Two field experiments were conducted at Ngala in the Lake Chad Basin during the 1981 and 1982 wet seasons to investigate chemical weed control in direct-seeded, irrigated rice (Oryza sativa L.). Pre-plant-incorporated, pre- and post-emergence herbicides and their mixtures were tested in replicated, completely randomized block trials. Rice variety IR 28 was drilled each season at 125 kg/ha and fertilized with N (150 kg/ha) and P2O5 (40 kg/ha). In both seasons, the post-emergence herbicides propanil + oxadiazon at 3·0 + 1·0 kg a.i./ha, propanil+fluorodifen at 1·4 + 1·8 kg a.i./ha, propanil + bentazone at 3·6 + 1·5 kg a.i./ha and propanil at 3·6 kg a.i./ha as well as thiobencarb applied pre-emergence at 5·0 kg a.i./ha controlled the weeds effectively and gave similar grain yields to that from hand-weeding twice, and significantly higher grain yield than the unweeded control treatments. Grain yields similar to those from the ‘two hand-weeding control’ were achieved also by use of the herbicide dymrone at 3·0 kg a.i./ha, either soil-incorporated pre-sowing or sprayed before weed emergence. The herbicides gave satisfactory control of the weeds, which were predominantly the broad-leaved weed Luffa echinata Roxb., together with the grasses Sorghum aethiopicum (Hack.) Rupr. ex Stapf., Echinochloa colona (L.) Link, Eleusine indica (L) Gaertn, Digitaria sanguinalis (L.) Scop., Oryza longistaminata (Chev. & Roehr.), and sedges Cyperus rotundus L. and Cyperus esculentus L. The studies indicate that effective weed control by these herbicides and herbicide mixtures are dependable alternatives to hand weeding for direct-seeded, irrigated rice cultivation in the Chad Basin area.
Present address: School of Agriculture, Tafawa-Balewa College, Ahmadu Bello University, PMB 0248, Bauchi, Nigeria.
Eleven treatments including PRE and POST herbicide combinations ( Table 1 ) were evaluated in a randomized complete block design (RCBD) with three replications at each location. Herbicides included in the study were butachlor (Butaveer ® , Chambal Fertilizers & Chemicals Ltd.), oxadiargyl (Topstar ® , Bayer Crop Science), pendimethalin (Stomp ® , BASF India Ltd.), bispyribac-sodium (Nominee gold ® , PI Industries), and azimsulfuron (Segment ® , Dupont India Ltd.).
Rice cultivar tolerance to different rates of azimsulfuron in dry-direct seeded rice, experiment 2 (2010–2011).
2.1.2. Experimental design and treatments
Analysis of variance P values for the crop injury and yield reduction for experiment 2 (2010–2011).
The traditional methods of weed control in rice include hand-weeding by hoe or hand pulling, but this is becoming less common because of labour scarcity at critical time of weeding and increasing labour costs (Chauhan, 2012, Kumar and Ladha, 2011). Moreover, seedlings of some grassy weeds such as Echinochloa crus-galli (L.) look similar to rice seedlings (Rao and Moody, 1987, Rao and Moody, 1988), making hand weeding more tedious, difficult, and less effective. However, adoption of DSR technology usually leads to shift in weed flora composition towards difficult-to-control weeds (Singh et al., 2013). In this situation, use of herbicides is becoming more popular in DSR because they are more effective, easy to apply, provide selective control, saves on labour and costs less.
*General ground coverage (45 DAS) by weeds in non-treated area at respective locations are given in parentheses ( ).
Chemical weed control is being recommended instead of manual weeding as it is cost-effective and less labour dependent. With the availability of herbicides and associated weed management technology, it is possible to improve the yield of direct-seeded rice through chemical weed control (Mishra & Singh, 2008 ). It mustn’t be overlooked that all pesticides are venomous; they must be handled safely so as to avoid excessive and costly wastes, environmental concerns, crop damage, and damage to adjacent crops by spray drift, injury to the applicator, excessive contamination and residues, and injury to beneficial organisms. It is advisable to rotate the herbicide combination in each year for delaying the development of herbicide resistance in weeds (V. P. Singh et al., 2016 ). This study is meant to determine the most suitable weed management practices to control weeds in the direct-seeded spring rice and to work out their economics. The study is also carried out to compare the performance and yield of spring rice under various weed management techniques.
The weeds pose to be more cumbersome in direct-seeded rice (DSR) than in puddled transplanting because the emerging weeds are more competitive as compared to simultaneously emerging DSR seedlings and lack of water in wet and dry direct-seeded rice make these crops more prone to initial weed infestation which lacks otherwise in transplanting (Kashiwar et al., 2016 ). Herbicide-based weed management in large-scale rice farming has become the smartest and most viable option. Therefore, it is must to use herbicide judiciously (Anwar et al., 2012 ). Other herbicides that were found effective in DSR were pyrazosulfuron and oxadiargyl as pre-emergence and azimsulfuron, penoxsulam, cyhalopfop-butyl, and ethoxysulfuron as post-emergence (Rao et al., 2007 ). Herbicides may be a viable alternative to hand weeding (Chauhan & Johnson, 2011 ). Traditionally, weeds were controlled through manual weeding. Manual weeding, though effective, is getting increasingly difficult due to labour scarcity and rising wages rates (Rao & Ladha, 2013 ).
Rice is the world’s most important staple food crop for more than half of the world’s population (Kumar & Ladha, 2011 ). The world’s total rice areais 167 million ha and production is about 782 million tons with productivity of 4.67 Mt ha −1 . The productivity of rice in Southeast Asia is 4.41 tons ha −1 and China is the largest producer (FAOSTAT, 2018 ). In Nepal, rice is the chief staple food followed by maize and wheat (Pradhanang et al., 2015 ). It is grown on 1,469,545 ha (around 47.5% of the total cultivated area) and despite being on the first position among all cereal crops in Nepal the production and productivity of rice is 5,151,925 tons and 3505 kg ha −1 respectively which is considered very low (FAOSTAT, 2018 ). Jhapa district ranked second in terms of the area of production (87,500 ha) and productivity (3.86 Mt ha −1 ) (Ministry of Agricultural Development [MOAD], 2017 ).
2.1. Experimental site
The experiment was conducted in the command area of Prime Minister Agriculture Modernization Project (PMAMP), Project Implementation Unit (PIU), Rice zone, Jhapa at Baniyani-6, Kechanakawal Rural Municipality. The site was situated at 16.093 km south-west of Bhadrapur, headquarter of Jhapa district. The experimental field was 61 meters above the mean sea level and located geographically at 27° 36.22ʹN latitude and 84° 19.073ʹE longitude.
A field experiment was conducted during the spring season of 2019 to determine the effects of various weed management practices on weed dynamics and productivity of wet direct-seeded spring rice at Baniyani, Jhapa. The experiment was laid out in a Randomized complete block design (RCBD) with five treatments and four replications. The treatments consisted of Pretilachlor 50% EC (dose: 900 ml a.i. ha −1 ) as a pre-emergence herbicide, Bispyribac sodium 10% SC (dose: 240 ml a.i. ha −1 ) as a post-emergence herbicide, hand weeding (at 30 DAS and 60 DAS), weed-free (weeding at every 15 days interval) and weedy check as control. The rice variety used in the experiment was Hardinath-1. The plot treated with Bispyribac sodium recorded a significantly higher number of effective tillers per meter square and grains per panicle at 90 DAS while it recorded the lowest weed density (9.17 weeds m −2 ) at 60 DAS. Sedges were dominant as compared to broadleaf and grasses. Sterility percentage and 1000-grain weight were not affected by various weed management techniques. Bispyibac sodium was found statistically superior in terms of grain yield (6.97 tons ha −1 ), straw yield (7.78 tons ha −1 ) and harvest index (47.29%). The experiment concluded that there was a reduction in yield by 67.9 percent due to the presence of weed as compared to weed-free. The economic analysis revealed that the post-emergence herbicide Bispyribac sodium was beneficial as compared to other weed management practices in terms of gross returns, net returns and BC ratio.
Rice is generally cultivated as transplanted, wet-seeded, and dry-seeded. Dry direct-seeded rice is sown directly into a prepared seedbed on dry soil or under zero-till condition. In wet-seeded rice, pre-germinated seeds are broadcasted or sown into the puddled soil surface. In contrast, wet‐seeding involves sowing pre-germinated seed, with a radicle varying in size from 1 to 3 cm, on or into puddled soil (Kumar & Ladha, 2011 ; Rao et al., 2007 ). Low wages and adequate availability of water favour transplanting, whereas high wages and low water availability favour DSR (Pandey & Velasco, 2005 ). High yielding and improved varieties, proper nutrient and weed management techniques drew the farmers towards DSR culture instead of the traditional transplanting system. DSR requires less labour, energy, time, irrigation water, reduces the emission of greenhouse gasses and ensures the proper growth of succeeding crops as well (Kaur & Singh, 2017 ; Kumar & Ladha, 2011 ). Puddling and transplanting operations consume a significant quantity of water; in some cases, up to 30% of the total rice requirement (Chauhan, 2012 ) and puddled soil creates a hardpan below the plough layer and reduces soil permeability and soil structure. This triggers the farmers to be shifted from manual transplanting to the DSR system (Hossain et al., 2016 ). When motorized broadcasting was used, the pre-germination period was shortened to ensure short radicles for ease of handling and to minimize damage, as was the case when drum‐seeders for row‐seeding are employed (Balasubramanian & Hill, 2002 ).
The soil texture of the research plot was found to be loam. The soil was acidic (pH: 4.6) in nature. The soil contains medium (0.17%) amount of nitrogen, low amount of phosphorus (21.98 Kg ha −1 ) and a low amount of potassium (103.2 Kg ha −1 ). The organic matter content of the soil was medium (3.50%) (Table 1). The experimental field was only used to plant the rice and kept follow rest of the year.