Diet formulation
In commercial practice the protein content of food given to broiler chickens and growing pigs is reduced in a stepwise manner to match the changing needs of the animals as they grow. Even if choice feeding is difficult to apply commercially, it has the potential to be used to define more precisely the changes in requirements to allow more accurate formulation of rations. In particular, the optimum level of amino acids for animals of particular genotypes at different stages of growth can be assessed more comprehensively by diet selection experiments than by complex factorial experiments in which different groups of animals are given all combinations of likely amino acids at different stages of growth (Forbes and Covasa, 1995).
Based on results with choice feeding, the optimum single foods to be given to birds under different environmental temperatures have been recommended, as have levels of methionine inclusion in broiler foods and lysine inclusion in food for growing pigs.
Fig. 8.4. Effect of allowing sheep and goats 13% (solid bars), 57% (grey bars) or 70% (open bars) excess straw over intake on voluntary intake, organic matter (OM) digestibility and digestible OM intake (from Wahed et al., 1990).
New genotypes
If diet selection can be used to define more precisely the optimum content of protein, amino acids and any other essential nutrient, for existing genotypes, then it can also be used for novel genotypes, such as may be produced by genetic engineering techniques in the foreseeable future. Leaving aside the moral issues, there are likely to be major technical problems with the feeding and management of animals whose potential for growth has been elevated considerably but whose ability to ingest, digest and metabolize the food to support such growth is unknown. Diet selection experiments are likely to lead more rapidly to a resolution of these problems than classical factorial experiments but the limits of diet selection as a technique for assessing optimal nutrition have yet to be defined, and much more basic research is needed at this stage so that we are not to be taken unawares if, and when, novel genotypes are produced for commercial use.
New genotypes of crop plants can also be assessed as animal foods by diet selection techniques.
Conclusions
Given the appropriate conditions, diet selection methodology provides a very powerful tool for nutritional and behavioural scientists and has great commercial potential.
Currently, however, there are only two situations in which farm animals are offered choices in practice. One is grazing, which is not covered specifically in Fig. 8.5. Relation between amount of food organic matter (OM) offered and amount eaten according to the model of Oosting and Nordheim (1999). Phase 1, all offered food is eaten;
phase 2, intake increases with amount offered, according to theory of efficiency of utilization of ME; phase 3, further addition of food does not result in increased intake. Symbols are observed intakes: 䡩, untreated; ●, urea-treated rice straw.
this book. The other is the apparently widespread practice of offering poultry a mixture of whole cereal and a pelleted balancer food, either in separate troughs or, more usually, in the same trough. However, there is little evidence to date that individual birds improve their diet by choosing disproportionate amounts of the two foods.
Questions still to be answered include:
● Is it better to offer a standard complete food in choice with whole grain or to formulate a high-protein balancer to complement the cereal?
● Is the welfare of animals better served by offering them a choice of two or more foods?
Ultimately, it is economics that will dictate whether choice feeding is imple- mented commercially. In most research, non-standard foods have usually been used to try to ensure that selection is for or against a nutrient rather than an ingredient. In commercial practice least-cost formulation determines the raw materials to be used as ingredients, and some of these have sensory properties rendering them more or less attractive to animals and distorting their choice away from constructing for themselves a balanced diet. It remains to be seen whether choice feeding will be suitable in practice, especially for pigs.
9 Integrative Theories of Food Intake Control
This chapter deals with mechanistic modelling of voluntary food intake and diet selection over periods of 1 day or more. The philosophy of modelling, together with many examples from ruminant digestion and metabolism, is included in the book edited by Dijkstra et al. (2005), while Yearsley et al. (2001) discuss modelling of food intake in herbivores, with particular reference to extensive systems.
Some of the theories that have been put forward to explain the control of intake in birds and mammals are described. In many cases they are developments of simpler hypotheses described briefly in Chapter 1, which postulated that feeding is controlled by a single factor, usually acting in a negative feedback manner. Stomach distension (Chapter 3), hypothalamic temperature (Chapter 5), blood glucose concentration (Chapter 4), body fat stores (Chapters 4 and 15) and plasma amino acids (Chapter 4) have all, in their turn, been proposed as the factor whereby intake is controlled to match requirements.
However, none of these theories outlined so far can explain how intake is controlled under all circumstances. Balch and Campling (1962), having reviewed the control of voluntary food intake in non-ruminants, concluded that: ‘… food intake is unlikely to be regulated by any single mechanism and … oropharyngeal sensations, gastric contractions and distension, changes in heat production and changes in the levels of circulating metabolites, may severally be implicated’.
In a complex situation it is often helpful to construct a model to study the likely consequences of integrating the various parts of the system in a quantitative manner. At the very least, the attempt at model-building forces the would-be modeller to consider the main factors involved in the system under study and can usually point to what critical experiments are required in order to fill in gaps in concepts or data.
In the first edition of this book (Forbes, 1995), prediction of food intake was given a chapter to itself. In this edition this subject is incorporated into the appropriate parts of the text, including this chapter, because it is the aim of
188 © J.M. Forbes 2007.Voluntary Food Intake and Diet Selection
in Farm Animals 2nd Edition (J.M. Forbes)
theorists and modellers to develop quantitative hypotheses that reflect the main features of the intake control systems and should thereby produce good predictions of intake.