Implications of fasting on the energy metabolism and feed evaluation in ruminants

Abstract

Fasting metabolism is adapted to ensure the orderly mobilization of endogenous substrates and fuel for maintaining vital activity. When no exogenous energy is consumed it gives rise to a glucose deficient condition; as a result gluconeogenesis from amino acids occurs which results in a higher excretion of N and a higher heat production. When therefore fasting metabolism is adopted as the reference level of metabolism when assessing energetic efficiency, efficiency appears higher below energy maintenance (Km) than above (Kf). When enough feed is given to meet the glucogenic demand of an animal, but no more, heat production falls to a minimum, which suggests that this value, rather than fasting heat loss, would be the appropriate base for measuring the energetic efficiency of exogenous feed. In general about one-third of energy maintenance (150 kJ/kg W 0 75 daily or 20 mmol of glucose/kg W 0 75 daily) should be used for obtaining this minimum metabolic state in ruminants. KEY WORDS: fasting, energy metabolism, feed evaluation, ruminants PHYSIOLOGY OF FASTING After prolonged fasting ruminant animals can no longer absorb exogenous nutrients and must utilize endogenous energy and protein to maintain vital activity. Depending on the duration of food deprivation, fasting induces the physiological adaptation characterized by hypoglycaemia (Blum et al., 1981; Rule et al., 1985), hyperlipidaemia (Di Marco et al., 1981; Rule et al., 1985), hyperketonaemia (Lyle et al., 1984 ; Rule et al., 1985) and hypoinsulinaemia (Rule et al, 1985; Ku Vera, 1988; MacDowell and Annison, 1991). These changes not only lower the metabolic activity of the active cell mass, but are also