Factors Limiting Feed

T HE mechanisms regulating feed intake appear to be the same mechanisms that control energy exchange. The physiological functions brought into play are numerous and involve chemosensitive areas, reflex responses and integrative function of the central nervous system. Aside from the primary physiological factors limiting feed intake, a close relationship exists in ruminants between the content of indigestible plant residues in the digestive tract and the total amount of feed consumed. Brief attention will be given first to the current concepts of physiological factors involved in the regulation of feed intake in ruminants. The second part deals with the importance of digestibility in regulating feed intake in terms of quantifying its effects and interrelationships. For a more complete discussion the reader is referred to a review by Fissmer (1941) of the early work and a review of recent work by Balch and Campling (1962). The stimuli for changes in food intake arise from animal functions and environmental conditions. In the ruminant the rate of energy expenditure, environmental temperature, qualitative characteristics of the diet and the physical effects of food in the gut are significant factors influencing the level and day-to-day changes in food consumption. Fundamental to an understanding of feed intake regulation is the concept expressed by Brobeck (1960) that feed intake is not an independent variable, since animals obtain all of the energy transferred to heat, work and storage (includes growth, fat deposition and milk) from the feed. Any one of these physiological functions may change in magnitude relative to the other. It is apparent, therefore, that the four variables are regulated individually as well as together. Adolph (1947) has stated that food intake is one of the best regulated of animal functions. However, the complexity of describing