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 th eory, often by applying greater degrees of stimulation than occur natu rally, and is then unable to explain how physiological changes in that stimulus can be responsible for controlling intake. We have found tha t the effects of two or three stimuli (sodium acetate, sodium propiona te, ruminal distension) applied together were additive. As to the site of this integration, receptors in the rumen wall are sensitive to bot h mechanical stimulation and acids, with transmission of impulses in v agal afferent fibers probably modulated by the osmolality of ruminal f luid. Thus, a certain degree of integration (''polymodal'') is likely to have occurred at the level of the transceiving organ. A second leve l of integration is ''polytopic.'' In this level stimulation of one vi sceral site modifies the effects of the same type of stimulus at anoth er site. A third level of integration occurs in the central nervous sy stem, whereby the effects of visceral stimulation might be balanced wi th signals from other stimuli (e.g., the special senses) to determine whether feeding should take place at any given moment. The thesis pres ented is that the central nervous system receives a nonspecific signal from the viscera; the animal might then learn to eat that amount of f ood that minimizes the competing discomforts of excessive abdominal vi sceral stimulation and shortage or imbalance of nutrients.