The results from experimental studies about genetic factors (‘major genes’) involved in disease resistance and their effects in different genetic backgrounds are reviewed and discussed. The major genes considered are (1) Tv-A and Tv-B, gene loci controlling resistance to infection from avian leukosis virus (ALV), (2) B blood groups which represent the major histocompatibility complex (MHC) of the domestic fowl and are indicators of genetic differences in resistance to Marek's disease (MD) and, only tentatively, (3) endogenous viral (ev) genes which can influence reactions to leukosis infection. For these experiments purebred and crossbred progeny from three genetically different experimental White Leghorn lines have been used extensively. The results concerned with leukosis are based on a comparison between groups of chickens which were homozygous resistant or susceptible to ALV infection. The variability in the amount of damage from leukosis points to interactions between the genotype of resistance to ALV infection and genetic characteristics of the line or lines involved. In this respect the negative influence of ev 6 on the immune response to ALV infection could play a role. To study the association of B blood groups and resistance to MD, an experimental line which still segregated at the B locus (B2, B13, B14 and B21) was crossed with two other lines which were homozygous, respectively, for B2 and B15. Mortality caused by MD in birds carrying the same segregating B blood groups showed a completely different pattern in the two crosses. Rather unexpectedly, the difference in MD mortality was largest between the two types of crossbred layers carrying B21, which points to an interaction between line and B blood group type. Further studies suggested that this interaction depends on genes within the MHC for which the lines crossed might differ. The availability of information from different genetic groups allowed an evaluation of the importance of each ‘major gene’ in relation to the comparative effects of other factors influencing the trait under consideration. Apart from becoming aware of possible interactions resulting from genetic differences between lines, simultaneous results from genetically different experimental lines also provide a safeguard against overestimating or underestimating the potential of such ‘major genes’ for practical utilization in poultry breeding.

Dietary flax seed may prove to be a key feed ingredient in establishing the egg as a food useful for promoting cardiovascular health in humans. Yolk linolenic acid (LNA; 18:3n−3) increases in a linear fashion in response to graded levels of ground flax seed and whole flax seed if grit is provided in the feed to promote in vivo seed grinding. In contrast, the metabolic derivatives of LNA, eicosapentaenoic acid (EPA; 20:5n−3) and docosahexaenoic acid (DHA; 22:6n−3), are deposited only in limited amounts regardless of the level or form of flax seed fed. The distribution of the omega-3 polyunsaturated fatty acids (n−3 PUFA) in egg yolk lipid classes varies between laying stocks and these differences appear to influence the oxidative and sensory qualities of the product. Dietary flax seed results in apparently benign fluctuations in body weight of mature laying hens. Undesirable changes in egg production have only been reported for young birds early in their reproductive cycles. Changes in egg quality include reduced shell weight and decreased yolk weight. It is proposed that yolk weight reduction is related to changes in circulating oestradiol brought about by either n−3 PUFA or the phyto-oestrogens characteristic of flax seed.

Skeletal integrity of end-of-lay birds is becoming a major concern for both egg producers and meat processors. Osteoporosis induced by continual demands for shell calcification causes loss in skeletal integrity as the layer gets older. This loss in bone structure, coupled with minimal exercise, leads to very fragile bones in older birds. Bone breakage as high as 20% has been reported in birds entering the processing plant, while some degree of bone breakage is found in a much higher proportion of eviscerated carcasses. Bone breakage on the farm, during transport and subsequent handling is obviously a welfare concern, while processors are also concerned about bone contamination of meat products. Factors affecting the development of osteoporosis are discussed and include genetics, level of egg production, environment and nutrition. There is no simple solution to the problem in terms of long-term or short-term nutritional modification. In the case of caged birds, bone strength characteristics can be improved by removing them to other systems for a period of time prior to transport, although this is rarely a commercial solution. However, so-called alternative housing systems are themselves responsible for skeletal defects in some birds.

This paper considers some of the factors affecting the fertility and hatchability of guinea fowl eggs. In addition to genetic components there is a wide range of environmental influences that affect these two measures of performance. These include severe nutritional deficiencies, certain drugs and feed contamination with pesticides and mycotoxins. After eggs are laid, faulty handling and storage conditions and incorrect fumigation procedures have harmful effects on hatchability, while correct incubation temperature, humidity and ventilation rates during the setting and hatching stages are essential for optimum results. The causes of low levels of hatchability can be identified by the careful examination of eggs which fail to hatch. However, accurate and complete records relating to the breeder flock and the history of the eggs from the nest through to the completion of incubation are essential for problem solving and in helping to obtain optimum results.

A serological survey of brucellosis in traditionally managed local domestic fowl was carried out using the microtitre serum agglutination test with Brucella abortus antigen. Sixty two of 172 domestic fowl sampled (30%) were positive. Because most traditionally managed animals are not segregated, this indicates a potential source of infection to other livestock. Most of the birds identified as positive had titres of 1:40. The public health importance of the disease is highlighted and control measures suggested.