Similarities and differences between intake control mechanisms in pigs, chickens and ruminants.

The title of this paper presupposes that farm animals do control their voluntary intake but this is really a question of degree. If control were absolute then we would expect, for example, that a fast would be followed by exact compensation when food was made available again; this is not the case, especially with fasts of more than a few hours. However, the fact that some compensation does occur indicates that there is some control and the mechanisms involved appear to be similar for the various groups of farm animals. For control to be exact, the nutrients yielded from the digestive tract would have to meet the requirements of the animal at all times and this could only be achieved by constant nibbling of food. That such continuous nibbling is not a feature of normal feeding behaviour is further evidence of the approximate nature of intake control. Indeed, it would clearly be impossible for an animal offered a single food source to control the intake of each nutrient to meet its requirements unless the composition of the food exactly matched the ratio of nutrients utilized by the animal. There is not space to attempt to cover the whole of the subject but rather to select some areas where the similarities or differences between the various farm species are of particular interest.