General rules concerning power requirements
The above information, qualified by the following rules, makes it possible to determine the kind and number of draft animals needed to power various field operations:
• Given the soil conditions of a region, the weight of the implement to be used and the average depth at which the implement will work, the animal(s) must be able to deliver, for an extended period of time, a force equal to and preferably more than the total resistance, or draft requirement of the work. Tables 1 and 2 gives the draft requirements of various field implements.
• Work that requires frequent "peak efforts" (pulling a plow through rooted or rocky soil; pulling a cart over hilly terrain) tires animals quickly. The operator must compensate by reducing the length of the work day, reducing the intensity of the work (for example, taking smaller cuts with the plow), or providing frequent rests. The alternative is to increase the number of animals used.
• Individual animals do not pull to capacity when hitched in pairs or multiple arrangements. Tests have shown that the individual is 7.5 percent less efficient when it works within a pair. The percentage increases to 15, 22, 30 and 37 percent when the animal works in a team of three, four, five and six animals, respectively.
• As the line of pull is lowered (or as the angle between the line of pull and the ground becomes more acute), less power is needed to move the load. Donkeys and short-legged cattle can produce more power than their weight would indicate because they are closer to the ground.
• Animals deliver maximum performance only when the harness (yoke, collar or breastband) is properly fitted and provides a broad, smooth surface against which to push. Test evidence suggests that bovine animals can deliver 2550 percent more horsepower when harnessed in a breastband or collar rather than a yoke. The difference is explained in the lower point of draft, the increased comfort of the harness, and in the fact that the animal pushes against a much larger surface area.
• Animals must be in good health and properly trained.