Effect of spacing and weed management practices on yield parameters and yield of direct-seeded rice

Rice (Oryza sativa L.) is the stable food for more than 60 per cent of the world population and its cultivation secures a livelihood for more than two billion people (Manisankar et al., 2020). Rice is grown on 167 million ha worldwide, with Asia accounting for 146 m ha. Above 90 per cent of the globally produced rice is consumed in Asia. In India, rice is grown in an area of 45.7 m.ha with a production of 124.3 m.t and an average productivity of 2.7 t ha^-1. In Tamil Nadu, rice is cultivated in an area of 2.2 m.ha with a production of 7.9 m.t and a productivity of 3566 kg ha^-1 (Indiastat, 2021).
       
In Tamil Nadu, rice is being cultivated under different ecosystems viz., transplanted puddled lowland rice, direct seeded lowland rice (Wet seeded rice in puddled soil and Dry seeded rice in un-puddled soil), dry seeded upland rice and deep water rice. Most of the farmers in the intensive cropping areas are shifting from conventional transplanting to System of Rice Intensification (SRI) and direct seeded rice due to shortage of labour and scarcity of water (Rathika and Ramesh, 2018). Direct seeded rice (DSR) is one of the crop establishment methods in the field that involves sowing the seeds directly rather than transplanting the seedling from a nursery (Kaur and Singh, 2017). It is the best alternative method, which, in the absence of transplant shock, requires only 35 to 57 per cent water. DSR has one week earlier crop maturity and lowers the laborious cost over transplanted rice (Patel et al., 2018). DSR is subject to more severe weed infestation than transplanted lowland rice, because in DSR weeds germinate simultaneously with rice (Ramesh and Rathika, 2020). Weeds are a major biotic constraint to the success of direct-seeded rice compared to conventional method of rice cultivation because emerging DSR seedlings are less competitive with concurrently emerging weeds, which compete with rice for moisture, nutrients and light (Kumar et al., 2008). The control of weeds during critical period of crop-weed competition is very important so as to avoid yield loss (Ramesh and Rathika, 2016). In direct seeded rice (DSR) under puddled condition, grasses cause maximum yield reduction followed by sedges and broad leaved weeds (Rathika et al., 2020). Mechanized drum seeders with increased row-to-row spacing allow for an improved integrated weed management strategies viz., chemical, mechanical and manual weeding, during the critical period of crop-weed competition for effective, long-term and sustainable weed control in the DSR system (Prakash et al., 2015). Application of combined weed management practices can ensure control weeds in direct seeded rice resulting higher uptake of nutrients by the crop to produce higher grain yields to the farmers. Hence the present investigation has been carried out to develop proper spacing along with weed management for direct seeded rice cultivation under sodic soil condition.

The field experiment was conducted at Anbil Dharmalingam Agricultural College and Research Institute, Thiruchirappalli, Tamil Nadu, India and study area location was 10°45'N latitude, 78°36'E longitude and 85 m above MSL. The soil of the experimental site was sandy clay loam in texture with pH of 9.0 and EC 0.2 dSm^-1. The initial soil fertility status showed that low organic carbon (0.47 per cent), low available N (223 kg ha^-1), medium available P₂O₅ (14.2 kg ha^-1) and high available K₂O (284 kg ha^-1). The field experiment was laid out in split plot design with three replications. The main plots were spacing of 20 × 15 cm, 20 × 20 cm and 25 × 25 cm. Sub-plot consisted of weed management practices in DSR viz., pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS (S₁), pyrazosulfuron ethyl 20 g  ha^-1 at  3 DAS + cono weeding at 20 DAS + hand weeding  (HW) at 40 DAS (S₂), pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + cono weeding at 20 DAS and 40 DAS (S₃), pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS +  bispyribac sodium 25 g ha^-1 at 20 DAS + cono weeding at 40 DAS (S₄), pyrazosulfuron ethyl 20 g  ha^-1 at 3 DAS + bispyribac sodium at 25 g  ha^-1 at 20 DAS +  HW at 40 DAS (S₅), hand weeding at 20 and 40 DAS (S₆) and unweeded control (S₇). The medium-duration variety TNAU Rice TRY 3 was chosen for this field experiment. The rice variety (TNAU Rice TRY 3) was developed by crossing ADT 43 with Jeeraga samba. Seed rates were required 25, 21 and 18 kg ha-1 while using 20 × 15, 20 × 20 and 25 × 25 cm of drum seeder, respectively. Sprouted seeds were directly sown in puddled soil under waxy conditions using different spaced drum seeders. Weed management practices were obtained through different weed management strategies such as pre-emergence, post-emergence herbicides, mechanical weeding (cono weeder) and manual weeding.
       
The observation on yield attributes viz., number of productive tillers m^-2, panicle length, number of grains panicle^-1, number of filled grains panicle^-1 and test weight were recorded at harvest stage. The grain and straw yields were calculated from each treatment of net plot was harvested, threshed, cleaned and weighted at 14 per cent of moisture content. The straw was harvested from each plot, sun dried for three days. Grain and straw yields were expressed as kg ha^-1. The data collected from the field experiment was statistically analyzed using the procedure given by Gomez and Gomez (1984).

Weed flora
 
The dominated weed species presented in the experimental site are Cynodon dactylon, Echinochola colona and Echinochloa crus-galli among the grasses, Cyperus iria, Cyperus difformis and Cyperus rotundus among the sedges and Eclipta alba and Ammannia baccifera among the broad leaved weeds. Similar weed flora have been observed in direct seeded rice under sodic soil condition as reported by Kokilam (2017); Rathika and Ramesh (2019); Palani et al., (2020).
 
Effect of spacing on weed dynamics
 
Plant spacing had significant effect on total weed density, total weed biomass and weed control efficiency in direct seeded rice (Table 1). Spacing of 20×15 cm recorded significantly the lowest total weed density (40.39 no. m^-2), total weed dry weight (85.52 g m^-2) and higher weed control efficiency (77.5%). This was on par with 20×20 cm of spacing which was registered the lowest total weed density (41.70 no. m^-2), total weed dry weight (87.28 g m^-2) and higher WCE (77.2%) at 45 DAS as compared with spacing of 25 × 25 cm. This might be due to spacing of 25 x 25 cm which allows weed seeds to germinate and to develop vigorous weed population under lower density of crops. However, 20 × 15 cm and 20 × 20 cm of spacing increases the light interception by the plants, which lowers the availability of sunlight for weed growth under the canopy of the crop and stimulate the ability of crop to suppress the weeds. This is in agreement with the findings of Khan et al., (2017) and Daba et al., (2022).
 

 
Effect of weed management practices on weed dynamics
 
Total weed density
 
The application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS +  bispyribac sodium 25 g  ha^-1 at 20 DAS +  cono weeding at 40 DAS registered the lowest weed density (7.67 no. m^-2) at 45 DAS and it was on par with pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium at 25 g  ha^-1 at 20 DAS + HW at 40 DAS and hand weeding at 20 and 40 DAS (7.95 and 8.08 no. m^-2, respectively). The next best treatment was significantly recorded the lower weed density by application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + cono weeding at 20 DAS and 40 DAS which was on par with pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + cono weeding at 20 DAS + hand weeding at 40 DAS (20.69 and 20.96 no. m^-2, respectively). This might be due to the fact that application of pre-emergence herbicides prohibits weed seed germination in the initial stage; similarly, post-emergence controls the weeds efficiently at the later stages and also adoption of cono weeding effectively incorporate the weeds while concurrently improved the soil aeration, which resulted in lower total weed density of grasses, sedges and broad leaved weeds. Unweeded control recorded higher weed density (192 no. m^-2). The corroborated with the findings of Rathika and Ramesh (2018) and Kokilam et al., (2020).
 
Weed dry weight
 
Application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS +  bispyribac sodium 25 g ha^-1 at 20 DAS +  cono weeding at 40 DAS registered the lowest weed dry weight of 12.63 g m^-2. It was on par with  pyrazosulfuron ethyl  20 g  ha^-1 at 3 DAS + bispyribac sodium  25 g  ha^-1 at 20 DAS + HW at 40 DAS and hand weeding at 20 and 40 DAS (12.95 and 13.19 g m^-2, respectively) at 45 DAS. This finding might be due to lower crop-weed competition throughout the crop weed competition period because of broad spectrum herbicides and periodical weed control strategies to DSR. This is in agreement with the findings of Sangeetha (2006) and Sivakumar et al., (2021). Unweeded control recorded higher weed dry weight of 405.54 g m^-2.
 
Weed control efficiency
 
Application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS +  bispyribac sodium  25 g  ha^-1 at 20 DAS + cono weeding at 40 DAS recorded higher weed control efficiency (97.0%). This was on par with pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS + HW at 40 DAS and hand weeding at 20 and 40 DAS (96.9 and 96.8%, respectively) at 45 DAS. This finding was similar to that reduction of weed biomass was attained due to pre emergence and post emergence supplemented with mechanical weeding and hand weeding in DSR. The corroborated with the findings of Kokilam et al., (2020) and Sivakumar et al., (2021).
 
Interaction effect on weed dynamics
 
The interaction effect of spacing and weed management practices were significantly influenced the weed density, dry weight of weeds. The lowest weed density and dry weight of weeds was found direct seeded rice sown at spacing of 20 × 15 or 20 × 20 cm with application of pyrazosulfuron ethyl  20 g  ha^-1 at 3 DAS + bispyribac sodium  25 g ha^-1 at 20 DAS + cono weeding at 40 DAS, which was statistically on par with application of  pyrazosulfuron ethyl  20 g  ha^-1 at  3 DAS +  bispyribac sodium  25 g  ha^-1 at 20 DAS +  HW at 40 DAS compared to unweeded control. The finding clearly indicated that density of crop population with appropriate weed management strategy suppress the weed dynamics under DSR situation. The interaction between spacing and weed management was in correlation with result of Nayak et al., (2014).
 
Effect on yield attributes
 
Among spacing, the highest yield attributes were significantly recorded with spacing of 20 × 20 cm (Table 2). This was followed by 20 × 15 cm. The highest number of productive tillers m^-2 (389), panicle length (27.2 cm), number of grains panicle^-1 (192), number filled grains panicle^-1 (187) and test weight (24.7 g) were registered under spacing of 20 × 20 cm, which was followed by spacing of 20 × 15 cm while the lowest yield attributes were recorded in spacing of 25 × 25 cm under sodic soil condition. The finding revealed that spacing of 20 × 20 cm provided significantly higher yield attributes compared to 20 × 15 cm and 25 × 25 cm of spacing. This might be due to lower intra-crop competition helps to avail the essential resources viz., solar radiation, nutrients, moisture and photosynthetic translocation which leads to higher yield attributes. This is in agreement with the findings of Chadhar et al., (2020).
 

       
Among the weed management practices yield attributes were significantly influenced under sodic soil condition. The highest yield component viz., number of productive tillers m^-2 (401), panicle length (26.5 cm), number of grains panicle^-1 (202), number filled grains panicle^-1 (191) and test weight (25.1 g) were recorded with application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS + cono weeding at 40 DAS which was on par with application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS +  bispyribac sodium 25 g ha^-1 at 20 DAS +  HW at 40 DAS and hand weeding at 20 and 40 DAS. The next best treatment significantly registered on pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + cono weeding at 20 and 40 DAS which was on par with  pyrazosulfuron ethyl 20 g  ha^-1 at 3 DAS + cono weeding at 20 DAS + hand weeding at 40 DAS and lower yield attributes were recorded with unweeded control. This is might be due to lesser crop weed competition resulted weed free condition during critical period of DSR and boosted the nutrient uptake, space and light positively increased yield attributes. This is in agreement with the findings of Prasanth et al., (2015) and Manisankar et al., (2019).
 
Grain and straw yield
 
The grain and straw yield of DSR drastically influenced by spacing (Table 3). The highest grain and straw yields (4546 and 6714 kg ha^-1) was achieved under 20 × 20 cm of spacing which was followed by 20 × 15 cm of spacing, while least grain and straw yield (3616 and 5975 kg ha^-1) was noted in 25 × 25 cm of spacing. The finding indicated that spacing of 20 × 20 cm substantially higher the grain and straw yield compared to 20 × 15 cm and 25 × 25 cm. The probable cause for the higher grain and straw yield under spacing of 20 × 20 cm could be due to diminished crop-weed competition and lower intra - crop competition is increased the number of productive tillers and other yield components which was led the yield of grain and straw yield of DSR. The similar result in agreement with Daba et al., (2022).
 

       
On the other hand frequency of weed management practices significantly increased the grain and straw yields of DSR. The highest grain and straw yield (4667 and 6898 kg ha^-1) was obtained under application of  pyrazosulfuron ethyl 20 g  ha^-1 at 3 DAS + bispyribac sodium 25 g  ha^-1 at 20 DAS + cono weeding at 40 DAS  which was on par with application of  pyrazosulfuron ethyl 20 g  ha^-1 at 3 DAS + bispyribac sodium 25 g  ha^-1 at 20 DAS + HW at 40 DAS and hand weeding at 20 and 40 DAS. The next greatest treatment significantly found under pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + cono weeding at 20 and 40 DAS which was on par with  pyrazosulfuron ethyl  20 g  ha^-1 at 3 DAS + cono weeding at 20 DAS + hand weeding at 40 DAS. This might be due to application of PE, PoE herbicides, mechanical weeding and hand weeding were lower the total weed density, weed dry weight acquired under weed free situation culminated in decreased the nutrient removal by weeds and improved the nutrient uptake of crop and finally enhanced the grain and straw yields of direct seeded rice. The corroborated with the findings of Prashanth et al., (2016) and Manisankar et al., (2021). The lowest grain and straw yields (2995 and 5231 kg ha^-1) was recorded with unweeded control.
 
Interaction effect on yield parameters and yield
 
The interaction effect of spacing and weed management practices on yield parameters, grain and straw yields was significant. The highest productive tillers m^-2, panicle length, total grains panicle^-1 and yield of grain and straw was found in the interaction between spacing of 20 × 20 cm and  application of pyrazosulfuron ethyl  20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS + cono weeding at 40 DAS  which was on par with application of pyrazosulfuron ethyl 20 g  ha^-1 at 3 DAS +  bispyribac sodium  25 g ha^-1 at 20 DAS + HW at 40 DAS. Lowest yield parameters, grain and straw yields were found in the interaction between spacing of 25 × 25 cm and unweeded control. Test weight was did not superiorly influenced by interaction of spacing and weed management practices. Khan et al., (2017) concluded that spacing and weed management practices influenced the yield component of rice which positively enhanced yield of rice.
 
Economics of DSR
 
Economic parameters have been computed using the current market prices of inputs and outputs (Table 4). Notably, employing a spacing of 20 × 20 cm along with the application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS + cono weeding at 40 DAS resulted in a higher gross income of ₹ 86,402 ha^-1, accompanied by a net income of ₹ 38,900 ha^-1 with B:C ratio of 1.82. On the contrary, utilizing a spacing of 25 × 25 cm along with unweeded control approach obtained a lower gross income of ₹ 44,631 ha^-1. This was accompanied by a net income of ` 4,242 ha^-1 and a B:C ratio of 1.11. This was in conformity with the result of Sivakumar et al., (2020).
 

It may be concluded that sowing in rows at 20 × 20 cm apart and application of pyrazosulfuron ethyl 20 g ha^-1 at 3 DAS + bispyribac sodium 25 g ha^-1 at 20 DAS + cono weeding at 40 DAS provided effective weed control and higher yield in direct-seeded rice.

The authors gratefully acknowledge the Department of Agronomy, Anbil Dharmalingam Agricultural College and Research Institute, Tamil Nadu Agricultural University, Thiruchirappalli for providing facilities for carrying out research work.

The authors declare that there is no conflict of interests regarding the publication of this article.