Toward a new theory of feed intake regulation in ruminants

Part I of this thesis contains a critical appraisal of the commonly accepted theory with regard to feed intake regulation in ruminants and the presentation of a new theory. This new theory assumes that feed consumption creates both benefits to the animal (in a non-reproducing animal the intake of net energy for maintenance and gain) and costs (the total oxygen consumption of the animal). It is hypothesized that, for the animal, the intake level where the ratio between benefits and costs becomes maximal, is optimal. Predictions of this optimum level for a wide range of feeds are shown to agree closely with observed voluntary feed intake in non-reproducing ruminants. Physiological processes related to the concept of an optimum feed intake are discussed. Maintenance of intracellular pH and associated energy costs may appear to be key factors in view of increases of the metabolic acid load consequent upon changes in intake. It is concluded that the concepts developed here may reflect a more universal principle governing the intensity of different forms of behaviour in ruminants as well as in monogastric animals.Part II reports results of a long-term feeding experiment with small West African Dwarf goats and a larger sheep breed given pelleted roughage. Between species, intake of digestible organic matter and fasting heat production appeared to vary as a function of metabolic weight.The effect of nutrient supplements on intake of low to medium quality roughages was investigated in supplementation and infusion experiments with the same species. Nutritive substances tested were by-pass protein, rumen microbial material, grass juice, intestinally digestible carbohydrates, and volatile fatty acid mixtures. Nutrient supplements usually depressed roughage intake but increased estimated intake of metabolizable energy (ME). From the theory presented in Part I it is inferred that such changes of intake are the result of changes of the efficiency of ME utilization

Oxygen efficiency and the control of energy flow in animals and humans

A new theory of feed intake regulation in ruminants has been developed as an alternative to the traditional view that feed consumption is limited by the physical and physiological constraints of the animal. Historical evidence that supports the traditional view has been examined. The new theory is based on the assumption that feed consumption has both positive and negative outcomes (benefits and costs). In a non-reproducing animal, benefits include yield of net energy for maintenance and gain. Costs are represented by the total oxygen consumption of the animal. The ratio between benefits and costs is calculated as the oxygen efficiency of feeding behavior, i.e., yield of net energy per liter of oxygen consumed. Voluntary energy intake corresponds to the feed consumption level at which oxygen efficiency is maximum. Literature examples were used to illustrate the predictive power of this theory. Differences in intake can be related to the efficiency of energy utilization, i.e., the animals' cost of processing feed. Current knowledge of the nature and causes of variation in processing costs is summarized. Secretory and absorptive processes associated with fiber consumption may explain why fiber can alter feed intake. Maximizing oxygen efficiency seems to be a unifying principle controlling other types of behavior such as locomotory behavior in humans. Possible physiological mechanisms controlling self-selected rates of locomotion are discussed in relation to the control of energy flow in ruminants