Impact of Different Seed Rates and Knocking Down Days of Sesbania on Weed Density and Yield of Dry Direct Seeded Rice

The weed is a major constraint of dry-direct seeded rice (DDSR) due to change in establishment methods and shifting weed flora towards competitive grasses and sedges. To minimize the weed density, its species and dry weight with brown manuring and for optimizing the yield of DDSR, the experiment was conducted during monsoon season of 2014 at Chitwan, Nepal. The experiment was done using a strip plot design to find the optimum seed rate and killing date of Sesbania under rice-Sesbania co-culture. Among different seed rates (60, 80, 100 kg ha) and knocking down days (21, 28, 35 & 42 DAS) of Sesbania, the optimum seed rate of Sesbania was 102 kg ha and killing date was 32 days. The individual plot size was 5 x 4 m. The growing of Sesbania with 100 kg seeds ha along with its knocked down at 28 DAS was seen best to minimize the weeds having better performance of rice. The experiment clearly demonstrated the importance of brown manuring on effective control of weeds and on grain yield of rice under dry direct seeded rice.


Introduction
Rice (Oryza sativa L.) is the principle source of the food for more than half of the world's population who depends for their daily sustenance (Chauhan and Johnson, 2011). In spite of the first position of rice in terms of importance, area and production in Nepalese context, the present productivity remains low (3.17 t ha -1 ), which is far below than that of other rice growing countries (FAOSTAT, 2012(FAOSTAT, & 2005. Rice is mainly cultivated by transplanting in puddle field, which results in the formation of hard pan and damages soil structure, though it helps in retention of more water and effective in weed control, but this needs more time, labour and energy. With the advent of resource conserving technologies, direct seeding is being emerged as a viable alternative to transplanted rice (Tripathi et al.2004). Farmers are keen to adopt direct seeded rice instead of transplanted rice as there is acute shortage of labour and sky rocketing wages of labourers at the time of transplanting. It is suggested that alternate method of planting i.e. Dry-DSR is gaining popularity regarding its high water use, labor use and energy use efficiencies (Kumar and Ladha, 2011). Thus being cost effective the dry direct seeding method also Research Article allows early sowing of wheat. The effective weed management is a major challenge for farmers when adopting dry DSR because of weed flora shifts toward more difficult-to-control and competitive grasses and sedges. Higher weed infestation is a major problem in dry-DSR causing major loss to rice production worldwide. It is important to manage the weeds to sustain the yield when transplanting is being replaced widely by dry-DSR. Weeds compete with rice plant for all critical growth factors such as space, light, temperature, water and nutrients and reduced the yield by 30-90% (Singh, Bharadwaj, Thakur, Pachauri, Singhand, & Mishra 2009). Sesbania co-culture is different from other types of weed management practices. It overcomes the cost associated with manual and herbicidal weeding along with removal of negative impact of chemical herbicide to the soil condition and plant or human life. Sesbania followed by 2,4-D was more effective in suppressing broad leaves and sedges and less effective on grasses (Kumar and Ladha, 2011).
Sesbania rostrata is a small semi-aquatic leguminous tree which forms a symbiotic relationship with Gram-negative rhizobia and leads to the formation of nitrogen fixing nodules on both stem and roots. This technology can reduce the weed population by nearly half without any adverse effect on rice yield (Kamboj et al., 2012). Growing of Sesbania as an intercrop with direct seeded rice up to 30 DAS reduced the weed infestation by 30% (Singh, Johnson, Mortimer, & Orrr, 2003). The atmospheric nitrogen fixation and facilitation of crop emergence in areas of soil crust formation are other benefits of this technique in addition to weed suppression (Gopal et al., 2010;. It helps in adding about 15 kg N ha -1 along with smothering of weeds and conserving moisture (Gaire et al., 2013). Thus, the major part is to evaluate its effect on weeds and on the performance of dry direct seeded rice through this experiment.

Materials and Methods
The experiment was carried out at Agronomy Farm of Agriculture and Forestry University (AFU), Rampur, Chitwan. The site is located 9.8 km South-West from Bharatpur, headquarter of Chitwan district. This location is situated at 27 0 37' N latitude and 84 0 25' E longitudes with the elevation of 256 m above mean sea level (Thapa and Dangol, 1988). Rice variety "US-312" via line sowing and seed of Sesbania rostrata via broadcasting were sown together manually on 7 th of June, 2014. The soil of experimental plot was sandy loam which had medium type of total N (0.2%), available P (46.62 kg ha -1 ) and organic carbon (1.9%) but low in available K (82.8 kg ha -1 ). Twelve treatment combinations were tested in Strip Plot Design and replicated three times. Each 20 m 2 plot had 20 rows of 5m length, with an inter-row spacing of 0.2 m. The fertilizer was applied in the form of urea, di-ammonium phosphate (DAP), and murate of potash (MOP) whereas the recommended dose of NPK in each experimental plot was 150:80:80 kg ha -1 . One third of nitrogen, full dose of phosphorus and potash were applied as basal dose at final land preparation. Remaining two third dose of Nitrogen was applied at tillering stage and panicle initiation stage in equal split. Zinc sulphate @ 25 kg ha -1 was also applied at final land preparation for correction of zinc deficiency in soil.

Weeds species
The major weed species found during experiment of this brown manuring research have been listed in Table 1. These have been categorized along with three dates of appearance and the weeds were Cynodon dactylon, Echinochloa colana, Setaria glauca, Paspalum scrobiculatum, Para grass, Melochia corchorifolia, Cyperus iria, Fimbristylis miliaceae, etc.

Effect on Weed Density
The seed rates of Sesbania had significant effect on total weed density and non-significant to the individual weed categories at 30 DAS (Table 2). Increasing seed rate of Sesbania caused to decrease total weed density and weed density of individual weed categories. Significantly the lowest total weed density was recorded with the seed rate of 100 kg ha -1 (92.92 weeds m -2 ) followed by lower seed rate of Sesbania. The total weed density with 100 kg ha -1 was 83.31% lower than weedy check and 75% lower than farmers' practice. The different knocking dates of Sesbania had not influenced the total weed density and weed density of individual categories. Knocking down of Sesbania at 21 to 42 DAS had reduced the weed density by 59.15% and 72.73% as compared to farmer's practice and weedy check respectively. These findings are in conformity with  i.e. broadcasting of Sesbania along with rice seeding and killing Sesbania by spraying 2,4-D around 30 days after seeding reduced the weed density by 37-42 per cent compared to the rice crop without brown manuring.
There was observed non-significant effect of different seed rates and knocking down days of Sesbania on weed density at 60 DAS (Table 3). With increasing seed rate and delaying knocking down decreased weed density. Comparatively higher weed density of all categories of weeds recorded on knocking down at 21 DAS. The weed density with different seed rates and knocking down days of Sesbania had 60 and 85% lower as compared to farmer's practice and weedy check respectively.  The density of grasses, sedges and total weed density had significantly influenced by varying seed rates of Sesbania at 90 DAS but broad leaf weeds were similar among all seed rates (Table 4). Significantly lowest total weed density was recorded with the seed rate of 100 kg ha -1 which was statistically lower than recorded on seed rate of 80 kg ha -1 and higher than seed rate of 60 kg ha -1 . Similar trend was observed with density of grasses but in case of sedges, lowest density was observed on 80 kg ha -1 seed rate of Sesbania followed by 100 and 60 kg ha -1 . But, the density of sedges with 100 kg ha -1 was statistically similar to the weed density observed with lower seed rates of Sesbania. Brown manuring acted as cover crop in suppressing weed growth effectively at the initial growth stage. Angadi et al.(1993), Sharma and Ghosh (2000) and Yadav (2004) also reported similar results. Gupta et al.(2006) reported that coculture of Sesbania in rice and its subsequent knock down by 2,4-D reduced the weed population by nearly half without any adverse effect on rice yield.

Grain Yield, Straw Yield, Above Ground Biomass and Harvest Index
The average grain yield in the experiment was 3671 kg ha -1 and it was not significantly influenced by both seed rates and knocking down days of Sesbania (Table 5). However, 100 kg ha -1 of Sesbania seed rate produced the highest grain yield (3956.63 kg ha -1 ) and lowest (3392.52 kg ha -1 ) with the seed rate of 60 kg ha -1 . The trend of yield was in increasing rate with increased seed rate of Sesbania. Higher grain yield in case of Sesbania 100 kg ha -1 may be attributed by release of more amount of nitrogen producing more number of productive tillers, more number of filled grains per panicle and less sterile spikelets. These results are in affirmation to those reported by Gopal et al. (2010) who reported brown manuring in direct seeded rice reduced weed population by nearly half without any adverse effect on rice yield and also, Sesbania surface mulch decomposed very fast to supply N and other re-cycle nutrients.
In case of knocking down days, knocking down of Sesbania at 28 DAS produced highest grain yield (3944.20 kg ha -1 ) as compared to other days of knocking down. Early knocking down of Sesbania (21 DAS) had lowest grain yield (3452.53 kg ha -1 ) because it produced less biomass of Sesbania and has less amount of nitrogen released contributing less to yield attributes as compared to delay knocking down.

Straw Yield
Both the seed rates and knocking down days of Sesbania had also non-significant effects on straw yield (Table 5). However, the highest straw yield (5152.77 kg ha -1 ) was recorded with the seed rate of 80 kg ha -1 whereas the lowest straw yield (4902.97 kg ha -1 ) was with the seed rate of 60 kg ha -1 . In case of knocking down days, knocking down at 21 DAS produced highest straw yield (5247.08 kg ha -1 ) followed by knocking down at 28 and 42 DAS respectively. The lowest straw yield (4814.23 kg ha -1 ) was observed with the knocking down day of 35 DAS.

Above Ground Biomass
There was also non-significant effect of both seed rates and knocking down days of Sesbania on above ground biomass ( Table 5). The lowest above ground biomass (7820.54 kg ha -1 ) was recorded with the Sesbania seed rate of 60 kg ha -1 and it was increased with increasing seed rate of Sesbania and highest was recorded with Sesbania seed rate of 100 kg ha -1 (8328.11 kg ha -1 ). Knocking down of Sesbania at 28 DAS had produced highest above ground biomass (8392.73 kg ha -1 ) which was decreasing with delaying knocking down and lowest (7963.90 kg ha -1 ) was found when knocked down Sesbania at 42 DAS. Early knocking down of Sesbania (21 DAS) had also produced lower above ground biomass as compared to 28 DAS.

Harvest Index (HI)
The average harvest index was observed 0.39. There was significant effect of both the seed rates and knocking down days of Sesbania on harvest index (Table 5). The harvest index was found significantly lower (0.37) with lower Sesbania seed rate (60 kg ha -1 ) than Sesbania seed rate of 100 kg ha -1 and statistically similar with Sesbania 80 kg ha -1 . Significantly, knocking down of Sesbania at 28 and 35 DAS had produced similar harvest index (0.40) which were significantly higher than early and delay knocking down days and lowest (0.36) harvest index was recorded with 21 DAS knocking down.

Optimum Seed Rate and Knocking Down Date of Sesbania
The relationship of seed rate of Sesbania in rice-Sesbania co-culture and grain yield follow a quadratic response with coefficient of determination of 0.79 (R 2 =0.79) and this association was highly significant (r=0.89**). Similarly the relationship of knocking down dates of Sesbania in rice-Sesbania co-culture and grain yield follow a quadratic response with coefficient of determination of 0.79 (R 2 =0.12) and this association was significant (r=0.35*). The optimum seed rate of Sesbania in rice-Sesbania coculture of 102.28 kg ha -1 (102 kg ha -1 ) was found with optimum knocking down days of 31.67 days (32 DAS) of sowing with selective herbicide (Table 5).

Conclusion
It was concluded that the growing of Sesbania with 100 kg seeds ha -1 along with its knocked down at 28 days was the best weed management practices which produced yield similar with farmers' practice. The optimum seed rate and knocking down days of Sesbania were 102.28 kg ha -1 (102 kg ha -1 ) and 31.67 DAS (32 DAS) respectively which was found suitable for controlling weeds and produced higher grain yield with higher economic return and being a good weed management practices for dry-DSR.