Plough 8 ha himself, or hire - and should he buy a second tractor?
A farmer owns a 55 kW tractor and a 3-furrow mouldboard plough with 0.35 m furrow width. He plans to plough 8 ha before the rains. The field runs about 180 m long. On a measured run the tractor averaged 5.5 km/h, and each headland turn takes about 45 s. Field efficiency is estimated at 75 %. The tractor burns 11 L/h at 14 currency units per litre, and the operator is paid 25 units per hour. Annual fixed costs (depreciation, interest, insurance, shelter) for the tractor and plough are 20,000 units. A contractor has quoted 750 units/ha to plough, but can only come after the first rains. Neighbours have asked him to plough about 90 ha a year in total if he has spare capacity.
1. Calculate the theoretical and effective field capacity, the hours and the diesel needed to plough the 8 ha. Check whether 75 % field efficiency is realistic for this field.
Read the worked answer
Working capacity
- Working width = 3 × 0.35 m = 1.05 m.
- Theoretical field capacity = (width in m × speed in km/h) ÷ 10 = (1.05 × 5.5) ÷ 10 = 0.578 ha/h.
- Effective field capacity = 0.578 × 0.75 = 0.433 ha/h.
- Time for 8 ha = 8 ÷ 0.433 = 18.5 hours of field time.
Diesel and labour
- Fuel = 18.5 h × 11 L/h = 203 L; cost = 203 × 14 = 2,842 units.
- Operator = 18.5 × 25 = 462 units.
- Variable cost total ≈ 3,304 units, or about 413 units/ha.
Is 75 % realistic? Check the turn penalty. A 180 m run at 5.5 km/h takes 180 ÷ (5.5 × 1000/3600) ≈ 118 s. Add a 45 s turn and the pass becomes 163 s, so the ratio is 118 ÷ 163 = 72 % before any overlap, blocked furrows or refuelling losses. So 75 % is optimistic but of the right order; plan on 20–22 hours rather than 18.5, and about 220–240 L of diesel. The check matters because the whole timeliness argument depends on it: even 22 hours is under three 8-hour days, comfortably inside the pre-rains window.
2. Should he plough the 8 ha himself or hire the contractor? Then evaluate the proposal to buy a second tractor to serve the 90 ha of neighbour work.
Read the worked answer
The 8 ha decision The 20,000 units of annual fixed cost is already committed - he owns the outfit and the money is spent whether or not this field is ploughed. For this job the relevant comparison is variable cost, about 413 units/ha, against the contractor's 750 units/ha. Ploughing it himself saves roughly 337 units/ha, about 2,700 units on 8 ha, and he can finish in under three working days. The contractor arrives only after the first rains, which would push planting back and cost more yield than the saving. Recommendation: plough the 8 ha himself now.
The second tractor Full costing, because a second machine brings new fixed costs:
- Capacity 0.433 ha/h → 90 ha needs 208 hours.
- Variable cost = 208 × (11 × 14 + 25) = 208 × 179 = 37,232 units.
- Fixed cost = 20,000 units.
- Total = 57,232 units for 90 ha = 636 units/ha, against 750 units/ha from the contractor. That is a margin of only about 114 units/ha, roughly 10,000 units a year. That margin is too thin to justify the capital. One major breakdown, a wet season, or two neighbours withdrawing their work erases it, and the machine stands idle in the dry season. Recommendation: do not buy the second tractor. Take neighbour work with the existing tractor only where it does not clash with his own peak operations, charge the full 750 units/ha, and secure written commitments for at least 120–150 ha before reconsidering expansion, so fixed cost per hectare falls well below 200 units.
3. In the field the plough rides up and leaves an uneven furrow bottom, and the tractor pulls to the right. Diagnose the likely causes and give the adjustment sequence.
Read the worked answer
Likely causes, cheapest and most common first
- Levelling or top-link setting. If the plough rides up, the cross-shaft (levelling box) or top link is wrongly set, so the plough is pitched nose-up and the points cannot reach depth. Level the plough front-to-back on a firm, flat patch until all three bottoms touch together; then set working depth.
- Depth control. A depth wheel set too high, slipping, or draught control reacting to hard spots will lift the plough out of the soil. Fix the depth wheel and use position control for a mounted plough.
- Worn or mismatched shares and points. A blunt or shorter point on one bottom makes that body ride out and pushes the plough sideways - which also explains the pull to the right. Compare all three points with a new one and check that the frame is square and the furrow widths equal.
- Line of draught. If the plough is offset from the tractor's centre line it will always pull to one side. Swing the cross-shaft or adjust the hitching point so the plough follows the centre line.
- Tyre pressure and ballast. Uneven rear tyre pressures make the tractor lean and the plough follow crookedly; too little front ballast lets the front lift when the plough bites. Equalise pressures to the specification and add front weights.
- Soil condition. Ploughing soil that is too wet smears the furrow and makes the bodies ride out; wait for the correct moisture.
Sequence: level first, then depth, then wear, then line of draught, then ballast - and re-check the first few runs before ploughing the whole field.