3rd International Poultry Meat Congress

of 2 or more yolks at the same time. Feed restriction to control body weight during rearing
and in lay decreases the proportion of yellow follicles (future egg yolks) and leads both to
increased egg production and longevity (Table 9). In a large field study, Alvarez and Hocking
[18] showed that in feed restricted birds the incidence of multiple follicles in the first half of
the laying period ranged from 10-30% of breeders that would in turn lead to a decrease in the
rate of lay of settable eggs. Genetic selection to decrease the propensity for multiple ovulations
would contribute to increased egg production and, in the longer term, allow flock owners to feed
breeders to optimise their welfare without recourse to severe feed restriction. Interestingly, feed
restriction is relatively ineffective in turkey hens but is used successfully in ducks, as discussed
in more detail by Hocking [19]. Realistically, genetic selection for single ovulations is difficult
and awaits the development of indirect DNA-based methods.
Table 9. Body weight, number of yellow follicles and the proportion of yellow follicles
developing as pairs of similar size and ovulable status at the onset of lay in a line of broiler
chickens [20].
Feeding Body weight, kg Yellow follicles, N Multiple follicles, %
Ad libitum 5.6 11.8 64
Feed restricted 3.9 7.4 27
Inevitability of Genetic Challenges
Genetic improvement in broiler traits that contribute to profitability of the industry with
continuous reduction of the negative impact on the environment, and adaptability to a range
of environments in balanced breeding programmes, will continue into the future [21]. Intense
natural and artificial selection results in selective sweeps [22] which increases the frequency of
linked recessive genes (alleles) through linkage to selected genes [23] exposing cryptic genetic
variation. Genetic selection results in linked (and unlinked) genes with a positive correlation
reaching fixation, leaving recessive genes with negative correlations or those that are deleterious
to become more frequent. Because selection is intense, and effective populations sizes are
relatively low, these genes may become homozygous giving rise to negative correlations and
unexpected correlated responses to selection. Hocking [24] argued, on the basis of genetic
theory, that unexpected consequences of genetic selection, such as leg and muscle defects, were
therefore inevitable and that genetic selection in a balanced breeding programme to reduce
these effects are essential. To a large extent breeding companies have been successful in doing
so over the last 20-30 years [13, 14]. In addition, genetic drift may increase the frequency of
deleterious alleles and to mitigate this effect, breeders maintain large effective population sizes,
minimize relationships between relatives and maintain several selected and unselected reserve
lines [1].Furthermore the negative correlations with broiler traits that do arise are generally of
low magnitude (e.g. Table 6) and do not compromise genetic progress in the major production
traits in balanced breeding programmes.
General Conclusions
Genetic selection combined with breeding structures that multiply genetic improvement into
commercial flocks is responsible for 85% of the change in feed efficiency since the 1950s and
has increased the yield of high value breast meat by about 2-fold during the same period. Poultry
meat has made, and will continue to make, a substantial contribution to reducing the impact of
meat production on the environment in the face of an increasing human population and rising
demand for meat linked to increased affluence.
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