The tractor is the main source of power on modern commercial farms and enables one man to do the work of many horses, oxen or people. Tractors today are extremely versatile machines and when fitted with the right implement can perform any operation on the farm. In addition to pulling ploughs and cultivators, a tractor may be used with implements for harvesting, bush clearing, cutting hedges, digging ditches and land leveling. By using the power take-off (PTO), machines can be driven as well as towed, and the tractor can be used as a stationary engine to drive maize shellers, pumps and circular saws. In the early days as a replacement for horses, tractors simply towed machines but in the 1940s, the Canadian engineer Harry Ferguson revolutionised the tractor by inventing the three-point linkage system. This allows an implement such as a plough to be attached directly to the back of the tractor. When the plough is in the ground the force of the plough acts on the back wheels of the tractor to give greater traction and less wheel spin. This means that a small light tractor could operate ploughs and other implements that previously had required a much larger and heavier tractor. A trailer’s wheels are situated at the back of the trailer to put weight on the drawbar of the tractor. This therefore puts weight on the back wheels of the tractor creating greater traction and less wheel spin and.

Small ‘walking’ tractors of 0.75 kW to 7.5 kW may be used for market gardens or orchards. These machines are fitted with handles which carry the controls used by the walking operator to guide the machine. They can be fitted with a plough body, cultivator tines or other fittings to provide the power for different tasks.

Regularly farm tractors have four wheels with either 2-wheel drive to the back wheels, or 4-wheel drive. These days, a standard fitting on a 2-wheel drive tractor is a differential lock which can be engaged to provide full power to both back wheels at the same time. This is useful when working in wet, slippery conditions.

Track-laying tractors or crawlers have the great advantage that they have a powerful grip without exerting much pressure on the soil. They can be used for heavy loads on almost any class of land. Their main disadvantage is the high cost of maintenance, particularly that of the tracks.


Under the old imperial system of measurement, tractors were rated by Horsepower (HP), but since metrication they are rated by Kilowatts (kW).

A kilowatt is three-quarters of one horsepower, and some examples of tractor ratings are shown in the tables on the following page:

Table 1: Tractor Ratings (horsepower and kilowatts)

    HP  kW  Average Fuel Consumption litres / hr  
 Small Tractor  30 – 45  22 – 34  2  
 Medium Tractor  60 – 80  45 – 60  4.5  
 Large Tractor  100  75  11  
 Very Large Tractor  275  205  20+  

The drawbar pull of a tractor is expressed in Kilonewtons (1 000 Newtons), and for different sizes of tractor would be:

· Light Tractor12 kN
  • Medium Tractor  17 kN
· Large Tractor19 kN

Purely as a point of interest, the figures given for tractors can be compared with the figures for various types of draft animal given in the table below.

Table 2: Figures of draft animals that can be compared to tractors

 Animal   Drawbar Pull kN  Average Speed Of Work Km/h   Power Developed kW  Horse Power
 Horse  0.6 – 0.8  3.6  0.75  1.00  
 Ox  0.6 – 0.8  2 – 3  0.56  0.75  
 Cow  0.5 – 0.6  2.5  0.35  0.45  
 Mule  0.5 – 0.6  3 – 3.5  0.52  0.70  
 Donkey  0.3 – 0.4  2.5  0.25  0.35  

(Ref: C.R. Howard, Institute for Agricultural Engineering)

The fuel input / work output efficiency for a farm tractor is about 13 – 15%, whereas for the ox, is about 8 – 9%. However, tractor fuel is highly refined, whereas, the fuel supplied to the ox in the form of food, is in a very crude form. Furthermore, whereas the fuel for a tractor is imported onto the farm, for the ox it is produced locally.

Implement Power Requirements

The table on the following page gives the power requirements of some of the common farm implements, and you can relate these to the power rating of the tractors of different sizes:

Table 3: Power requirements of some farm implements

 Implement  Size  Engine kW Required  
    2 disc  35  
 Plough – mounted  3 disc  50  
    4 disc  75  
 Plough – trailed  5 disc  90  
    6 disc  100 
 Plough – reversible  3 disc  45  
    2 tine  45 – 60  
 Ripper  3 tine  60 – 80  
    5 tine  75 – 90 
    7 tine  90 – 100  
    1.60 metres wide  40  
 Disc Harrow – mounted, offset  1.83 metres wide  45  
    2.10 metres wide  50 
    2.29 metres wide  60  
    2.06 metres wide  56  
 Disc Harrow – trailed, offset  2.30 metres wide  65  
    2.75 metres wide  75  
 Disc Harrow – rome type  2.30 metres wide  80 – 100  
    2.60 metres wide  100 – 120 
 Spring Tine Cultivator  9 tines  40  
    12 tines  45 
    2 row  36  
 Gang Tiller  3 row  40  
    4 row  50 
    6 row  60  


Figures 1 and 2: Tractor Small Tractors; 58 horsepower John Deere (right) and Massey Ferguson 135(left)

Source: howardandsons                                                               Source: payloadglobal

Figures 3 and 4: Medium Tractors; Massey Ferguson 60 HP, 42 KW (left) and Ford 6600, 60HP, 42KW (right)

Figure 5: Large tractors, John Deere 93 HP, 72 KW


Figure 6 and 7: 3t tipping trailer (left) and 6t tipping trailer (right)

Source: image.ec21                                                 Source: i03.i.aliimg

Figure 8: A Grain Trailer

Source: jpmtrailers.com

Figure 9: 6t Duty Dumper