Limitations of conventional models and a conceptual framework for a nutrient flow representation of energy utilization by animals

Abstract

Conventional models of energy utilization by animals, based on partitioning metabolizable energy (ME) intake or net energy (NE), are reviewed. The limitations of these methods are discussed, including various experimental, analytical and conceptual problems. Variation in the marginal efficiency of utilizing energy can be attributed to various factors: diet nutrient composition; animal effects on diet ME content; diet and animal effects on ME for maintenance (MEm); experimental methodology; and important statistical issues. ME partitioning can account for some of the variation due to animal factors, but not that related to nutrient source. In addition to many of the problems associated with ME, problems with NE pertain to: estimation of NE for maintenance (NEm); experimental and analytical methodology; and an inability to reflect variation in the metabolic use of NE. A conceptual framework is described for a new model of energy utilization by animals, based on representing explicit flows of the main nutrients and the important biochemical and biological transformations associated with their utilization. Differences in energetic efficiency from either dietary or animal factors can be predicted with this model. Modelling: Energy utilization: Nutrient flow representationMathematical models can integrate theories and observations into a coherent framework that can be useful for both conceptual and computational purposes. Animal models have been developed for a variety of species and applications: pig growth (Whittemore & Fawcett, 1976;Black et al. 1986; Moughan et al. 1987;Pomar et al. 1991a; Technisch Model Varkensvoeding, 1991); reproducing sows (Pomar et al. 1991b;Pettigrew et al. 1992); poultry production (Zoons et al. 1991;Hruby et al. 1994); growing sheep (Gill et al. 1984); growing fish (Machiels & Henken, 1986;; growing and reproducing beef cattle (Buchanan-Smith & Fox, 2000); and dairy cattle (Baldwin et al. 1987). Some of these models are based exclusively on empirical observations, such as direct relationships between daily lysine and energy intake, and average daily gain and backfat thickness in growing pigs, established using multiple linear regression (Carr et al. 1979). Application of such empirical models is limited to animal, environmental, and management conditions similar to those used in the trials on which they are based. Furthermore, this approach to representing animal production offers little insight into the mechanistic biological principles of which the measured performance is a consequence.In contrast to the empirical approach, highly complex mechanistic biochemical models have been developed to simulate nutrient metabolism at the level of individual tissues, using differential equations to represent (noncausally) relationships between the various metabolite flow rates. These mechanistic models are most useful for demonstrating biological and biochemical principles, especially at the cellular and inter-cellular levels. Wholeanimal models of this type have been developed, for example in mon...

Associations between feed efficiency, sexual maturity and fertility-related measures in young beef bulls

“…Metabolic differences also influence the timing of sexual maturity (Pruitt et al, 1986). These differences are known to influence body composition (Birkett and de Lange, 2001) and should be taken into consideration when modeling feed intake throughout performance assessments ). Feed efficiency (Owens et al, 1993) and fertility (Coe, 1999) vary with age, so young bulls evaluated for performance while still in the period of sexual development could display differences in feed efficiency and reproductive parameters such as organ biometry, semen quality, scrotum IR thermography, testicular ultrasonography and histomorphometry.…”

Characterization of useful energy content in swine and poultry feed ingredients

“…For example, dietary fat is energetically used more efficiently for body lipid deposition than for generating adenosine tri-phosphate (ATP) to support various metabolic functions, while for enzymatically digested carbohydrates these efficiencies are similar (Table 3). The aforementioned limitations also contribute to the substantial variation in estimates of k f and k p across studies involving growing pigs (Birkett and de Lange 2001a): 0.60 to 0.80 for k f and 0.44 to 0.60 for k p . It should be noted that some of the observed variability in estimates of k f and k p can be attributed to methodological problems, in particular the statistical interpretation of individual data sets where MEm, REl and REp are partly confounded (Birkett and de Lange 2001a).…”

“…The aforementioned limitations also contribute to the substantial variation in estimates of k f and k p across studies involving growing pigs (Birkett and de Lange 2001a): 0.60 to 0.80 for k f and 0.44 to 0.60 for k p . It should be noted that some of the observed variability in estimates of k f and k p can be attributed to methodological problems, in particular the statistical interpretation of individual data sets where MEm, REl and REp are partly confounded (Birkett and de Lange 2001a). Moreover, animal type has been shown to influence k p , which appears related to the varying ratio of increments in protein synthesis and protein deposition (e.g.…”

“…The difference between DE and ME contents in feeds is largely determined by urinary excretion of nitrogenous compounds. This difference is a function of feed digestible protein content and the fraction of absorbed protein that is not retained in the body, eggs, or milk (Whittemore 1983(Whittemore , 1987Birkett and de Lange 2001a). The latter implies that feed ME content varies with feed protein level and partitioning of absorbed protein between protein retention and use as an energy source.…”

“…The main limitation of this approach is that diet effects on the energetic efficiency of post-absorptive nutrient metabolism are not considered. Given the known differences in energetic efficiency of using the various classes of nutrients for different metabolic purposes (Black 1995;Birkett and de Lange 2001a; Table 3) it is no surprise that DE and ME systems are rather poor in predicting feed efficiency and other aspects of animal performance. For example, the GE supplied by digestible fat is converted more efficiently to body lipid than the GE that is supplied by enzymatically digested carbohydrates, such as starch and sugars.…”