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genotype that produced the greatest egg mass with beak treatment produced
the least with intact beaks. The re-ranking was shown to be due to mortality
from beak-inflicted injuries.
Results from Emsley et al. (1977) suggest that flightiness is associated
with higher productivity. They estimated genetic correlations between egg
production and flightiness score which showed that greater excitability was
mildly associated with higher rates of lay. Kashyap et al. (1981) , attempting
to break the correlation between production and flightiness, selected layers
using a selection index for aggregate economic gain based on genetic para-
meters estimated by Emsley et al. (1977) , which gave positive weight to egg
number and negative weight to excitability or flightiness. Nevertheless, their
results showed a positive response in excitability. Bennett et al. (1981) rea-
nalyzed those with retrospective indices and showed that genetic changes in
excitability were actually greater than what would have been predicted by
theory.
The Environment of Selection
Biswas and Craig (1970) compared White Leghorn hens of lines selected for
high- and low-social-dominance strains in single-hen cages and floor pens
and in crowded and un-crowded conditions in both cages and floor pens.
Significant interactions between genetic stock and environment were present
with reversals in ranking between environments. In the first study, high
social dominance hens matured earlier and laid more eggs per hen housed
when kept in single-hen cages but were later in maturity and laid fewer eggs
when kept in within-strain social groups in floor pens. In the second study,
changes in rank of performance were also found for age at first egg and
hen-housed egg production when the strains were compared in cage and
floor pen environments. High-strain hens were lower in performance than
low-strain hens when compared in intra-strain groups in floor pens or in
multiple-hen cages but were more or equally productive when kept in indi-
vidual cages.
A significant genotype by cage-environment (single versus multiple)
interaction for days' survival was observed in a random-bred population of
White Leghorns ( Muir, 1985 ). However, such an interaction can be caused
by a change in variance or a change in ranking. Muir et al. (1992) showed
that the interaction was mainly due to re-ranking of genotypes. Thus, the
bird which does the best in one environment is much poorer in the other and
vice versa.
Although there was no significant interaction for eggs per hen housed,
Muir (1985) showed that this result was due to a quadratic relationship of
surviving group size with rate of lay, whereby cages with seven surviving
birds had a higher rate of lay than those with eight or nine. Estimation of
genetic correlations assumes a linear relationship between genotype and
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