Integration of regulatory signals controlling forage intake in ruminants.

Numerous factors have the potential to affect the amount of forage or pasture eaten by ruminant animals, including gut capacity, ability of tissues to metabolize nutrients, ruminal acidity, and osmolality. Much research into the control of food intake has tested one particular theory, often by applying greater degrees of stimulation than occur naturally, and is then unable to explain how physiological changes in that stimulus can be responsible for controlling intake. We have found that the effects of two or three stimuli (sodium acetate, sodium propionate, ruminal distension) applied together were additive. As to the site of this integration, receptors in the rumen wall are sensitive to both mechanical stimulation and acids, with transmission of impulses in vagal afferent fibers probably modulated by the osmolality of ruminal fluid. Thus, a certain degree of integration ("polymodal") is likely to have occurred at the level of the transceiving organ. A second level of integration is "polytopic." In this level stimulation of one visceral site modifies the effects of the same type of stimulus at another site. A third level of integration occurs in the central nervous system, whereby the effects of visceral stimulation might be balanced with signals from other stimuli (e.g., the special senses) to determine whether feeding should take place at any given moment. The thesis presented is that the central nervous system receives a nonspecific signal from the viscera; the animal might then learn to eat that amount of food that minimizes the competing discomforts of excessive abdominal visceral stimulation and shortage or imbalance of nutrients.