Effects of physical processing on the nutritive value of poultry diets

Commercial production of poultry diets normally involves the admixture of a number of different feed ingredients. Some of these, particularly the protein supplements, will have undergone some degree of physical processing prior to receipt at the feed mill. For example, oil extraction, either by hydraulic press or solvent extraction, is applied to most oilseeds. In addition to removal of most of the oil, anti-nutritive factors such as protease inhibitors are inactivated to varying degrees during such processes. However, for the purposes of this paper, the term ‘physical processing’ is interpreted as relating to the treatment of materials during, or immediately following, admixture with the purpose of providing a balanced diet suitable for consumption by poultry. This process normally involves some degree of grinding of the material which improves uniformity of admixture, provides particles of a size perceived to be suited to the target group and may make nutrients more available for digestion in the bird. Subsequently, the feed may be subjected to heat conditioning and/or pelleting by one of several processes. Heat conditioning has been adopted widely as a means of reducing spoilage and pathogenic microorganisms (Cox et al., 1986) and pelleting has become widespread, particularly for broilers, as a means of improving growth and feed efficiency when compared with mash diets. The term ‘nutritive value’ is not easy to define. In effect it describes the interaction between the diet and the target species for the promotion of a particular physiological function, such as growth or egg production, under particular environmental conditions. Clearly two major components of any diet are energy and protein (amino acids) and the nutritive value of diets may be broadly defined in terms of these. However, for poultry, energy may be described in terms of true metabolizable energy (TME), apparent metabolizable energy (AME) or productive energy (PE). Equally, protein may be described in terms of crude protein (N × 6.25), the amino acid profile, ileal digestible amino acids, etc. Ultimately, a wide range of minerals and vitamins also contributes to nutritive value and should perhaps be considered within the framework of this title. However, for the purposes of this chapter the author will focus on aspects