Field capacity and field efficiency of machines
Theoretical and effective field capacity, field efficiency and its losses, typical values, timeliness and machine planning, with worked sums.
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How long will it take to plough, sow, spray or harvest a field? The answer decides whether a crop is sown on time, whether a farm needs one combine or two, and whether to own a machine or hire it. Engineers answer it with field capacity, the rate at which a machine covers land, and field efficiency, the share of that rate actually achieved once turns, overlaps, refilling and breakdowns are counted.
Field capacity and efficiency calculations are among the most common numerical questions in farm machinery and agricultural engineering exams.
Definitions
The factor 10 converts km/h × m into hectares per hour: 1 km/h × 1 m = 1,000 m2 per hour = 0.1 ha per hour.
Effective field capacity (EFC) is the actual rate of work, measured as the area covered divided by the total time spent in the field on the job, including turning, unloading, refilling and adjustments.
Material capacity is used for harvesters and processors: tonnes of crop handled per hour.
Why efficiency is less than 100 per cent
- Turning at the ends of the field, more significant in short or irregular fields.
- Overlap: working at less than full width to avoid gaps; GPS guidance reduces overlap to a few centimetres.
- Filling seed, fertiliser or spray tanks, and unloading grain.
- Adjustments, cleaning blockages and minor repairs.
- Slower speeds in heavy patches, wet soil or lodged crops.
Time spent travelling to and from the field and on major breakdowns is usually excluded from field efficiency but included in planning.
Typical values
The ASABE Machinery Management Data give ranges such as:
| Machine | Typical field efficiency | Typical speed (km/h) |
|---|---|---|
| Mouldboard plough | 70–90% | 5–10 |
| Chisel plough, field cultivator | 70–90% | 6.5–11 |
| Disc harrow | 70–90% | 5.5–10 |
| Row-crop planter | 50–75% | 6.5–11 |
| Grain drill | 55–80% | 6.5–11 |
| Boom sprayer | 50–80% | 10–20 |
| Combine harvester | 65–80% | 3–10 |
Efficiency is highest for operations with long, uninterrupted runs and no materials to handle, and lowest for those with frequent filling or unloading.
Timeliness and machine planning
Operations done late cost yield: sowing after the optimum date, or harvesting after crops are ready, often reduces yield or quality by a fraction of a per cent per day. Timeliness costs therefore favour larger or more machines, while ownership costs favour fewer and smaller ones. The number of machines needed depends on:
- the area to be covered;
- the days available for the operation (allowing for weather; many places have only a fraction of calendar days suitable for fieldwork);
- the hours per day that can be worked;
- the machine's effective field capacity.
Planning a harvest
A combine with a 6 m header harvests at 5 km/h with 70 per cent field efficiency.
- TFC = 5 × 6 ÷ 10 = 3.0 ha/h.
- EFC = 3.0 × 0.70 = 2.1 ha/h.
A farm must harvest 300 ha of wheat. Experience shows that in the harvest window of 15 calendar days, about 60 per cent are suitable for combining, for 10 hours each.
- Hours available = 15 × 0.60 × 10 = 90 hours.
- Area one combine can harvest = 2.1 × 90 = 189 ha.
- Combines needed = 300 ÷ 189 ≈ 1.6, so 2 combines, or 1 combine plus contract harvesting of about 111 ha.
Field efficiency from measurement. On a 10 ha field, a planter with a 6 m width at 8 km/h (TFC = 4.8 ha/h) takes 3.0 hours including turns and refilling: EFC = 10 ÷ 3.0 = 3.33 ha/h; field efficiency = 3.33 ÷ 4.8 ≈ 69 per cent.
Field patterns
The path a machine follows affects both efficiency and soil compaction:
- Back-and-forth (shuttle) pattern: the machine turns at each end of the field; simple, but each turn costs time, and ploughs with one-way bottoms leave dead furrows unless reversible ploughs are used.
- Circuitous (around-the-field) pattern: the machine works around the field from the outside in, or inside out, turning at corners; useful for harvesting and mowing.
- Headland pattern: the ends are left as headlands for turning and worked last.
- Controlled traffic farming: all machines use the same permanent wheel tracks, reducing compaction to a small fraction of the field.
Improving field efficiency
- Field shape and layout: longer runs and fewer short rows reduce turning; headlands planned for turning.
- GPS guidance and auto-steer: less overlap and less operator fatigue.
- Larger hoppers and tanks, quick-fill systems and unloading on the go (grain carts).
- Well-maintained machines with fewer stoppages.
- Matching speed to conditions and to the machine's capacity.
Key points
- TFC (ha/h) = speed (km/h) × width (m) ÷ 10.
- EFC is the actual area covered per hour in the field; field efficiency = EFC ÷ TFC.
- Losses come from turning, overlap, filling and unloading, adjustments and slow working.
- Typical efficiencies: tillage 70–90 per cent, planting and spraying 50–80 per cent, combines 65–80 per cent.
- Plan machine numbers from area, suitable days, hours per day and EFC, considering timeliness costs.
What the exams ask
- School-leaving and entrance papers: meaning of field capacity, and why efficiency is less than 100 per cent.
- Degree and research-fellowship entrances: TFC, EFC and field efficiency calculations, factors affecting efficiency, and typical values.
- Agricultural engineering examinations: machine selection and scheduling, timeliness costs, and fleet planning.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is my planter's efficiency so low?
Planters stop often to refill seed and fertiliser, and planting usually involves careful turns and short rows. Larger hoppers, bulk filling and efficient field patterns raise efficiency.
Does driving faster always save time?
Up to a point. Beyond the machine's design speed, quality suffers, such as uneven seed depth or grain losses in a combine, and breakdowns increase.
What is the difference between field capacity and material capacity?
Field capacity is area per hour; material capacity is the mass of crop handled per hour, important for combines, balers and forage harvesters, where crop yield affects how fast they can go.
Sources
- ASABE. Agricultural Machinery Management Data (ASAE D497.7). American Society of Agricultural and Biological Engineers, 2011.
- ASABE. Agricultural Machinery Management (ASAE EP496.3). American Society of Agricultural and Biological Engineers, 2006.
- Hunt, D. and Wilson, D. Farm Power and Machinery Management, 11th edition. Waveland Press, 2016.
- Kepner, R. A., Bainer, R. and Barger, E. L. Principles of Farm Machinery, 3rd edition. AVI Publishing, 1978.
- Witney, B. Choosing and Using Farm Machines. Longman, 1988.
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