Introduction

Rice (Oryza sativa L.) is most important staple food for 60% of the World’s population. Rice contributes 43% of total food grain production and 46% of total cereal production in India. In 2018, rice cultivated on an area of 43.79 mha with an annual production 112.91 mt and productivity about 2.58 t ha^-1 (Anonymous, 2018). Rice is the predominant crop of West Bengal and produces more than 13% of total rice production in the country. Rice crop is adversely affected by several types of biotic and abiotic tress during their growth and development. Among biotic stress, weed competition is one of the chief yield limiting factors in rice yield. Weeds compete with crops for soil moisture, light, nutrients and space. Weeds are the greatest competitors in their initial growth stages than at later stages and hence the growth of crops becomes slow down and finally grain yield decreases (Jacob and Syriac, 2005). It was reported that weed infestation in transplanted rice reduced the grain yield of 27-68% (Yadav et al., 2018).

Weed control by mechanical and cultural means is an expensive method in transplanted rice. Especially during peak period of labour crisis, occasionally weeding becomes delayed causing severe decrease in grain yield. Therefore, due to high labour cost, non-availability of labour in time and time taken for manual weeding, farmers are enforced to decide for cheaper alternative of chemical weed control. Chemical weed control offers economic and efficient weed control if applied at proper dose and stage (Kumar and Sharma, 2005). The weed flora is very much diverse, so single application of one herbicide does not provide satisfactory weed control throughout the crop growth period. Regular use of a single herbicide may develop herbicide resistance in weeds. Therefore, use of combined application of herbicides in the form of tank-mix or ready-mix having different mode of action have a capacity to control broad spectrum weed as well as delaying the development of herbicide resistance. Application of herbicide mixtures may be useful for controlling broad-spectrum weeds in rice. Post emergence application of mixture of pyrazosulfuron ethyl +molinate at 2000 g a.i. ha^-1 is very effective against broad spectrum of weeds (Acharya and Bhattacharya, 2013). Similarly, post-emergence tank-mix application of bispyribac-Na with pre-mix chlorimuron+metsulfruon recommended for controlling complex weed flora and enhancing productivity of transplanted rice (Kaur et al., 2016). New herbicide XR-848 Benzyl Ester+Cyhalofop Butyl 12% EC (w/v) at 150 (25+125) g ha^-1 at 2-3 leaf stage control weed and produced more grain yield of rice (Mahapatra et al., 2017). Pyrazosulfuron ethyl+Pretilachlor (ready-mix) at 3.5 kg ha^-1 as post emergence is very effective against the composite weed flora in transplanted rice (Mondal et al., 2018). Early post-emergence triafamone+ethoxysulfuron (pre-mix) is very much effective against complex weed flora in transplanted rice (Yadav et al., 2019).

In view of the above, the present study is undertaken to look out the broad spectrum weed control through a new early post-emergence ready-mix herbicide combination Fenoxaprop 5%+Chlorimuron 0.6%+Pretilachlor 50% ME on transplanted kharif rice.

Materials and Methods

This study was conducted during Kharifseasons of two consecutive years i.e.2017 and 2018, respectively at the Agricultural Farm, Institute of Agriculture, Visva-Bharati, Sriniketan, Birbhum district in West Bengal, India. The location is situated at20^°39^'N latitude  and 87^°42^/E longitude with an average altitude of 58.9 mamsl under typical semi-arid tropical climate. The total rainfall received for the experimental period was 426.6 mm during 2017 and 675.3 mm during 2018. The soil of the experimental site was sandy loam in texture. The fertility status of the soil was 152.8 kg ha^-1alkaline permanganate oxidizable nitrogen (N) (Subbiah and Asija, 1956), 13.2 kg ha^-1 available phosphorus (P) (Bray and Kurtz, 1945), 149.8 kg ha^-1 1 N ammonium acetate exchangeable potassium (K) (Hanway and Heidel, 1952) and 0.51% organic carbon (Walkley and Black, 1934). The pH of the soil was 6.15 (1:2.5 soil: water ratio) (Prasadet al., 2006). There are overall 10 treatments viz. Fenoxaprop 5%+Chlorimuron 0.6%+Pretilachlor 50% ME (800 ml ha^-1), Fenoxaprop 5%+Chlorimuron 0.6%+Pretilachlor 50% ME (1000 ml ha^-1), Fenoxaprop 5%+Chlorimuron 0.6%+Pretilachlor 50% ME (1200 ml ha^-1), Chlorimuron ethyl 25% WP (24 g ha^-1), Fenoxaprop-p-ethyl 9.3% w/w EC (625 ml ha^-1), Pretilachlor 50% EC (1500 ml ha^-1), Pyrazosulfuron ethyl 10% WP (150 g^-1 ha), Hand weeding  at 15 and 30 days after transplanting (DAT), weed free and weedy check. The treatments were replicated thrice in randomized complete block design keeping individual net plot size of 4×5 m². Rice variety ‘Sahbaghi’ was transplanted on 26^th July, 2017 during first year and 28^th July, 2018 during the second year at 20×10 cm² plant geometry using 2-3 seedlings hill^-1 and it was 115-120 days crop. A general dose of N:P₂O₅:K₂O was applied uniformly to each plot @100:50:50 kg ha^-1 along with farm yard manure @ 5 t ha^-1. One fourth quantity of nitrogen and full amount of phosphorus and potassium were applied in each plot as basal on the day of transplanting. Rest three forth quantity of nitrogen was applied in two splits as top dressing i.e. half at active tillering stage and rest one fourth at panicle initiation stage. After sowing, a light irrigation was given to the crop for uniform germination. Herbicides were applied 10 days after transplanting (DAT) in the experimental plot as per treatment in both the years. Herbicide was sprayed on 5^th August during 2017 and 7^th August during 2018 with the help of battery operated knap-sack sprayer fitted with flat-fan nozzle using spray volume of 500 l ha^-1. Data on density and dry matter of weeds were recorded at 30 days after herbicide application i.e. 40 DAT with the help of 0.25 m² quadrate selected randomly in each plot. After identifying, the weed species were grouped into grassy, broadleaved and sedge separately. Weed density was calculated on the basis of the total number of an individual weed species m². On the basis of weed data, different weed indices were computed using the standard procedure as following details:

2.1.  Weed control efficiency (WCE)

Weed control efficiency was computed by adopting the following formula given by Mani et al. (1973) as follows:

WCE (%)=(Weed dry weight in weedy check (g)-Weed dry weight in treatment (g))÷Weed dry weight in weedy check (g)×100

2.2.  Weed index (WI)

Weed index is the per cent reduction in crop yield under a particular treatment due to the presence of weeds in comparison to weed free plot as suggested by Gill and Kumar (1969). This is used to assess the efficacy of a herbicide. Lesser the weed index, better is the efficiency of a herbicide. It is expressed in percentage and was determined with the help of following formula:

WCE (%) =((X-Y)÷X)×100

Where, WI = Weed index; X = Crop yield from weed free plot and Y = Crop yield from the treated plot for which weed index is to be worked out. Crop was harvested at physiological maturity and data on yield attributes and yield were recorded. Before statistical analysis, the data on density of weeds and dry weight of data were subjected to square root (√ x+0.5) transformation to improve the homogeneity of the variance (ANOVA) separately for each year. The significant treatment effect was judged with the help of ‘F’ test at the 5% level of significance.

Results and Discussion

3.1.  Weed flora

Weed flora composition of the experimental field during two years of investigation consisted of grassy weeds like Echinochloa crus-galli and Panicum repens sedges like Cyperus iria, Cyperus difformis, Fimbristylis miliacae and Cyperus rotundus and broad leaved weeds like Eclipta alba, Ludwigia parviflora, Marsilea quadrifolia and Monochoria vaginalis at the initial stages before application of the herbicide.

3.2.  Effect on density of weeds

The results of experiments furnished in Table 1 revealed that significant reduction in all the weed control treatments with respect to weed density of grassy, broadleaved and sedge over the weedy check (Table 1).

Table 1: Effect of weed control treatments on total density and dry weight weed at 40 days after transplanting

The highest reduction in density of weeds were recorded under two hand weeding due to complete removal of the weeds whereas fenoxaprop+chlorimuron +pretilachlor at 1200 ml ha^-1 found more superior among the herbicides treatments in reducing the weed density of grassy weed (2.67 number m^-2), broad leaved weed (5.72 number m^-2) and sedges (6.29 number m^-2) during 2017 whereas during 2018, it was 2.64 number m^-2 for grassy weed, 4.77 number m^-2 for broad leaved weed and 6.56 number m^-2 for sedges followed by fenoxaprop+chlorimuron+pretilachlor at 1000 ml ha^-1 as compared to weedy check.Sole application of chlorimuron ethyl, fenoxaprop-p-ethyl, pretilachlor and pyrazosulfuron ethyl were less effective in controlling complex weeds flora as compared to their ready-mix application during both the years of experiment. The ready-mix herbicide fenoxaprop+chlorimuron+ pretilachlor provided higher order of performance in terms of reducing weed density of broad-leaf, sedge and grassy weed. These findings are in accordance with Teja et al. (2016), Yadav et al. (2018) and Yadav et al. (2019). This combination exhibit property of foliar activity that inhibits the synthesis of fatty acids in the meristem tissues, inhibits plant enzyme acetolactase synthase thereby, blocking branches chain of amino acid biosynthesis and inhibits growth and reduces cell division. Due to this, phloem transport of the plant is hampered. A secondary effect is stunted growth on account of cessation of cell division and slow plant death (Meena et al., 2019).

3.3.  Effect on dry weight of weeds

All the herbicides reduced the dry weight of grassy weeds, broad-leaf weeds and sedges significantly as compared to the untreated weedy check at 40 DAT (Table 1). The dry weight of grassy weeds Echinochloa crus-galli and Panicum repens,dry weight of broad leaved weeds Eclipta alba, Ludwigia parviflora, Marsilea quadrifolia and Monochoria vaginalis as well as dry weight of sedges Cyperus iria, Cyperus difformis, Fimbristylis miliacae and Cyperus rotundus decreased with increase in dose of fenoxaprop+chlorimuron+pretilachlor during both the years.Fenoxaprop+chlorimuron+pretilachlor1200 ml ha^-1 and 1000 ml ha^-1 were superior to chlorimuron ethyl (24 g ha^-1), fenoxaprop-p-ethyl (625 ml ha^-1), pretilachlor (1500 ml ha^-1) and pyrazosulfuron ethyl (150 g ha^-1) and weedy check during both the years. Many researchers reported that application of herbicides used in combination have been very effective against complex weed flora in transplanted rice (Teja et al., 2016, Yadav et al., 2018 and Yadav et al., 2019).

3.4.  Weed control efficiency

The data on weed control efficiency against total grassy weed, broad leaved weed and sedge weed were presented in Table 2.

Table 2: Weed control efficiency of different treatments against total grassy, broad leaved andsedge weed at 40 days after transplanting

The minimum weed control efficiency was observed in weedy check (0.00%) whereas the highest (100.0%) was recorded in a plot treated with weed free plot and hand weeding at 40 DAT. Weed control efficiency of fenoxaprop+ chlorimuron+pretilachlor increased with increase in its dose from 800 ml ha^-1 (79.61 and 78.81%) to 1000 ml ha^-1 (81.60 and 82.95%) and 1200 ml ha^-1 (84.90 and 84.18%) against total grassy weeds during first and second year, respectively. Similarly, weed control efficiency increased with increase in its dose from 800 ml ha^-1 to 1200 ml ha^-1 against total broad leaved and sedge weeds. Weed control of efficiency of fenoxaprop+chlorimuron+pretilachlor at 1000 ml ha^-1 and 1200 ml ha^-1 was higher than chlorimuron ethyl (24 g ha^-1), fenoxaprop-p-ethyl (625 ml ha^-1), pretilachlor (1500 ml ha^-1) and pyrazosulfuron ethyl (150 g ha^-1). Thus, based on two years data, 1200 ml ha^-1 or 1000 ml ha was recognized the most optimal dose of fenoxaprop+chlorimuron+pretilachlor. This result was in accordance with Yadav et al. (2019) who reported that penoxsulam+butachlor (ready-mix) reduced the weed infestation in transplanted rice as well as enhanced weed control efficiency against diverse weed flora in comparison with the weedy check.

3.5.  Effect on crop

There was significant effect of herbicidal treatments on number of panicles m^-2, test weight, number of grains panicle^-1 and grain yield of the crop during both the years (Table 3).

Table 3: Effect of different treatments on yield attributes, grain yield and weed index of transplanted rice

Conclusion

Fenoxaprop+chlorimuron+pretilachlor 1000 ml ha^-1 applied as spray in 500 L water ha^-1 at 10 DAT provided effective control of complex weed flora in transplanted rice and increased grain yields.

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