Mathematical models which simulate the uptake, metabolism and partitioning of dietary nutrients, can be used ultimately to derive relationships between the monetary value of production and levels of nutrient intake for different types of pigs growing under different physical and financial conditions. Several biological models simulating growth in the pig have been developed and applied in commercial practice (Whittemore and Fawcett, 1976; Stombaugh and Oko, 1980; Moughan, 1981; Phillips and MacHardy, 1982; Tess et al. 1983; Whittemore, 1983; Moughan and Smith, 1984; Black et al., 1986; Emmans,
1986; Moughan, et ai, 1987; Watt et al., 1987; Burlacu et al., 1988; Pettigrew et ai, 1989; Pomar et ai, 1991; Bridges et ai, 1992; Ferguson et ai, 1994; de Lange, 1995; Knap, 1996; van Milgen et ai, 2000) As the field of pig growth modelling develops, there is a tendency for models to become more causal (less empirical) and to further differentiate among the dietary nutrients and their ultimate metabolic fates (Boisen and Verstegen, 2000; Black, 2000a; Birkett and de Lange 2001). Models can provide predictions of the net utilization of individual nutrients, such as amino acids, glucose, fatty acids and volatile fatty acids. It is critical, in any biological pig growth model, to accurately simulate amino acid flow and thus be able to predict net body protein deposition and the supply of net energy from degraded amino acids. Implicit in this is the need to simulate the ingestion, digestion, absorption and metabolism of amino acids. The absorption and metabolism of amino acids in mammals is complex and highly integrated with continuous flux within and between body cells and compartments (see Chapters 1, 3, 4 and 5). It is useful, however, and inherently necessary when constructing a model of metabolism, to view amino acid metabolism as several discrete physiological processes (Table 11.2) which underlie or are causative to amino acid utilization. These processes, and how they can be modelled are discussed briefly below. The reader is referred to the recent reviews by Moughan (1999), Black (2000b) and Whittemore et ai (2001) for a more comprehensive treatment of the topics.
In Table 11.2, a distinction is made between the 'maintenance' or 'basal' processes and those associated with growth. However, the maintenance and growth processes are highly interrelated. For example, gut endogenous amino acid losses are often considered part of the maintenance cost, but increase substantially with food dry-matter intake and are thus correlated with growth. For the growing pig, 'maintenance' is a concept rather than a reality and by definition does not exist as a sole state for a growing animal. If an animal is forced to a state of zero dietary nitrogen balance, it is no longer growing, and at least in the short term is in a highly catabolic state with respect to body lipid reserves.
Body protein accretion Inevitable amino acid catabolism Gut endogenous amino acid loss Turnover of body protein
Synthesis of non-protein nitrogen-containing compounds Preferential amino acid catabolism aA distinction is made between basal or maintenance processes (i.e. those occurring in the hypothetical state whereby body tissue is neither being gained, nor lost) and those processes associated with the accretion of new body tissue. The rate of a process at 'maintenance' is defined as that rate commensurate with a daily food intake under which body weight is neither being gained nor lost. Rates of the processes during growth are variable. It should be noted that for most of the metabolic processes there is actually a natural continuum between maintenance and growth and that the distinction between stat8s is arbitrary and reliant on definition.
Nevertheless, it is considered helpful to conceptualize and represent overall metabolism in two parts: maintenance and growth. At zero nitrogen retention there are still costs associated with body protein metabolism and these are the classical 'basal' or 'maintenance' costs. For positive nitrogen retention, there are extra costs incurred associated with maintaining the proteinaceous body tissues, but these may be better classified as 'support costs for growth' (Table 11.2), rather than 'maintenance'. The partitioning of overall amino acid metabolism into a relatively constant 'basal' or 'maintenance' component and a separate, more variable component associated with production (protein deposition plus support costs) is consistent with the early arguments of Folin (1905) reiterated by Mitchell (1959).
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