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The data stated in pertinent literature on the optimum tractive powers and speeds of different breeds of draft animals vary considerably. For donkeys, 25-40 kp and 0.55-0,7 m/s are usually indicated, for horses 35-80 kp and 0.55-1.1 m/s and for oxen 30-80 kp and 0.6 - 0.85 m/s. While information on camels is scarce, they seem to have a somewhat higher tractive power and a slightly slower speed than oxen. As a rule, draft animals can be worked for some minutes or even hours at levels exceeding the optimum values, as long as they are allowed to rest for an adequate length of time afterwards in order to regain their full strength.
Since draft animals are living sources of energy, the physical parameters are accompanied by numerous other determining factors, which, however, defy precise description and prediction. In addition to environmental parameters such as temperature, humidity, the time of day and the season, such nonphysical factors include, for example, the animal's age, state of health and momentary mood, between it and the drover, and the work cycle it is expected to cope with.
For example, the maximum tractive-force requirement for plowing is often rated as For example, the maximum tractive-force requirement for plowing is often rated a 1/7 - 1/10 of the animal's weight, whereby the lower value relates to freshly cleared land with soil that still contains roots capable of causing considerable and unforeseeable fluctuations in the amount of tractive effort required. An animal power - even one in which the load varies rhythmically, i.e. foreseeably - never subjects the animal to such pronounced, heavily taxing fluctuations.
The maximum tractive power that can be achieved for a very limited time is stated in the literature as ten times the optimum level (for slowing down from a dead run) down to two or three times the optimum level (for overcoming inertia).
Draft animals harnessed in teams perform at about 10-20% below their optimum standard.
Consequently, doubling the number of draft animals does not equate to a doubling of efficiency. In span labor, both the per-head tractive power and, to a lesser extent, the speed will drop off noticeably.
The situation with regard to the optimum achievable work output is similar to that concerning the ascertainment of the animal's performance level. For European draft horses, the daily work period is taken as 8 to 10 hours, depending on the breed. The optimum daily work output is the product of the number of work hours and the optimum performance level. While the optimum daily work output can sometimes be exceeded, the days required for recuperation will, in the long run, cause the amount of work done under excessive strain to drop below the optimum level.
In historical literature on hauling operations, numerous references can be found to the effect that imponderables such as how well the wagoner understands his horses, how much prudence he exercises in giving them a rest at the proper time and place, and even the "tone of his voice and the bite of his whip", greatly limit the validity of any approximation formula.
For several consecutive hours of overexertion, it was often assumed ("according to Maschek") that the three factors power, speed and daily working time were of identical influence. Accordingly, a 10% increase in tractive power and a 15% rise in speed would necessitate a 25% decrease in daily working time (based on the respective optimum values).
The consequences of short-term overexertion with regard to the daily working time were considered negligible, and it was assumed that a doubling of the tractive effort (e.g. for pulling vehicles uphill) could best be compensated for by cutting the speed in half. It is important to note that only well-trained horses will automatically behave accordingly. "Young, hot-headed" horses, however, tend to increase their speed beyond the optimum level whenever more tractive effort is expected of them. One of the many tasks of the driver was to prevent such behavior and, hence, the untimely exhaustion of the animals.
The influence of uphill pulling on an animal's tractive power was tentatively described on the basis of the observation that, on a slope of 30° or more, the draft animal needs its entire strength just to move its own weight. It was taken for granted that the tractive effort would decrease proportionally on a more gradual slope (i.e. by one-third for a slope of 10°).
The braking ability of well-trained horses on slopes was assumed to be about half as high as the tractive power. The potential increase in tractive power due to the influence of the animal's own weight on downhill slopes which would constitute an important factor in an analysis of the "Delou" (described later on) - was not investigated, since it was of no interest to the hauling trade.
Consequently, the data to be found in pertinent literature permit only very rough estimates regarding the work output that can be achieved using the appropriate draft animals for animal-power applications. Since animal powers will, as a rule, only be used where an adequate amount of experience has already been gained in the utilization of draft animals for soil-tilling purposes, the empirical data gathered in this field will permit much more accurate assessments of the likely animal-power efficiency levels, and should therefore be given preference over data indicated in the literature.
Despite a scarcity of available data, it may be stated with all certainty that the performance of draft animals in developing countries is considerably lower than the levels observed in Europe. The tractive power of an African plow horse for example, is rated at 260 W. whereas the old European power unit "1 HP" corresponds to 750 W. or nearly three times as much.
Much the same applies to the daily work period, which amounts to 3 to 6 hours for African horses. Thus, the work output that can be achieved with African horses amounts to only about 20% of the corresponding European standards. As for ox harnessing, the difference is less pronounced, but still significant.
Since the utilization of draft animals in Africa is of very limited tradition, it cannot yet be said to which extent the lower draft-animal performance level is attributable to climatic factors or to the less-than-optimum breeding status of African draft animals.
While the development of a high-performance engine may take years, the breeding of draft animals for European conditions has been- going on since the early Middle Ages: animal energy = living energy.
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