Excluding disease, the single most important factor affecting the albumen quality of the freshly laid egg is the age of the bird that laid it. With advancing flock age, Haugh unit scores decrease and the variability of the scores increases. An induced pause in egg production is beneficial in largely restoring albumen quality in aging hens. Both strain differences and strain/age interaction effects in Haugh unit scores of fresh eggs have often been observed, but these are normally small and have little practical significance. Albumen quality of the egg is not greatly influenced by bird nutrition. Environment and housing, even heat stress, appear to have almost no direct effect on the albumen quality of the egg at oviposition.

When ambient temperatures are high, delays in egg collection and/or cooling increase the rate of Haugh unit score decline. The maintenance of albumen quality during egg storage is dependent on the eggs being cooled quickly following lay and subsequently being held at low temperatures, preferably down to, but not below, 0°C. Oiling of eggs within 24 h of lay is very effective in retarding deterioration of the albumen but does not replace the need for cool storage. Information on genetic effects on the albumen quality of stored eggs is equivocal, although there is general agreement that any effects are small and of little commercial significance. Of greater importance is the finding that within strains there can be great variation in Haugh unit scores and that strains laying white-shelled eggs are much less variable than those producing brown-shelled eggs.

Some new scientific developments are reviewed with respect to commercial application in poultry breeding. In the case of conventional breeding, ‘Best Linear Unbiased Prediction’ (BLUP) of breeding values and ‘juvenile selection’ schemes are evaluated. In the area of molecular genetic technology, ‘DNA fingerprinting’ and certain aspects of ‘gene transfer’ are considered.

The forceful arguments for the use of BLUP in dairy cattle breeding are not relevant to poultry. Its application is determined mainly by the balance between the additional cost of data processing and the value of the increase in genetic progress. The advantages of juvenile selection schemes for pure breeding are more obvious than for crossbreeding. The way in which the emphasis on crossbred records depends on the genetic correlation between purebred and crossbred information is also shown.

The prospects for DNA fingerprinting as an aid to selection will depend much on the identification of linked ‘trait genes’ having a marked influence on performance. In the case of gene transfer between species with the objective of influencing performance, viability appears to be a major obstacle to application in animal breeding. Such problems may be less serious when transferring genes for disease resistance. The application of these new developments is complicated by the fact that the aim of poultry breeding is to produce crossbreds. The new developments that have been assessed have to compete in terms of economic efficiency with the particularly favourable conditions which poultry provide for the application of conventional breeding procedures.

Temperature as one of the most important single physical factors determining or influencing embryo development and hatchability is reviewed and possible temperature manipulations are discussed as a management tool.

Pre-incubation egg holding temperature is reviewed as a factor acting on variability in developmental stage at oviposition. A mathematical approach for determining the temperature threshold or physiological zero for embryonic development is outlined. Temperature requirements during incubation are discussed, taking into account the ultimate aims and objectives of the incubation process, i.e. hatchability and chick quality for survival, growth and reproduction, challenging at the same time the idea that the best hatchability is synonymous with the highest chick quality.

Factors affecting limits of variation in incubation temperature, with particular reference to strain or line differences, are discussed.

Accessory reproductive fluids and organs of male domestic fowl, turkeys, ducks, quail, guinea fowl and pigeons were examined. Male domestic fowl, turkeys and ducks produced different quantities of accessory reproductive fluids – lymph-like fluid and/or foam – during natural copulation. Quail released a unique foam, which is produced constantly in the cloacal region regardless of the ejection of semen and the time of day. The natural ejaculate of this species is always associated with the release of foam. In contrast, guinea fowl and pigeons released neither lymph-like fluid nor foam at ejaculation when semen was collected using the lumbar massage method. The tissues and organs which are involved in the production of these accessory reproductive fluids are the paracloacal vascular body (PVB), lymphatic folds (LF), dorsal proctodeal glands (DPG), ejaculatory groove region (EGR) and tissues in the vicinity of the papilla of the ductus deferens (TVP). Guinea fowl possessed the PVB, DPG and TVP even though they produce no accessory reproductive fluids. The only exception was the pigeon which has neither the tissues nor the organs listed above. The accessory reproductive fluids have a favourable effect on the function of freshly ejaculated spermatozoa in domestic birds.

The time between a hatching egg being produced and set in an incubator can be divided into a laying house period and a storage period. The period in the laying house affects hatching results by the stage of embryonic development at oviposition as well as by the subsequent environmental conditions. Advanced embryonic development at oviposition or prior to storage seems to improve hatchability, especially when storage is prolonged. Stage of embryonic development at oviposition is influenced by parental age, strain, individual bird variation and clutch length. After oviposition and before storage embryonic development can be influenced by the type of nest, egg collection pattern and temperature.

During storage, hatchability is influenced by the length of the storage period, temperature, humidity, gaseous environment and the orientation of the eggs. A decrease in hatchability can be detected in eggs stored for 2–3 days or more. Storage temperature should be decreased with extended length of storage. Temporary heating before incubation and enclosing eggs in plastic bags during storage improves hatchability, especially when storage is prolonged. A high humidity during storage also improves hatchability, probably due to a reduction in water loss. The changes in albumen pH during storage are discussed in so far as they provide a possible explanation for relationships between environmental conditions during storage and hatching results.

A brief discussion is presented of the indirect effects (particularly through litter condition and leg problems) and the direct effects on carcase composition and quality of variation in protein:energy ratio, nutrient density, type and quality of added fat, fish oils, enzyme addition to the feed, mineral and vitamin levels and the use of certain drugs.