Drones in agriculture
What farm drones do today, from crop sensing to spraying and seeding, how to calculate spray rates and work rates, and the rules, risks and real limits.
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Small unmanned aircraft have moved from novelty to everyday farm equipment in a decade. Drones photograph crops in wavelengths the eye cannot see, map fields to the centimetre, and, increasingly, spray, spread fertiliser and sow seed. In some countries spraying drones now treat tens of millions of hectares a year.
Drones are a fast-growing topic in agricultural engineering and precision agriculture courses and in essay and interview questions on digital agriculture. This article covers what they do, the arithmetic behind using them, and the rules and limits that decide whether they are worth it.
Types of drone
- Multirotor drones (four, six or eight rotors) take off vertically, hover and fly slowly and precisely. Almost all spraying and most imaging drones are multirotors. Their main limitation is short endurance: typically 10 to 30 minutes per battery.
- Fixed-wing drones fly like small aeroplanes. They cover much larger areas per flight for mapping but need space to launch and land and cannot hover.
- Hybrid VTOL designs take off vertically and cruise on wings.
The history of farm use is longer than it seems: unmanned remote-controlled helicopters have been spraying rice in Japan since the late 1980s. Cheap, stable multirotors and better batteries made drones widespread from the 2010s; large-scale adoption of spraying drones since then has been led by East Asia.
Sensing: seeing the crop
Cameras carried by drones record:
- Visible (RGB) images, for counting plants, spotting gaps, lodging, weeds and damage.
- Multispectral images, typically in green, red, red-edge and near-infrared bands. Healthy leaves reflect strongly in the near-infrared and absorb red light, so indices such as the NDVI (normalised difference vegetation index) map vigour across a field.
- Thermal images, which show canopy temperature: water-stressed plants close their stomata and run warmer.
- LiDAR, which measures height and structure, used in orchards and forestry.
Because drones fly low, their images are very detailed: a ground sampling distance (the ground size of one pixel) of a few centimetres is common, much finer than most satellites. The trade-off is coverage: drones suit fields and farms, satellites regions and seasons.
The value lies in decisions, not pictures: a vigour map that directs scouting to the problem patches, a stand count that decides whether to replant, a prescription map for variable-rate fertiliser.
Mapping
Flying a grid with overlapping photos allows photogrammetry software to stitch an orthomosaic (a distortion-free map) and a digital elevation model. Uses include planning drainage and land levelling, measuring field and plot areas, assessing flood or hail damage for insurance, and laying out irrigation.
Spraying, spreading and seeding
Spraying drones carry tanks commonly from about 10 to over 50 litres and apply pesticides, foliar nutrients and other liquids; spreader attachments broadcast granular fertiliser and seed (rice, cover crops, pasture). Their advantages:
- They reach fields that are wet, steep, terraced, or with tall crops where ground sprayers cannot go, and they do not compact soil or damage the crop with wheels.
- They are fast to deploy on small and scattered fields.
- They reduce operator exposure compared with backpack spraying.
- They allow precise spot spraying of mapped patches.
Their constraints: small payload means frequent refills, low water volumes make product formulation and droplet size critical, and spray drift must be controlled through nozzle choice, flight height, speed and wind limits.
Spray rate and work rate of a spraying drone
A drone flies at 18 km/h with an effective swath of 5 m. Its pump delivers 4 L/min through the nozzles. The tank holds 20 L.
Application rate. The standard sprayer formula:
Rate (L/ha) = flow (L/min) × 600 ⁄ (speed in km/h × swath in m) = 4 × 600 ⁄ (18 × 5) = 2,400 ⁄ 90 ≈ 26.7 L/ha.
(The factor 600 converts minutes, kilometres and metres to hectares.) To apply a label rate of 15 L/ha instead, the pilot can reduce flow to 15 × 90 ⁄ 600 = 2.25 L/min, or fly faster or with a wider swath if the spray pattern allows.
Theoretical field capacity = speed × swath ÷ 10 = 18 × 5 ⁄ 10 = 9 ha/h.
Area per tank = 20 L ÷ 26.7 L/ha ≈ 0.75 ha, which takes 0.75 ⁄ 9 h ≈ 5 minutes of spraying.
If refilling the tank and swapping the battery take another 5 minutes, and turning at field ends wastes a little more, the effective field capacity falls to roughly 4 to 4.5 ha/h, about half the theoretical figure. Logistics, not flight speed, usually limit how much a drone covers in a day.
Rules and safety
Drones are aircraft, and in most countries both the flight and the application are regulated:
- Aviation rules set by national civil aviation authorities: registration of the drone, operator training and certification, weight classes, maximum heights, distance from people, airports and roads, and whether flights beyond the pilot's line of sight are allowed. Heavy spraying drones usually need specific approvals.
- Pesticide rules set by national pesticide regulators: a product may be approved for ground application but not for aerial application, and application from aircraft is restricted or prohibited in some jurisdictions except under derogations. The label must permit drone use, at the stated rate and water volume.
- Data and privacy rules apply to images of neighbours' property and people.
Operators should check the current rules where they work; they are changing quickly as authorities catch up with the technology.
Do they pay?
Drones pay where they solve a real constraint: inaccessible or waterlogged fields, labour shortages, small fragmented plots, or the need for detailed, timely information. In many places service providers own the drones and charge per hectare, which lets small farmers use them without buying one. Information drones pay only if their maps lead to decisions that change inputs or yields; a beautiful NDVI map that changes nothing is a cost.
Limits to keep in mind:
- Short battery life and small payloads.
- Weather: wind, rain and heat limit flying and spraying windows.
- Skills: flying is easy; processing and interpreting data, and calibrating sprayers, are not.
- Evidence: field data on drift, deposition and efficacy at low water volumes are still building for many crop–product combinations.
Key points
- Multirotor drones dominate farm use; fixed-wing drones cover larger areas for mapping.
- Sensing uses RGB, multispectral (NDVI), thermal and LiDAR data; value comes from decisions taken on the maps.
- Mapping produces orthomosaics and elevation models for planning and damage assessment.
- Spraying drones reach wet, steep or tall crops without soil compaction, but have small payloads and drift risks.
- Spray rate (L/ha) = flow (L/min) × 600 ÷ (speed km/h × swath m); field capacity (ha/h) = speed × swath ÷ 10.
- Both aviation and pesticide rules apply; the product label must permit aerial or drone application.
What the exams ask
- Agricultural engineering and precision-agriculture courses: types of drones, sensors and vegetation indices, photogrammetry, spray calibration and field capacity calculations.
- Research-fellowship and postgraduate entrances: comparing drones with satellites and ground sensors, variable-rate application, and drift.
- Civil-service and policy examinations: essays on digital agriculture, regulation of drones, custom-hiring services and access for small farmers.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can a drone replace a tractor sprayer?
For small, wet or hilly fields and spot treatments, often yes. For large, flat, open fields, ground sprayers still cover more area per hour at lower cost per hectare, with fewer drift concerns.
What is NDVI?
The normalised difference vegetation index, (NIR − Red) ÷ (NIR + Red), calculated from near-infrared and red reflectance. Dense healthy vegetation gives values near 0.8–0.9; bare soil near 0.1–0.2. It shows relative vigour but not the cause of a problem.
Do I need a licence to fly a farm drone?
In most countries, yes for anything beyond small recreational drones, and spraying usually needs further approvals. Rules differ by country and change, so check with the national aviation authority and pesticide regulator.
Sources
- FAO and ITU. E-agriculture in Action: Drones for Agriculture. Food and Agriculture Organization of the United Nations and International Telecommunication Union, 2018.
- Tsouros, D. C., Bibi, S. and Sarigiannidis, P. G. "A review on UAV-based applications for precision agriculture." Information 10, 349 (2019).
- Hunt, E. R. and Daughtry, C. S. T. "What good are unmanned aircraft systems for agricultural remote sensing and precision agriculture?" International Journal of Remote Sensing 39, 5345–5376 (2018).
- Chen, H., Lan, Y., Fritz, B. K., Hoffmann, W. C. and Liu, S. "Review of agricultural spraying technologies for plant protection using unmanned aerial vehicle (UAV)." International Journal of Agricultural and Biological Engineering 14, 38–49 (2021).
- European Union. Directive 2009/128/EC on the sustainable use of pesticides, Article 9 (aerial spraying).
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