Soil microbial population as influenced by drone and robotic herbicide application and mechanical intercultivation under normal and high density planting system cotton

Introduction

Cotton also known as 'white gold' is an important commercial fibre crop of India and Telangana. In India cotton occupies 11.23 million hectares area with 30.69 million bales and 465 kg ha⁻¹ of production and a productivity, respectively. In Telangana, cotton area is 1.81 million hectares with a production and productivity of 5.55 million bales and 521 kg ha⁻¹, respectively, UPAg, (2024-25). Cotton plays a major role in sustaining the livelihoods of about 40-50 million people in cotton processing and trade in India. The microbial populations were found in rhizosphere soil with high plant population due to more root activity. Traditionally, cotton is grown at wider spacing between rows and plants in a row in well drained, highly fertile, heavy soils. Due to slow initial growth, cotton faces severe weed competition at early stages and the yield losses may range from 50 to 85% (Venugopalan et al., 2009) [4] . Critical period of crop weed competition of cotton is around 15 to 60 days (Sharma, 2008) [8] . Weed competition is severe during early stages than three weeks after sowing. Pre emergence pendimethalin and post emergence quizalofop ethyl + pyrithiobac sodium application controls the weeds effectively in cotton (Ali et al., 2013) [1] . Using UAV and knapsack sprayer showed significant effect on weed parameters compared to control (Chen et al., 2019) [3] . Herbicide application significantly influences the microbial populations immediately after application for a short period (Tyagi et al., 2018) [12] , pendimethalin application higher than recommended dose reduces the microbial population significantly, but untreated soils had highest microbial populations (Oyeleke et al., 2011) [7] , pre and post emergence herbicides reduced the microbial populations compared to non-herbicide applied treatments and followed similar trend as untreated plots at later stages (Shivashankar et al., 2016) [9] , beyond 40 days of herbicide application and up to harvest microbial populations increased considerably, indicating there was no long term effect of herbicides application on soil flora and fauna (Tyagi et al., 2018) [12] . Thus, studying the effect of herbicides on soil microbial populations is more important as they govern the soil biological processes.

Materials and Methods

A field experiment was conducted at Regional Agricultural Research Station (RARS), Palem, Nagarkurnool under Professor Jayashankar Telangana Agricultural University, Rajendranagar, Hyderabad, Telangana. The experiment was laid out in strip plot design with two vertical factors: D1-normal planting (90 × 60 cm) and D2-high density planting (80 × 20 cm) with six horizontal factors W1-Drone spraying of pendimethalin 38.7% CS 677.25 g a.i ha⁻¹ as PE fb pyrithiobac sodium 6% w/w + quizalofop ethyl 4% w/w EC 125 g a.i ha⁻¹ as POE at 2-3 leaf stage of weeds mechanical intercultivation at 60 DAS, W2robotic spraying of pendimethalin 38.7% CS 677.25 g a.i ha⁻¹ as PE fb pyrithiobac sodium 6% w/w + quizalofop ethyl 4% w/w EC 125 g a.i ha⁻¹ as POE at 2-3 leaf stage of weeds fb mechanical intercultivation at 60 DAS, W3-manual spraying of pendimethalin 38.7% CS 677.25 g a.i ha⁻¹ as PE fb pyrithiobac sodium 6% w/w + quizalofop ethyl 4% w/w EC 125 g a.i ha⁻¹ as POE at 2-3 leaf stage of weeds fb mechanical intercultivation at 60 DAS, W4-robotic-mechanical intercultivation at 20, 40 and 60 DAS, W5-weed free check (Mechanical intercultivation at 20, 40 and 60 DAS + intra row hand weeding) and W6-weedy check, in a sandy loam textured soil, with neutral reaction, low in organic carbon, available nitrogen and high in available phosphorus and potassium. The soil samples were collected from the experimental plot and the enumeration of bacteria, fungi and actinomycetes was counted by using serial dilution technique (Subba Rao, 1988) [11] at 30, 60, 90 DAS and harvest. Nutrient agar media used for bacteria (Allen, 1953) [2] , Rose Bengal media for fungi and actinomycetes (Martin, 1950) [6] isolation. The data were statistically analyzed after log transformation as per the procedure given by Gomez and Gomez (1984) [5] .

Results and Discussion

Influence of plant density on microbial populations

At 30 DAS, high density planting recorded significantly higher bacterial (1.43, 1.43 × 10 6 CFU g⁻¹ soil), fungi (1.14, 1.14 × 10 3 CFU g⁻¹ soil) and actinomycetes (1.20, 1.19 × 10 4 CFU g⁻¹ soil) populations compared to normal planting, bacteria (1.40, 1.40 × 10 6 CFU g⁻¹ soil), fungi (1.13, 1.12 × 10 3 CFU g⁻¹ soil) and actinomycetes (1.17, 1.17 × 10 4 CFU g⁻¹ soil) populations during 2023 and 2024, respectively. Similar trend was observed during both the years of experimentation at 60 and 90 DAS. At harvest there was no significant influence of plant density on soil microbial populations.

Influence of weed management methods on microbial populations

At 30 DAS during 2023 and 2024, significantly higher bacterial (1.50, 1.49 × 10 6 CFU g⁻¹ soil), (1.50, 1.49 × 10 6 CFU g⁻¹ soil) fungi (1.19, 1.18 × 10 3 CFU g⁻¹ soil), (1.18, 1.18 × 10 3 CFU g⁻¹ soil) and actinomycetes (1.29, 1.29 × 10 4 CFU g⁻¹ soil), (1.29, 1.29 × 10 4 CFU g⁻¹ soil) population was found with W6weedy check and W5-weed free, respectively were on par with each other. Among weed management methods, W4-robotic mechanical intercultivation recorded significantly higher bacterial (1.48, 1.47 × 10 6 CFU g⁻¹ soil), fungi (1.18, 1.18 × 10 3 CFU g⁻¹ soil) and actinomycetes (1.29, 1.29 × 10 4 CFU g⁻¹ soil) populations over rest of weed management treatments viz., W1drone spraying, W2-robotic spraying and W3-manual spraying. At 60, 90 DAS and harvest, microbial populations were non significantly influenced by the weed management methods. Application of pendimethalin and pyrithiobac sodium + quizalofop ethyl significantly reduced the microbial population (bacteria, fungi and actinomycetes) compared to weed free and weedy check, initially for a short period of time. Later, there was no significant difference in microbial population, due to herbicide degradation by microbes as a carbon source for multiplication and there was no long term effect of herbicides application on beneficial microbes. These results are corroborated by Shivashankar (2016) [9] , Dubey et al. (2018) [4] and Siddagangamma et al. (2021) [10] .

Conclusion

The experimental results concluded that high density planting recorded significantly higher soil microbial populations (bacteria, fungi and actinomycetes) than normal planting and among the weed management methods herbicide application through drone, robot and manual knapsack spraying significantly reduced the microbial populations than robotic mechanical intercultivation and weed free and weedy check for an initially short period of time and had no long term adverse impact on microbial populations.

Acknowledgement

AcknowledgementThe authors immensely thank the Department of Agronomy, Regional Agricultural Research Station (RARS), Southern Telangana Zone (STZ), Palem, Professor Jayashankar Telangana Agricultural University, Indian Institute of Rice Research, Rajendranagar, Hyderabad and Marut Drone Tech. Pvt. Ltd., Hyderabad and Robot partner, Flic Farm Pvt. Ltd., X-machines brand, Hyderabad, India.

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