Oxen, water buffalo, horses, and mules are the primary draft animals. Camels and asses (donkeys) are more typically used as pack animals although they are used for pulling carts and tillage tools in some countries. Viewing the draft animal as a machine, we see that it can feed itself, maintain itself, be programmed for automatic control, and reproduce itself—four functions a ti actor cannot perform. As a mechanism, the animal is a jointed framework held together with ligaments and muscles. The engine consists of the digestive organs while the products of combustion are removed by the excretory system. The brain and nervous system provide a control system with a memory bank, logic circuits, and feedback loops. Joints and moving parts have a sealed lubrication system. Protecting the mechanism is a covering of hide which is resistant to damage and self-healing as far as minor dents and scratches are concerned.
Unlike the farm tractor, however, draft animals can be purchased in a very limited range of horsepower, cannot be worked continuously, and continue to burn fuel when not working.
Ail FAO publication states that when used as draft animals, bovines have an energy efficiency of 9 to 10 percent and members of the horse family 10 to 12 percent. However, other experts state that horses have an efficiency of about 20 percent. Tfie ditference between investigators is probably due to the breed, type of feed, and whether the work was done by the test animals on treadmills or draft devices. As a reference point, the efficiency of energy conversion for gasoline and diesel engines is 20 to $5 percent.
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