Determining power requirements
Before attempting to determine the kind and number of animals required for any particular farm, animal owners should be familiar with the concepts of pulling (draft) capacity and power. They should also consider the work characteristics of draft animals.
In order to move any object, an animal must exert a force equal to the weight or resistance of that abject. For example, it takes 50 kilograms (kg) of force to move a 50kg log. If the movement is accomplished by pulling, rather than by lifting, or carrying, the force is called draft.
The draft capacity of an animal increases with its weight. A generally accepted rule is that an animal can exert a constant pull on a load which equals approximately one-tenth of its body weight. The rule applies when the animal is allowed to assume a natural pace and expected to produce an extended rather than a concentrated effort. A 300-kg bull, for example, can pull a 30-kg log all day, but if it is made to pull 130 kg, it will work only three or four hours before it tires. If it is made to pull at a faster pace, it also will tire sooner.
A draft animal must be able to produce bursts of extra force as well as work at a steady pace. Under normal conditions, 30 kg of force may be required to pull a log, but if it must be dragged up a hill, or if the log catches in a gully, the force needed (draft requirement) increases. Horses, mules and oxen are preferred draft animals because they can pull loads over long distances at reasonable speeds and, when necessary, provide extra pulling capacity.
Power is the combination of pulling capacity and speed, or pace. Under normal conditions, a large horse will pull a 150-pound (lb) load at a steady rate of 2-1/2 miles per hour (mph). This rate of work is defined as one horsepower (hp). A bull of the same weight, pulling the same load, will assume a normal pace of 1-1/2 mph. At the end of an eight-hour day, the horse will have moved the load further, or produced more work than the bull, and is thus said to be more powerful than the bull.
Tests have shown that light horses, bulls, buffalo, mules and camels all provide about threequarters horsepower, cows about onehalf horsepower, and donkeys onethird horsepower. But it must be remembered that these are the rates at which the animals normally deliver force, not the maximum force they can produce in a given instant. In pulling tests, horses have, for several seconds, exerted pulls up to twice their weight and bulls have
pulled up to their actual weight. But the intensity of such efforts uses up the animal's strength and reduces the total time it is able to work.
Animals vary not only in their ability to pull loads, but also in the number of hours they will work. In the tropics, breeds of oxen will pull between one-seventh and onetenth of their weight for four to five hours per day. Donkeys will pull about one-fifth of their weight for three to four hours. In tests performed in Africa, bulls worked longer when the load was decreased slightly and the work done in two sessions, two to three hours in the morning and two to three hours in late afternoon. Donkeys refused to work beyond three or four hours regardless of how the work was distributed and in spite of a reduction in the size of the load.
(Source: CEEMAT, Manuel de Culture avec Traction Animale. 1968.)