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stress at 47 weeks of age, with results as reported by
Hester et al. (1996a,
1996b)
. Blood physiology and egg production were monitored before, dur-
ing, and after each of these periods. Packed cell volume immediately after
housing indicated that the selected line adapted to the new watering system
more quickly than the other lines. During cold stress the commercial and
control lines showed an increase in heterophil to lymphocyte ratio in 12-bird
cages while the selected line did not. Egg production before, during,
and after stress indicated that the selected line withstood social, handling,
and environmental stress better than the control and in some case the
selected line. Similar observations with heat stress showed that the selected
line withstood heat stress better as indicated by a lower mortality than the
control or commercial lines. Egg production before, during, and after heat
stress indicated that the selected line withstood social, handling, and environ-
mental stress better than the control line and in some cases the commercial.
Cheng et al. (2001a, b)
studied the same line compared with the revised
selected line (line with low production and high mortality) housed in single-
bird cages. At 21 weeks, the selected line had a higher CD4 to CD8 lympho-
cyte ratio and lower concentrations of dopamine and epinephrine and lower
ratios of epinephrine to norepinephrine and heterophil to lymphocyte ratios.
The bidirectional selection has resulted in the change in the immunity and
the expression of the neuroendocrine systems. In social stress studies using
the same lines plus a commercial line, DeKalb XL,
Cheng et al. (2002,
2003)
and
Cheng and Fahey (2009)
reported that the selected line has higher
resistance to various social stressors than the other two lines. These changes
magnify the differences in productivity and survivability observed in the
lines under basal and challenged conditions.
Conclusions from Group-Selection Studies
General conclusions were summarized by
Muir (2003)
and
Muir and Cheng
(2004)
. Selection to reduce the major economic and welfare problems of
feather pecking and cannibalism among laying hens may be successfully
achieved by either direct selection against pecking injuries or by means of
correlated responses associated with group (kin) selection for adaptation to
multiple-hen cages as indicated by mean hen-day rate of lay and survival.
Multi-Level Selection Experiments
Although (
Griffing 1977
) had developed equations to determine optimal
weights for between- and within-group deviations, those methods were only
published in a conference proceedings and were largely unknown. Further,
the results were academic at the time because methods to estimate genetic
parameters associated with the model did not exist as Henderson's
(
Henderson 1975, 1984; Henderson & Quaas, 1976
) mixed model methods
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