Nearly half of the global population depends primarily on rice (Oryza sativa L.) as a key source of nutrition (Kumar et al., 2016) and it serves as a staple food in India as well (Pathak et al., 2019). India has the world’s largest area under rice cultivation, where West Bengal is the leading rice-growing state, producing 16.65 million tonnes from 5.58 million hectares, with an average productivity of 2,984 kg ha^-1 (boro season: 3,570 kg ha^-1), accounting for 13.62% of India’s total production (Government of India, 2022).
       
Boro rice cultivation in West Bengal relies heavily on irrigation and fertilizers, which often encourages severe weed infestations, causing significant yield losses. Unchecked weed growth in boro rice can reduce yields by 33-45% (Manhas et al., 2012). Early growth stages are especially vulnerable due to slow rice growth at low temperatures, giving fast-growing weeds an advantage. Although cultural, physical and biological methods exist, manual weeding remains common due to its effectiveness and safety. However, labour shortages and high costs make this less viable. On the other hand, herbicides provide rapid and cost-effective weed control. However, repeated large-scale use of the same herbicides in rice fields have caused weed resistance, reduced beneficial soil microbes and contaminated water resources. Therefore, there is a need for new herbicide formulations at optimal doses that are both ecologically safe and economically viable for farmers.
       
Quinchlorac (3,7-Dichloroquinoline-8-carboxylic acid) (Fig 1) is a post-emergent, systemic herbicide classified under Group 4 (synthetic auxins) with selective, broad-spectrum action. It disrupts weed growth by mimicking natural auxins, increasing ethylene production, inhibiting cell wall formation (Koo et al., 1997) and generating reactive oxygen species, intensifying its effect on grassy weeds (Grossmann, 2010; Sunohara et al., 2011). It was first introduced in 1992 for controlling Echinochloa spp. in rice (Talbert and Burgos, 2007). But since then, it was not widely popular due to its certain disadvantages.


       
The new soluble concentrate (SC) formulation of quinclorac is claimed to overcome these limitations by offering ease of handling, minimal visible residues and no need for agitation during application. Therefore, in this experiment, the new formulation at different doses was compared with manual hand-weeding and widely used rice herbicide, bispyribac sodium, which is also a post-emergent-systemic herbicide, to assess the impact of quinchlorac 35% SC on weed flora, soil nutrient, microbial status and production economics in transplanted boro rice.

The field study was carried out in 2021-22 and 2022-23 during the rabi season at Jaguli Instructional Farm of Bidhan Chandra Krishi Viswavidyalaya in Nadia, West Bengal, India. The experiment was carried out on the sandy loam textured soil of order Inceptisol with proper drainage facility, neutral pH, medium organic carbon, low available nitrogen, medium available phosphorous and potassium. The experimental design employed was randomized complete block design (RCBD) consisting of seven treatments, replicated thrice. The plot size was 4 m × 3 m each. Seedlings were transplanted in the main field at 43 DAT in 2-3 cm puddled soil depth, at 20 cm × 15 cm spacing. Recommended dose of 120 Kg N, 60 kg P₂O₅ and 60 Kg K₂O were applied respectively through Urea, Single Superphosphate (SSP) and Muriate of Potash (MOP) in the experimental plot. From transplanting to the maximum tillering stage, a shallow water depth of 2-3 cm was maintained. Thereafter, the water level was increased to 5 cm and kept at this depth until the flowering stage. Subsequently, the irrigation depth was gradually reduced and finally discontinued 10 days before harvest. In this process, 8 irrigations were given.
       
The treatments are T₁: Quinchlorac 35% SC @ 175 g a.i. ha^-1 (500 ml ha^-1), T₂: Quinchlorac 35% SC @ 210 g a.i. ha^-1 (600 ml ha^-1), T₃: Quinchlorac 35% SC @ 245 g a.i. ha^-1 (700 ml ha^-1), T₄: Standard Bispyribac Sodium 10% SC @ 25 g a.i. ha^-1 (250 ml ha^-1), T₅: Quinchlorac 35% SC @ 490 g a.i. ha^-1 (1400 ml ha^-1), T₆: Hand weeding twice at 20 and 40 DAT, T7: Weedy check. The herbicides applications were done at 15 days after transplanting (DAT). The observations for weed density, weed biomass and weed control efficiency were taken at 30, 60 and 90 DAT respectively. Weed control efficiency (WCE) was calculated on the basis of weed dry weight as per the formula (Mani et al., 1973) given below:

 
Where;
WCE = Weed control efficiency (%).
DWT = Dry weight of weed in treated plot (g m^-2).
DWC = Dry weight of weed in control plot (g m^-2).
       
Visually the phytotoxicity symptoms were observed in the experimental field at 7, 15, 22 and 30 days after application of the herbicides and rated using a 0-10 rating scale based on the percentage of crop injury, where 0 = no injury (0%), 1 = 1-10%, 2 = 11-20%, 3 = 21-30%, 4 = 31-40%, 5 = 41-50%, 6 = 51-60%, 7 = 61-70%, 8 = 71-80%, 9 = 81-90% and 10 = 91-100% injury (Rao, 2000). Severe yield loss can be seen after 5 scale. The soil samples were collected from the inter-row space of the field at a depth of 0-15 cm and the microbial counts and soil nutrient status were analysed chemically before treatment application (baseline) and during harvest. The microbial counts were analysed using the pour plate method in accordance with Pramer and Schmidt (1965). The available nitrogen is analysed by using alkaline potassium permanganate (Subbiah and Asija, 1956), available phosphorus by using 0.5 M sodium bicarbonate solution (Olsen et al., 1954) and available potassium by using 25 ml neutral ammonium acetate (1N). Following the steps outlined by Gomez and Gomez (1984), the collected data for all parameters from the experimental site were statistically analyzed. The weed data were subjected to square root transformation to fulfil the assumptions required for valid statistical inference, as recommended by Panse and Sukhatme (1978).

Weed growth
 
It was observed that in the experimental site, the grassy weeds (58%) dominated, followed by the sedge weeds (35%). The dominance order of the weeds present in the field were Cynodon dactylon (L.)> Cyperus rotundus (L.)> Echinochloa colona (L.)> Cyperus difformis (L.)> Echinochloa crusgalli (L.)> Leersia hexandra (L.)> Fimbristylis  miliacea> Marsilea quadrifolia> Alternanthera philoxeroides > Ammania baccifera (L.)> Ludwigia parviflora> Monochoria vaginalis. Presence of similar types of weed flora were being reported by many researchers in boro rice such as Banerjee et al., (2008); Prashanth et al., (2016); Bhattacharya et al., (2025) and Ghosh et al., (2025).
 
Weed growth
 
The treatment T₆ (hand weeding twice at 20 and 40 DAT) recorded the lowest weed density as well as biomass, followed by treatment T₅ and T₃ (Table 1 and 2). Irrespective of treatments, the increasing population of weeds with the crop period might be due to the regeneration of grasses after its initial control by hand weeding or herbicides. This reduction could be attributed to the diminishing effectiveness of hand weeding over time or the gradual loss of herbicidal activity due to microbial degradation of chemical residues in the soil. A similar pattern was observed for weed control efficiency, with hand weeding being most effective, followed by T₅ and T₃; the higher dose in T₅ resulted in superior weed control among herbicidal treatments, while T₁ and T₂ remained statistically at par across observations in both seasons. These findings corroborate earlier reports by Moorthy and Saha (2002); Reddy et al., (2006); Parthipan and Ravi (2014); Anusha et al., (2016) and Arthanari et al., (2017).




 
Soil microbial status
 
The results (Table 3) indicated that the highest application rate of quinchlorac (T₅) generally led to the reduction in the populations of bacteria, fungi and actinomycetes when compared to the initial levels. Conversely, lower herbicide concentrations were associated with the increased microbial abundance, with the highest populations observed in the hand-weeded treatment and the weedy check. These findings align with the conclusions of Rathod et al., (2017) and Rani et al., (2024) who reported minimal long-term adverse effects of herbicides on soil microbiota, except when applied at rates exceeding recommended levels. This resilience may be attributed to the capacity of soil microorganisms to metabolise herbicides and utilise them as sources of essential nutrients, as suggested by Bera et al., (2016).


 
Soil nutrient status
 
Available nitrogen, phosphorus and potassium has changed more or less among various treatments during harvest of the rice (Table 4). Its availability value in the harvested soil is less in T₆, followed by weedy check and more for the herbicidal treatment having higher dose of Quinchlorac (T₅). It is due to the fact that the higher herbicidal dose has even created phytotoxicity in the soil microbiota, which lowers their proliferation in the soil, as a result, less utilization of the available nutrients by the micro-organisms as compared to the non-herbicidal treatments. Also, the application of higher herbicide dose resulted in effective weed suppression (Table 1 and 2), thereby improving the availability of soil nutrients for crop utilization. Although the hand-weeded treatment has controlled weeds efficiently, its negligible adverse effect on soil microbiota allowed greater microbial growth and nutrient immobilization, resulting in comparatively lower soil nutrient status in T₆. Similar impact of herbicide on soil nutrient status was found by Jaysawal et al. (2023) and Hirwe et al., (2025).


 
Phytotoxicity
 
The treatment T₅ caused the highest crop injury (10-60%) up to 30 days after application (DAA), in the form of chlorosis, necrosis, wilting and stunted growth, due to its higher dose, which effectively controlled weeds but adversely affected the crop (Table 5). Other herbicidal treatments exhibited minimal phytotoxicity, limited to the initial days after application, owing to their lower doses. Similar phytotoxic effect is shown on rice crop by various post-emergent herbicides as reported by Anwar et al., (2012), Mahapatra et al., (2017) and Shivashenkaramurthy et al., (2020).


 
Production economics
 
The production economics under different weed control methods are presented in Table 6. Among the treatments, twice hand weeding incurred the highest cost of cultivation and also produced the maximum gross and net returns; however, it resulted in a comparatively lower benefit-cost (B:C) ratio due to the high labour cost involved. Among all treatments, the highest B:C ratio was recorded with T₃, whereas the lowest was observed in the weedy check. This goes in line with the findings of Reddy et al., (2006) and Ramesha et al., (2017).

Hand weeding twice at 20 and 40 DAT effectively controlled weeds in boro rice; however, it resulted in a lower benefit-cost ratio. The treatment T₅ (Quinclorac 35% SC @ 1400 ml ha^-1) achieved the highest level of weed control but caused severe phytotoxicity to the crop. In contrast, treatment T₃ provided effective weed suppression with minimal crop injury. Therefore, application of Quinclorac 35% SC @ 700 ml ha^-1 at 15 DAT (T₃) appears to be an effective and economical option for weed management in boro rice, subject to further validation across varieties and seasons.

The authors declare that there are no conflicts of interest regarding the publication of this manuscript. All authors have read and approved the final version of the manuscript and have no competing financial interests, personal relationships, or conflicts of interest that could have appeared to influence the work reported in this manuscript.