Introduction

Rice (Oryza sativa L.) is the most important staple food crop for more than half of the world’s population. It is grown in more than 100 countries across the globe. In India, it is cultivated over  43.79 mha area with a  production of 112.91 mt of milled rice and an average productivity of  2578 kg ha^-1  (DES, 2018). In Andhra Pradesh, rice is grown in an area of 2.1 m ha with an annual production of 12.0 mt and a productivity of 5.70 t ha^-1 (Anonymous, 2017). Godavari delta with alluvial soils is the rice bowl of Andhra Pradesh, is one of the major contributors of rice production in India. Rice-rice is the predominant cropping system in this deltaic region under canal irrigated conditions. Furthermore, continuous cultivation of rice for longer periods and often with poor agronomic management especially improper or inadequate weed management practices, results in substantial yield loss (Tripathi, 1992; Dwivedi et al., 2001) and decline in factor productivity (Yadav, 1998). On the other hand, organic farming or traditional farming is gaining importance across the world and more particularly in India. It is one of the better options to improve the nutrient availability in deltaic alluvial soil by way of improving the microbial activity in the soil (Ramanjaneyulu et al., 2013). Available literature shows that, higher number of tillers m^-2 were produced in organically enriched soils (Kumar et al., 2014). Of many problems in organic farming, weed management is the major challenge as weeds have to be effectively controlled through non-chemical methods in organic production system. Traditionally, hand weeding has been in vogue in rice ecosystem. Of late, demand and usage of herbicides for timely and effective weed control has gained importance. However, neither chemicals can be used nor only one weed management method can be adopted as weed menace is the main problem in organic farming. The total annual loss in agriculture due to different pests is about 6000 crores in India. More than one third of the total loss (33–45%) is attributed by weeds alone (Mukherjee, 2006 and Manhas et al., 2012). This loss may be still more which is not quantified in case of organic production system. Furthermore, controlling weeds in rice-rice sequence is a difficult task due to poor land preparation following less turnaround time and monocropping, in low lying areas under uncontrolled canal irrigated conditions in Godavari deltaic regions (Satyanarayana, 2017). Hence, the present study is intended to study various options by adopting either a single method or combinations of methods through non-chemical practices, for effective and timely control of weeds in organic rice production system on deltaic alluvial soils.

Materials and Methods

The field experiment was conducted during kharif, 2017 and rabi, 2017–18 seasons under coastal irrigated ecosystem in deltaic alluvial soils at Regional Agricultural Research Station, Maruteru, West Godavari district, Andhra Pradesh, India. The experimental site is located at 16.38⁰ N latitude, 18.44⁰ E longitude with an average elevation of 5 m mean sea level altitude. It was laid out with seven treatments in a randomized block design (RBD) with three replications. The most common weed species existing in this Godavari delta alluvial soils are Echinochloa colona, Echinochloa crusgalli, Dactyloctenium aegyptium, Cyperus rotundus, Cyperus difformis, Cynodon dactylon, Commelina benghalensis, Ipomoea auatica, Monochoria vaginalis, Marsilea quadrifolia, Ludwigia parviflora, Ammania baccifera, Alternanthera echinata and Bergia capensis. The treatments are mainly comprised of combination of cultural and mechanical weed management practices viz., T₁: Hand weeding twice at 20-25 DAT and 45-50 DAT, T₂: One mechanical weeding at 20-25 DAT along with one manual hand weeding at 45-50 DAT, T₃: Location specific green leaf manure (Sesbania Sp.) incorporation @ 5 t ha^-1, T₄: Reduced spacing (20×12 cm² during kharif and 12×12 cm² during rabi) and incorporation of previous crop residue (paddy straw) @ 2 t ha^-1 along with one manual hand weeding, T₅: Locally available aquatic weed (Eichhornia sp.) incorporation @ 2 t ha^-1 and one manual hand weeding, T₆: Incorporation of neem cake 15 days before planting @ 5 t ha^-1 and one manual hand weeding and T₇: ITK treatment on weed control practiced by farmers (incorporation of mango leaf @ 2 t ha^-1). The experimental soil was clay loam in texture, slightly alkaline in reaction, low in organic carbon (0.43%) and available nitrogen (188 kg ha^-1), medium in available phosphorus (34.4 kg ha^-1) and high in available potassium (225.4 kg ha^-1). A medium duration rice variety MTU 1075 was planted at a spacing of 25×15 cm² during kharif season and a short duration rice variety MTU 1010 at 15×15 cm² during rabi season. All other standard agronomic and plant protection operations recommended for organic production systems for Godavari Agro-climatic zone of Andhra Pradesh were performed (Anonymous, 2016). The data were subjected to RBD analysis by using OPSTAT software and values were compared by using f-test and CD (Gomez and Gomez., 1984).

Results and Discussion

The results of experiments furnished in Table 1 revealed that, highest grain yield of 3680 kg ha^-1 was recorded with incorporation of green leaf manure (Sesbania sp.) (T3).

Table 1: Effect of non-chemical weed management practices on growth, yield attributes, yield and weed index in rice-rice cropping system (Kharif, 2017 and rabi, 2017-18)

It was at par with incorporation of locally available aquatic weed (Eichhornia sp.)+one manual hand weeding treatment (3613 kg ha^-1), but, significantly superior to rest of the treatments, during kharif season. Similar trend was observed in rabi season during which incorporation of green leaf manure (Sesbania sp.) treatment recorded highest grain yield of 3587 kg ha^-1. But, it was found to be on par with that of T5 (locally available aquatic weed (Eichhornia sp.) incorporation @ 2 t ha^-1+one manual hand weeding 3407 kg ha^-1), T₁ (hand weeding twice at 20-25 DAT and 45-50 DAT recorded 3367 kg ha^-1) but, significantly outyielded remaining weed management options. The superior performance of T3 treatment in both the seasons of experimentation was owing to considerably more no. of tillers m^-2 (392 during kharif and 373 during rabi) and panicles m^-2 (271) during rabi. Similar results are in confirm with Sarkar et al. (2015). Further, different weed management practices failed to show significant effect on plant height of rice in both the seasons, number of panicles m^-2 and straw yield during kharif season and panicle weight during rabi season which means all treatments were equal in their performance (Table 1). Though, reduced spacing (20×12 cm² during kharif and 12×12 cm² during rabi)+incorporation of previous crop residue (paddy straw) @ 2 t ha^-1 along with one manual hand weeding resulted in considerably less number  of tillers and panicles, it has produced nearly 500 kg ha^-1 (16%) less grain yield as compared to that of T₃. The yield reduction is due to high density population might have hindered the tillering ability which in turn resulted in less no. of effective tillers. This result could be supported by the findings of Mubshar et al. (2012). Though, straw yield was insignificant during kharif season, significantly higher straw yield was registered during rabi season with the treatment i.e. incorporation of locally available aquatic weed (Eichhornia sp.) along with one manual hand weeding (5327 kg ha^-1) and next best treatment was reduced spacing and incorporation of previous crop residue (paddy straw) along with one manual hand weeding treatment (5373  kg ha^-1). This might be due to more plant population leading to more of vegetative growth rather than economic yields. These findings are in accordance with Al-Mamun et al. (2013). The grain yield of transplanted organic rice was very much influenced by weed control treatments with mulching with biodegradable produces compared to other practices of weed control. These results were in confirmation with research findings of Rathod and Somasundaram (2017). Less is the weed index better is the weed control. In the current experiment, lowest values of weed index (1.0 and 0.3) were recorded with T₂ (One mechanical weeding at 20-25 DAT along with one manual hand weeding at 45-50 DAT). It means, this operation has effectively controlled the weeds better than other treatments. While, the highest weed index was recorded with location specific green leaf incorporation (Sesbania sp.) (13.8%, 6.8% and 20.6%) which is on par with locally available aquatic weed incorporation (Eichhornia sp.)+one hand weeding (11.6%, 1.2% and 12.8%) during kharif and rabi, respectively (Table 2). The T₄, T₆ and T₇ treatments registered negative weed index values due to limited weed control in these treatments which means these practices are ineffective in controlling the weeds in rice ecosystem under organic production system (Table 1).    

Though incidence of leaf folder and brown plant hopper and also diseases like sheath blight and leaf blast, was below economic threshold level in all the treatments, however, the treatment viz., green leaf manure incorporation treatment and neem cake incorporation at 15 days before planting treatments fared well with very low incidence than others (data not shown).

The data on economics in Table 2 showed that net returns (` 15250 and 14083 ha<sup>-1</sup>)and B:C ratio (1.50 and 1.46) were highest with green leaf manure (Sesbania sp.) incorporation. It was closely followed by locally available aquatic weed (Eichhornia sp.) incorporation+one manual hand weeding (` 12917 and 10333 ha^-1 of net returns and 1.40 and 1.33 B:C ratio during kharif and rabi, respectively.

Table 2: Effect of non-chemical weed management practices on the economics of rice-rice cropping system (kharif, 2017 and rabi, 2017-18)

Conclusion

Organic farming especially under monocropping of rice-rice system under uncontrolled canal irrigation faces severe weed management problems. The experimental results clearly indicated incorporation of locally available green leaf manure (Sesbania sp.) is the best way to maximize the crop productivity (average grain yield of two seasons: 3633 kg ha^-1) besides controlling the weeds to some extent. Further, research has to be continued to find out effective, low cost and yield maximizing weed management modules  under organic production systems in different rice growing ecologies across the country.

Acknowledgement

The author(s) are grateful to the Director of Research, ANGRAU, Guntur and Associate Director of Research, RARS, Maruteru for providing necessary facilities for carrying out the present investigation.

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