INTRODUCTION
Bangladesh is one of the largest producer and consumer of rice in the world with an annual production of 33.88 tons occupying 77.15% of the total cropped area of the country (BBS, 2013). But in the future, maintaining an increased production of rice will be a challenge especially in the context of decreasing cultivable rice land and scarcity of irrigated water of the country. Transplanted aman currently takes the greatest percentage of arable land, which is about 49.63% of total rice cultivated area, and contributes 40.57% of the total rice production (BBS, 2012).
Transplanting is the most popular crop establishment method in Asia’s irrigated rice growing areas (paddy fields). Traditional manual transplanting is one of the most time consuming, water-demanding, and laborious operations in rice cultivation, and thus direct seeding is becoming an attractive alternative to transplanting. Rice is directly seeded essentially by two methods (dry and wet seeding) based on physical conditions of the field and seed (sprouting or dry).
Direct seeding, using a drum seeder, is one of these methods of crop establishment. In order to achieve desired performance from direct wet-seeded rice, seeding has to be done much earlier than when most farmers would transplant, which could lead to crop maturity much before possible flash floods. In this method, the sprouted rice seeds are sown on well puddled soil in rows.
Weeds are one of the major biotic constraints to rice production (Yeasmin et al., 2008), and weed management has been a huge challenge for rice farmers and researchers as well (Anwar et al., 2011). In Bangladesh, weed infestation reduces grain yield by 70-80% in aus rice (early summer rice), 30-40% in transplant aman rice (autumn rice), 22-36% in boro rice (winter rice), and 40% in transplanted aman rice (BRRI, 2008; Haque et al., 2011). To reduce the cost of rice production, the adoption of alternative methods of weed control have been urgently needed, which includes mechanical weed control, biological weed control, and chemical weed control in combination with manual weeding. Moreover, integrated weed management systems utilizing cultural control technique can be developed for effective weed management (Hwang et al., 2017).
Mechanical weeding and herbicides are the alternatives to hand weeding. However, herbicides are now gaining popularity among farmers. Currently, numerous pre-emergence herbicides are effective in controlling weeds when used together with hand weeding (Ahmed et al., 2003). A technique developed for environmentally-friendly weed management, the combined applications of rice bran with pine leaves in rice fields, has increased yields of rice and has provided higher yields than hand weeding (Lee et al., 2015). Thus, more effective weed management systems are needed to optimize the aman rice yield.
The present study was undertaken to evaluate alternative crop establishment methods and weed control options for aman rice with the following objectives: (i) to evaluate the performance of different crop establishment methods on the productivity of aman rice and (ii) to compare the different weed control methods in transplanted aman rice.
MATERIALS AND METHODS
The field experiment was carried out on the experimental field of the Bangladesh Rice Research Institute (BRRI) at Shyampur Regional Station, Rajshahi, 2012. The experimental field was located in the High Gangetic River Floodplain and its soil belongs to Agro-ecological zone (AEZ)-26 (BARC, 2005). Bangladesh has a tropical monsoon climate. There are three major cropping season in Bangladesh: Rabi, Kharif-I, and Kharif-II. A popular transplanted aman rice variety developed by Bangladesh Rice Research Institute, BRRI dhan44, was used as planting material in the experiment for the growing season of Kharif-II (July to middle of October) in 2012. The following three crop establishment methods were chosen as common treatments for the entire study: direct wet seeding by drum seeder (T1), hand broadcasting (T2), and transplanting (T3). In addition, the following four weeding options were studied: Hand weeding (30 DAT and 45 DAT) (W1), BRRI weeder+HW (hand weeding) (W2), Herbicide (Pretilachlor 500 EC 1 L ha-1)+HW (W3), and no weeding (W4). The experiment was laid out in a split-plot design with crop establishment methods as the whole-plot factor and combination of weed management as the subplot factor. The treatments were replicated thrice. The unit plot size was 3 m×2 m. Two to three 30-day old seedlings were transplanted per hill with planting spacing of 20 cm×15 cm. The plots were fertilized with 105, 15, 60, 15, and 1.5 kg ha-1 of N, P, K, S, and Zn as urea, triple super phosphate (TSP), murate of potash (MoP), gypsum, and zinc oxide, respectively. All the fertilizers except urea were applied at final land preparation in full amount. Urea was applied in three splits: 30% at 7-10 days after transplanting (DAT), 30% at 20-30 DAT, and the last 40% at panicle initiation stage. Pretilachlor was applied at the early period of crop growth (30 DAT and 45 DAT). Maturity of rice occurred at different times irrespective of crop establishment methods and weed management practice. The crop was harvested at different dates at maturity.
Procedures of sampling and data collection
Plant density was measured at 20 DAT. Rice seedlings from the direct-seeded method were counted inside a quadrate (50 cm×50 cm) placed at random in two locations per plot. Plant height was taken from 5 randomly selected crop plants in a plot at 20, 40, and 60 DAT. Plant samples were collected from outside the area harvested for grain yield. Above ground dry matter (DM) was taken of rice plant at 20, 40, and 60 DAT.
Economic analysis
A simple economic analysis was done based on the different operational (variable) costs of cultivation under each planting method, from seeding to harvest, but no fixed cost was considered. Cost of land preparation, labor, inputs, irrigation, intercultural operations, and price of the product of rice (grain and straw) were collected from the farmers and local markets to compute total variable costs, gross return, gross margin, and benefit-cost ratio (BCR). The gross margin and BCR were computed as follows:
Gross margin=Gross return-total variable cost
Statistical analysis
The recorded data were compiled and tabulated for statistical analysis. Analysis of variance was conducted with the help of a computer package, MSTAT-C. The mean differences among the treatments were adjudged by Duncan’s New Multiple Range Test.
RESULTS AND DISCUSSION
The highest rice plant density was observed with the T3 as compared to T1 and T2 methods, respectively (Table 1). Similarly, the greatest dry matter weight and plant height were exhibited using the T3 method. This was similar to the results of Chowdhury et al. (1995). Among three crop establishment methods, the tallest plant was observed with the T3 method (120.38 cm) followed by T2 (111.75 cm) (Table 2). This is similar to the results of Chowdhury et al. (1995), which indicated that plant height significantly increased with concerning all crop establishment methods as evidenced from the present study. Transplanted rice had a significantly higher number of tillers (12.31) than rice planted by hand broadcasting (9.58) or direct wet seeding using drum seeder (8.76). The highest number of panicles (8.74) and filled grains panicles-1 (92.8) were established with a T3 method whereas the lowest number of panicles (6.32) and filled grains panicles-1 (87.1) were found with direct wet seeding by drum seeder. The above result of the present study shows similarity with the research reports of Prasad et al. (1999) and Ali (2005). Weight of 1000-grains was the highest (27.95 g) using T3 method followed by T2. The highest grain and straw yields were found in T3 (5.12 t ha-1) and (6.14 t ha-1). The results indicated that the highest yield obtained with a transplanting method might be due to the tallest plant; higher numbers of tillers and panicles. The maximum filled grains were recorded under this method, which ultimately resulted in the higher yield. This was similar to the results of Moody (1991). A partial budget showing cost of cultivation among T1, T2, and T3 methods in aman growing season according to the various weed management practices are presented in Table 3. Among three planting techniques, the highest ($ 644.71) and the lowest ($ 276.37) marginal returns were found in the crop establishment method of T3 and T1, respectively (Table 4). In aman season, T2 gave the highest BCR of 2.03 where for T1 and T3 showed the BCR of 1.53 and 1.96, respectively (Table 4).
Among the weed management treatments, rice plant densities were statistically similar at 60 DAT when the highest density was found with W1 and the lowest was with W4 (Table 5). It was found that the W1 produced the highest dry matter weight (1.54, 11.66, and 31.66 g) at 20, 40, and 60 DAT, respectively. The results also revealed that hand weeding had a remarkable effect on plant height as the heights were 26.28, 49.16, and 74.43 cm at 20, 40, and 60 DAT, respectively. It is evident from the Table 6 that there were no significant differences in plant height between the weed management practices of hand weeding (W1) and herbicide application combined with hand weeding techniques (W3) (115.98 and 115.11 cm, respectively) at harvest. This might be due to the fact that the presence of weeds will lead to a reduction in the average yield of the crop and individual plant height. A most tillers were found in W1 (11.20) than in W3 (10.69) and W2 (9.94) while W4 treatment registered the fewest tillers in this study (Table 6). This might be due to the careful removal of weeds by hand which reduced crop damage and yielded an increased number of plants m-2. It was found that the number of panicle ranged from 6.65 (W4) to 8.52 (W1). This might be attributed to better root development in transplanted seedling which produced healthy panicles with a higher number of grains. Weed management treatment of W1 produced a significantly higher number of filled grains (105.3) whereas W4 showed the fewest filled grains per panicle (72.2) (Table 6). The W1 treatment produced the highest grain and straw yields (4.97 t ha-1) and (5.96 t ha-1), respectively, while they were respectively the lowest (3.45 t ha-1 and 4.14 t ha-1) in the W4 treatment. Treatment W1 involved the maximum cost of production ($ 503.58) along with the highest BCR of 1.53 where the lowest cost of production ($ 365.22) was involved in W4 (Table 7). The net return from rice cultivation was found to be the maximum ($ 772.58) with the weed control practices in W1 followed by W3 ($ 723.22). The economic return when there was no weeding throughout the growing season due to lower grain and straw yield having the lowest net return of $ 520 along with lower BCR value of 1.42 than W2 (1.51). The results correlated with the findings of Ahmed et al. (2005).The results correlated with the findings of Ahmed et al. (2005).
A significant variation was found due to the interaction effect of establishment methods and various types of weed management at 20 DAT where the maximum plant density (94.66) was noted from the interactions between T3W1 (Table 8). The rice plant density at 40 and 60 DAT were not statistically different due to whole interaction treatments due to its non-significant variation. However, plant dry matter was significantly affected by crop establishment along with various weed management methods at 40 and 60 DAT although it did not significantly differ at 20 DAT (Table 8). Plant height varied from 20.58 to 28.33 cm at 20 DAT, 40.95 to 52.84 cm at 40 DAT and, 64.63 to 76.47 cm at 60 DAT. The data on number of panicles per hill significantly varied from 5.90 to 9.82. These data were obtained from the treatment combinations of T1W4 and T3W1, respectively, at harvest (Table 9). Prasad et al. (2001) also found significant variation due to crop establishment methods which supports the present findings. They reported that transplanting technique increased all the growth and yield attributes of rice significantly over seeding and puddle sowing of sprouted seeds. The maximum number of filled grains (108.50) was observed in T3W1 while the treatment combinations of T2W1 obtained the statistically similar maximum filled grains (107.00) followed by T1W1 and T2W3 (100.50 and 97.60, respectively). Among the treatment combinations, grain yield was the highest (6.10 t ha-1) in T3W1 followed by T3W3 (5.17 t ha-1). Among other yield and yield contributing characters, plant height, the number of tillers per hill, and straw yield were significantly identical due to all interaction treatments between crop establishment methods and various application option of weed management.
Economic analyses
T3 recorded higher gross return ($1314.68 ha-1) and net returns ($ 644.71 ha-1) than T1 and T2 (Table 4). Significant improvement in grain and straw yield could have contributed to the higher gross and net return. The BCR ratio was for T1 (1.53), T2 (2.03), and T3 (1.96) respectively. The W1 treatment recorded higher gross return ($ 1276.58 ha-1) and net return ($ 772.75 ha-1) than other treatments due to significant improvement of grain yield (Table 7). The BCR ratio was almost same for W2 (1.51) and W1 (1.53). The higher BCR ratio (1.64) was recorded in W3 because of less treatment cost, higher gross, and net returns. No weeding (W4) recorded lower net return of $ 520 ha-1 attributable to severe yield reduction. W4 showed lower BCR (1.42) might be due to less net return and higher cost of production than that of others (Table 7). It may therefore be concluded that the hand weeding approach whenever necessary was the best treatment than others. As the similarity with W1, it may be mentioned that the use of herbicide is an alternative in controlling weeds by hand weeding.
CONCLUSION
It can be concluded from the results that variety BRRI dhan44 performed better when grown using a transplanting method (T3) alone, hand weeding (W1) alone, or with their combination (T3 W1). Therefore, the cultivation of BRRI dhan44 with T3 alone, and W1 management alone, or with their combination (T3 W1) method is recommended for better yield during aman season. Further study is required to conclude the most effective method of cultivation for BRRI dhan44 with other agronomical management practices.
