium album by the second year could become problematic without herbicide rotation. The inverse weed density-yield relationship (R² = 0.74) can serve as a rough economic threshold for weed management decisions [15,16] .

Conclusion

This two-year research across four crop seasons showed clear differences in weed dynamics and productivity among the five establishment methods. SRI emerged as the most effective approach, combining the lowest weed density with the highest mean grain yield of 5.36 t ha⁻¹. Conventional transplanting performed reasonably but required more water and labour without additional yield advantage. Direct seeded rice suffered from severe weed pressure, especially during the first 30 days. DSR-dry recorded weed densities nearly double those of SRI and correspondingly lower yields. Without highly effective weed management, direct seeding appears risky under semi-arid conditions.

Zero-till wheat maintained competitive yields while reducing tillage costs, though gradual appearance of perennial broadleaf weeds warrants attention. The negative linear relationship between weed density and yield provides a practical benchmark: every 10-plant increase per square metre costs roughly 0.18 t ha⁻¹ in grain output. Future work should extend these findings across wider soil types and climatic zones in Iran.

References

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