Concern about the cholesterol content of the human diet has led to many attempts to reduce the cholesterol content of eggs. Although this has involved a number of different approaches, including genetic selection and nutritional or pharmacological manipulation, the overwhelming evidence is that egg yolk cholesterol level is very resistant to change. The present review argues that this is because of the particular mechanisms involved in yolk formation. Yolk precursors are synthesized in the liver of the laying hen and transported in the plasma to the ovary where they are taken up into the developing follicles by receptor-mediated endocytosis. As a consequence, the cholesterol content of the yolk is primarily dependent on the cholesterol content of triglyceride-rich lipoproteins. Studies in mammals have shown that inhibition of cholesterol synthesis can reduce the rate of synthesis and secretion of lipoproteins by the liver, but has little effect on the composition of the lipoproteins that are secreted. This response, and the key role of cholesterol in the synthesis of steroid hormones, would seem to preclude the possibility of any substantial reductions in egg yolk cholesterol levels through manipulation of cholesterol metabolism. Possible alternative approaches are briefly discussed.

A literature search was carried out for papers which described the intermittent lighting of laying hens and included information on mortality and behavioural responses. A review of 36 sets of mortality data indicated that intermittent lighting generally improved bird liveability, compared with that under conventional lighting. Biomittent lighting, which involved low total daily amounts of illumination, gave the greatest improvement. This brings into question welfare recommendations for a minimum of 8 h illumination per 24-h cycle. Asymmetrical and Biomittent intermittent lighting generally reduced vices, obesity and heat stress. Hens submitted to asymmetrical and Biomittent intermittent lighting had similar endocrine periodicities and patterns of activity to conventionally illuminated birds.

Semen diluents are presently being used both for short- and long-term storage of domestic fowl semen. They are being developed to improve the reproductive efficiency of males and to lower the cost of artificial insemination. The development of semen diluents began with the use of simple NaCl solutions. Now, more complex diluents containing different osmotic regulators, energy sources, buffers and chelating agents are on the market. The discovery of cryoprotectants, such as glycerol and dimethylsulphoxide, has facilitated the preservation of chicken semen indefinitely at −196°C. The diluents which have been developed for short-term storage of semen are currently being used commercially wherever the technique of artificial insemination is being used seriously, especially in the turkey industry. Even though diluents for long-term semen storage are now available, their commercial application is not yet feasible.

This paper reviews the development of the various diluents which has resulted in the ability to preserve domestic fowl semen for short and long periods. Also discussed are some of the research data that have been collected on the viscosity of semen and semen diluents and what effects this has on semen quality.

Reports and observations relating to the skeleton and bone biology of the domestic fowl from the mid 16th century onwards have been assembled. Reference is made to early observations on osteology, pneumatic bones, bone growth, vital staining, bone cell microscopy, vitamin D in relation to rickets, genetics, archaeology and the folklore associated with the clavicle.

There has been a long and continuing interest among poultry breeders and scientists in relationships of domestic fowl with their jungle fowl ancestor. Recent commercial practices in poultry breeding have greatly changed and improved the meat and egg stocks available for the production of food. There is concern, however, that artificial selection for traits of economic importance has caused a narrowing of the genetic base and subsequent reduction in genetic variability. Because continued progress in the achievement of increased growth or egg production depends heavily on genetic variability, its present status is of interest. Current techniques in molecular biology allow estimations to be made of genetic distance among populations. This paper reviews DNA fingerprinting in poultry using results from this technique to survey genetic diversity among jungle fowl, domestic fowl representative of stocks from several decades ago and today's commercial broilers and layers. The genetic distance between wild jungle fowl and layers has been found to be similar to that between wild jungle fowl and broilers. Within the two types of modern commercial fowl there is less genetic diversity among broiler than layer stocks.

The possible influence of age at change in cycle length and differences in effective photoperiod (Δp) on the response of layers to ahemeral light cycles have been examined. A brief account of the physiology of pattern of lay in the hen is given.

When an ahemeral light cycle is applied, mean rate of lay in relation to a normal 24-h cycle changes. The change in mean rate of lay depends upon the rate of ovarian follicular maturation of the individuals in the flock at the time of change to an ahemeral cycle. Early in lay, when rate of follicular maturation is rapid, changing to a long light cycle reduces rate of lay, whereas changing to a short cycle could increase rate of lay. As rate of follicular maturation declines with age, changing to a long cycle late in lay leads to an improvement in rate of lay. Based on published reports, a theoretical curve of the response to a 28-h cycle is given showing the significance of age at which application is made. From the available literature it appears that, provided Δp ≥ 0, response to ahemeral light cycles is not affected.