Activity is brought about by muscular contractions in which chemical energy stores are converted into mechanical energy, which in turn is converted into heat as the work is performed. In this sense it is wasted energy but some activity is essential – for example, foraging by freerange animals, which involves further energyexpenditure. This has led to the development of intensive production systems for egg layers and for growing chickens, pigs and calves, where activity is minimized.
Although chemical energy can be mobilized very quickly for vigorous work, this may not be reflected immediately in the animal’s oxygen consumption, but the delay is only of short duration and the so-called oxygen debt is usually made up in a few minutes by increased respiration. Changes in oxygen consumption of an animal thus provide a good indication of the heat produced by activity. Even mild exercise can cause a considerable increase in oxygen consumption, and therefore in heat production, and at higher levels of activity increases of up to ten times the resting oxygen consumption can be sustained for prolonged periods, e.g. in draft animals, sheep being herded, racehorses and animals in flight from predators.
There have been few direct measurements of the metabolic cost of activity in farm animals. Most estimates take the form of comparisons of heat produced under different conditions, such as standing vs. lying, walking vs. standing still, walking uphill vs. walking on the level. These comparisons are surprisingly consistent, even between species. When cattle and sheep stand up, the effort involved in getting up causes increased oxygen consumption of some 30% over a few minutes, after which the standing:lying ratio is of the order of 1.12–1.20:1. The metabolic cost of continued standing over lying has been estimated as 0.07 to 0.14 watts kg 1 body weight (6–12 kJ kg 1 day 1 ). In horses, which have the ability to sleep whilst standing, there is little difference in oxygen consumption between standing and lying.
The cost of movement on treadmills has been measured for animals and humans. The results for horses, cattle and sheep may be very crudely summarized as the increase in heat production per kg body weight in moving a distance of 1 m; it is 1.5–3 J kg 1 m 1 for horizontal movement and 25 35 J kg 1 m 1 for vertical upward movement. Speed of the movement has little effect on these estimates of total energy cost, because the effort of rapid movement has to be sustained for less time to cover the same distance. All these treadmill measurements may seriously underestimate the practical energy cost to animals of moving over soft or otherwise difficult ground. Experiments on animals dragging loads suggest that the mechanical work performed (i.e. force distance) multiplied by three provides an approximate estimate of the extra heat produced by the animal. The metabolic cost of activities of humans, who are cooperative subjects, has been extensively studied and may provide a guide as to what may be expected in animals.
There have been few direct measurements of the metabolic cost of activity in farm animals. Most estimates take the form of comparisons of heat produced under different conditions, such as standing vs. lying, walking vs. standing still, walking uphill vs. walking on the level. These comparisons are surprisingly consistent, even between species. When cattle and sheep stand up, the effort involved in getting up causes increased oxygen consumption of some 30% over a few minutes, after which the standing:lying ratio is of the order of 1.12–1.20:1. The metabolic cost of continued standing over lying has been estimated as 0.07 to 0.14 watts kg 1 body weight (6–12 kJ kg 1 day 1 ). In horses, which have the ability to sleep whilst standing, there is little difference in oxygen consumption between standing and lying.
The cost of movement on treadmills has been measured for animals and humans. The results for horses, cattle and sheep may be very crudely summarized as the increase in heat production per kg body weight in moving a distance of 1 m; it is 1.5–3 J kg 1 m 1 for horizontal movement and 25 35 J kg 1 m 1 for vertical upward movement. Speed of the movement has little effect on these estimates of total energy cost, because the effort of rapid movement has to be sustained for less time to cover the same distance. All these treadmill measurements may seriously underestimate the practical energy cost to animals of moving over soft or otherwise difficult ground. Experiments on animals dragging loads suggest that the mechanical work performed (i.e. force distance) multiplied by three provides an approximate estimate of the extra heat produced by the animal. The metabolic cost of activities of humans, who are cooperative subjects, has been extensively studied and may provide a guide as to what may be expected in animals.
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