Exogenous enzyme supplements are used widely in poultry diets in an attempt to improve nutrient utilisation, the health and welfare of the birds, product quality and to reduce pollution as well as to increase the choice and content of ingredients which are acceptable for inclusion in diets. Considerable advances have been made during the last decade in the manufacture, activity, quality, thermostability and specificity of supplemental enzymes for use in poultry diets. This paper attempts to give an overview of the advantages and disadvantages, as well as the interesting difficulties and variable effects observed when enzymes are added to poultry diets.

The use of non-starch polysaccharide (NSP) degrading enzymes as feed additives has consolidated around their application to barley- and wheat-based diets for broiler chicks. Only with this application has a good understanding of their mode of action been developed and reasons for their effectiveness established. Most of the effect of NSP degrading enzymes can be ascribed to reversing the increase in digesta viscosity caused by polysaccharides leached from grain cell walls. Because digesta viscosity is greatest in the youngest birds and decreases with age, the value of supplementary enzymes for older birds is limited to secondary and less well defined effects. These include the release of nutrients otherwise entrapped in the cellular matrix of the feed. Maize alone amongst the commonly used cereals does not appear to release soluble NSPs in amounts sufficient to produce a detectable depression in performance. However, maize is not a homogeneous commodity and for some samples this may not be the case. Use of NSP degrading enzymes with legume seeds can improve nitrogen retention but effects are generally small. The structures of the soyabean NSPs, particularly the pectic polysaccharides, appear unique and existing commercial formulations are not recognised to contain enzymes capable of their degradation. With the present knowledge of soyabean polysaccharide structure, more informed formulation of enzyme products for use with soyabean should enable better responses to be achieved. When coupled with the lesser response from maize, this could allow the effective use of enzyme additives with maize-soyabean diets.

A series of factors have contributed to the progressive diversification of reproduction strategies to produce day-old chicks in distinct geographical locations. Some of these strategies are adaptable by simple technology transfer and education (e.g. darkout housing to control photoperiod, appropriate feeding and management of breeder flocks) while others require continued efforts from geneticists and physiologists to develop new genetic lines and tools (eg. selection of breeds adapted for hot climates and gene banking). Reproduction strategies in broiler breeders will also probably continue to diversify depending on economic and social components, including labour and feed costs as well as local regulations. As a consequence, the final choice between natural mating or artificial insemination (for caged or floored breeders) is itself multifactorial and may lead to the selection of parent stock lines optimised for one or the other strategy, according to country and living standards. Finally, the diversity in consumer demand for poultry will itself favour the emergence of specialised birds and management techniques directly adapted to local needs.

New strains of avian leukosis virus (ALV) and new threats to the poultry industry such as the appearance of subgroup J ALV arise as a result of virus mutation. Avian retroviruses are classified as exogenous (transmitted as infectious virus) and endogenous (transmitted genetically). Exogenous species are the avian leukosis-sarcoma viruses, reticuloendotheliosis virus, and lymphoproliferative disease virus of turkeys. Endogenous virus families include the ev loci, EAV, ART-CH and CR 1. Evolutionary relationships between different species of retrovirus are determined from their nucleotide sequences and construction of phylogenetic trees. Similar analyses are conducted to determine the relationships between endogenous and exogenous retroviruses and the various retroelements from which retroviruses are believed to have evolved. Retroviruses are subject to several types of mutation. Point mutations are the most common and occur mainly because of errors during reverse transcription. The frequency of point mutations is of the order of one mutation per virion per replicative cycle. Point mutations occur most frequently in the env gene of ALVs resulting in changes in antigenicity and host range. Antigenic variation enables mutant viruses to avoid the host immune response. Mutation also occurs from genetic recombination among exogenous and endogenous ALVs, and between exogenous ALVs and non-retroviral host genes such as cellular oncogenes, giving rise to acute avian leukaemia and sarcoma viruses. Thus, continuing viral mutation will result in the appearance of ALVs with new disease producing properties.

Chicken genetic resources comprise a wide range of breeds and populations including red jungle fowl (the assumed progenitor of all domestic breeds), native and fancy breeds, middle level food producers, industrial stocks and specialised lines. Based on the suggestion that the more distant a breed or population is the more likely it is to carry unique genetic features, the assessment of genetic distances by means of molecular marker information may provide useful information for initial evaluation of chicken genetic resources. During the last two decades several molecular marker classes have become available. Variable numbers of tandem repeat loci, in particular microsatellites, have been successfully used in chicken diversity studies. Genetic diversity measures using the highly polymorphic variable number of tandem repeat lociyield reliable and accurate information for the study of genetic relationships between chicken populations. First results of the European project on chicken biodiversity (AVIANDIV) obtained from microsatellite typing in DNA pools of 51 diverse chicken breeds showed that jungle fowl populations, traditional unselected breeds and broiler lines appear to be widely heterogeneous populations that may include a large portion of the genetic diversity of the tested breeds. In contrast, highly selected strains (layers and experimental lines) are characterised by a lower polymorphism. They behave as outliers from the set of breeds sampled. Single nucleotide polymorphism is a new and very promising molecular marker system which offers opportunities to assess the genetic diversity in farm animal species differently by investigating the mode and extent of changes in certain positions in the genome.

Pulmonary hypertension syndrome (PHS, ascites) in broiler chickens provides an outstanding example of the pathophysiological interplay between the lungs and heart. Recent research has confirmed the hypothesis that PHS susceptible broilers have an inherent potential to outgrow their pulmonary vascular capacity. Definitive proof that pulmonary hypertension (PH) is initiated as a consequence of an excessive pulmonary arterial resistance was obtained through measurements of pulmonary wedge pressures. Definitive proof that a pulmonary diffusion limitation causes the hypoxaemia characteristic of susceptible broilers was demonstrated by the rapid return to full arterial blood oxygenation in pre-ascitic broilers when they were provided with 100% oxygen to breathe. Experiments involving acute and chronic unilateral pulmonary occlusion provided definitive proof that the entire pathogenesis of PHS can be replicated by reducing the pulmonary vascular capacity. These experiments conclusively demonstrate that the resistance of broilers to PHS depends upon the capacity of their pulmonary vasculature to accept the requisite cardiac output at blood flow rates and pressures sufficiently low to avoid triggering PH and systemic hypoxaemia. The chronic unilateral pulmonary artery occlusion technique was successfully applied to develop a broiler line that is highly resistant to PHS induced by fast growth and exposure to rigorous cool temperatures, demonstrating for the first time that a dominant gene codes for a highly significant proportion of the PHS susceptibility. We now possess the knowledge necessary to create broiler lines capable of maintaining extremely rapid growth under rigorous field conditions while retaining very low susceptibility to PHS.

Fertility in poultry is generally measured as the “percentage fertile eggs” laid by hens during a period of time following artificial insemination or natural mating. This parameter has presented some problems for the statistical analysis of fertility and its main limitations derive from the binary classification of an egg as fertile or infertile. An alternative measurements of breeding efficiency is the number of soermatozoa that associate with eggs at fertilisation. These can be estimated in laid eggs and present a robust and fundamental measurement of the efficiency of sperm transfer into hens. In several applications they have revealed differences in the efficiency of mating or artificial insenination that were not discernible by traditional estimations of fertility.