Nature of hunger and satiety control systems in ruminants.

Understanding mechanisms in control of feed intake and regulation of energy balance in the ruminant is of great importance if improvement in efficiency of production is to be continued. Although there is evidence in both ruminants and monogastrics that body energy content is regulated, under certain circumstances the system can be overridden with the result being either excessive weight gain or loss. In either case, this can lead secondarily to metabolic disturbances and inefficient production. The regulator of body energy content is interfaced with the controller of feed intake, thus providing a means for changes in energy output to be balanced by changes in energy input. Feeding behavior can be influenced by a variety of external and internal factors; for example, sensory cues, such as palatabili ty of a feed, can either enhance or detract from its acceptance by the animal. Certain hormones and metabolites also influence feeding behavior, and gastrointestinal factors, e.g. distention, may play a role in signaling the controller of feed intake. While it is accepted that the central nervous system is the primary site responsible for the control of feed intake and regulation of energy balance, mechanisms in receiving and compiling the information from the periphery and then generating the appropriate response are not understood. Areas in the brain, such as the hypothalamus, have been identified as sites having a primary role in hunger and satiety, and certain neural transmitter Received July 11, 1980. 1981 J Dairy Sci 64:1140-1152 systems have been shown to be involved in information transfer. Recently, however, peptides of the central nervous system have been found to play important roles in control of feed intake; there is evidence that opiate peptides are involved in the initiation of feeding (hunger) and cholecystokinin peptides are involved in the termination of feeding (satiety). There is much yet to be learned about roles of neural peptides, but it is likely that this new information will greatly increase our understanding of the role of the central nervous system in feeding behavior. To best use available resources, ways must be found to overcome certain limiting factors of feed intake. The ruminant is ideally suited for utilizing a variety of "waste" products as feedstuffs, but the bulk and unpalatabil i ty of many of these products precludes their extensive use. Also, in certain disease conditions and metabolic or nutritional deficiencies, feed intake often is depressed, thus further complicating the problem and resulting in poor efficiency of production. By understanding the basic mechanisms in control of feed intake and regulation of energy balance, it should be possible to develop methods of overriding limiting factors of feeding.