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UAV-based Herbicide Application for Efficient Weed Management in Direct-seeded Rice
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Background: Weeds are the key pest, which reduce crop productivity. Direct-seeded rice is very prone to early-stage weed infestation during initial crop development. Cultural and mechanical methods are difficult due to unavailability of labour and poor work efficiency. Hence, use of herbicides is an effective method for timely weed control. However, the efficacy of agrochemicals depends on the efficiency of spraying equipment. Conventional knapsack sprayers are used most often to spray herbicides but they are highly time-consuming, labour intensive and labour drudgery practices. Hence, the current experiment was aimed to study the efficiency of unmanned aerial vehicles (UAVs) on weed management.
Methods: A field experiment was conducted at Tamil Nadu Agricultural University, Coimbatore during summer, 2022 to evaluate the efficacy of UAV-based herbicide application in direct-seeded rice. The study comprised of five weed management treatments such as pre emergence application of pretilachlor with safener and pyrazosulfuron through both UAV and knapsack sprayer and unweeded check as control. The experimental plot was laid out in a randomized block design with four replications.
Result: The experimental findings showed that the application of pre-emergence pretilachlor significantly reduced the density and dry weight of grasses, sedges and broad leaved weeds in knapsack application. Moreover, it was found on par with UAV application of same herbicide without any phytotoxicity to rice seedlings. From the experiment, the application of pre-emergence pretilachlor through UAV is found to be more effective for timely weed control during early stages in direct-seeded rice, which reduce the labour intensity and drudgery than knapsack application.
MATERIALS AND METHODS
In summer 2022 (February-March), sprouted seeds of CO 51 rice variety were sown on puddled soil using a drum seeder by adopting a spacing of 20 × 10 cm. The recommended fertilizer dose of 150:50:50 kg/ha of nitrogen, phosphorus and potassium was supplied in the form of urea, single super phosphate and muriate of potash.
The unmanned aerial vehicle used in the present study was a hexacopter system. The UAV was embedded with a spraying system with four nozzles. The knapsack sprayer used in the present study was battery operated system. A flat fan nozzle was used in both the spraying system.
The experimental treatments were laid out in randomized block design with four replications to overcome field heterogeneity. The treatments comprised five weed management practices T1- Pretilachlor with safener (30.7% EC) @ 450 g a.i/ha UAV spray, T2- Pretilachlor with safener (30.7% EC) @ 450 g a.i/ha knapsack spray T3- Pyrazosulfuron (10% WP) @ 25 g a.i/ha UAV spray, T4- Pyrazosulfuron (10% WP) @ 25 g a.i/ha knapsack spray and T5- Unweeded check. The pre-emergence herbicides were applied at 3 days after sowing (DAS). The pre-emergence herbicides were applied using 40 L ha-1 and 500 L ha-1 spray volume for drone and knapsack system, respectively. The application of herbicide by UAV spraying system is illustrated in Fig 1.
Biometric observations on weed
Data on density (number per m2) and dry weight (g m-2) of the grasses, sedges and broad-leaved weeds were recorded for each experimental unit from a randomly selected unit area using quadrants (0.25 m2) at 20 days after sowing. In order to record the weed dry weights, the weeds were cut at ground level, shade dried and then oven-dried at 78±2°C until a constant dry weight was recorded.
Summed dominance ratio (SDR)
The SDR of the weed species was computed using the following formula suggested by Janiya and Moody (1989).
Whereas the relative weed density and relative weed dry weight were calculated by using the following formula.
Weed control efficiency (WCE)
Weed control efficiency was calculated as per the procedure given by Mani et al., (1973) and expressed in percentage.
The data on weed density and weed dry weight were analyzed statistically by adopting Fisher’s method of ANOVA suggested by Gomez and Gomez (1984). The data on weed density and weed dry weight were subjected to square root transformation before analysis. Statistical significance was tested by the F test at a critical difference (CD) of 0.05 level of probability.
RESULTS AND DISCUSSION
Weed density and weed dry weight
Data on weeds were grouped into grasses, sedges and broad leaved weeds and presented in Table 2. The application of pre-emergence pretilachlor caused 73.91% and 65.30% reduction in the density of grasses in knapsack (T2) and UAV (T1) treatments, respectively. The lowest density (32.08 No./m2) of grasses was recorded in knapsack application of pre-emergence pretilachlor (T2) and it was closely followed by UAV application of same herbicides (T1). The lowest sedge population (10.00 No./m2) was recorded in knapsack application of pre-emergence pretilachlor (T2) and pyrazosulfuron with 74.36% and 67.72% reduction respectively, over unweeded check. The density of broad leaved weeds was significantly reduced by pre-emergence herbicides and the reduction of BLW was 51.43% and 49.52% for pretilachlor application through knapsack and UAV treatments and pyrazosulfuron recorded 38.13% and 32.34% reduction of BLW on UAV and knapsack plots, respectively. The wide spectrum control of grasses, sedges and BLW were recorded in pre-emergence application pretilachlor in both application without any phytotoxicity to rice seedlings. This might be due to the application of pretilachlor at 3 DAS which effectively controlled the germinating weeds at the plumule-initiation stage by inhibiting the cell division and the presence of fenclorim protects the crop from phytotoxicity under low spray volume of 40 L/ha. The results were in line with the findings of Suganthi et al., (2005). The density of grasses, sedges and BLW was significantly higher in the unweeded check (T5).
All the applied pre-emergence herbicides caused significant reductions in dry weight of grasses, sedges and BLW (Table 2). The pretilachlor application reduced the weed dry weight over control by 72.83%, 66.77% and 58.91% for grasses, sedges and BLW respectively, for knapsack treatment (T2) and it was comparable with UAV application of same herbicides (T1). This might be due to application of pretilachlor was more effective in controlling grasses, BLW and sedges at early stages (Sunil et al., 2010). There was no significant difference between UAV and knapsack application and confirmed that carrier volume was not influenced the herbicide efficacy under saturated soil conditions. Similar results were reported by Paul et al., (2023). Chen et al., (2019) also reported that the application of pre-emergence herbicides through UAV had similar weed control efficiency with conventional knapsack sprayer.
Weed control efficiency
Weed control efficiency was worked out based on the weed dry weight and was significantly influenced by weed management practices at 20 DAS (Fig 3). The highest WCE (67.83%) was recorded in the pre-emergence application of pretilachlor by knapsack sprayer (T2) and was followed by UAV application of same herbicides (64.43%) (T1). The UAV-based herbicide application used low spray volume and recorded similar control efficiency of knapsack application.
Correlations between weed dry weight and rice dry matter production
A strong negative correlation was observed between weed dry matter and dry matter production of rice (Fig 4). The relationship indicated that the increased weed dry weight in unweeded check significantly reduced the dry matter production of rice up to 21.8% at 30 DAS. Application of pre-emergence herbicides had better weed control and dry matter production than unweeded plot and pretilachlor applied plots recorded the highest dry matter production. The research outcomes indicated that weed competition significantly affected rice crop in the early stages of growth, leading to a reduced accumulation of dry matter production in unweeded check. The results were corroborated with the findings of Ravishankar et al., (2008).
CONFLICT OF INTEREST
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