Genetic Influences on the Behavior of Chickens Associated with Welfare and Productivity - Genetics and the Behavior of Domestic Animals - page 330

realized heritability estimates of 0.37, 0.44, and 0.27, respectively. Of partic-
ular interest from the behavioral standpoint is that food consumption and
weight gain had a realized genetic correlation of 0.71. In a further study, the
similar results were reported from their 10 generations of chickens ( Pym
et al., 1984 ). Therefore, selection for weight gain in broilers also involves
considerable selection for appetite and vice versa. Food consumption records,
collected in addition to body weight information, involve considerable
expense in data collection but would allow selection for the economic trait
of food-conversion ratio also. Presumably because of the extra expense
involved and the fact that body weight and food-conversion ratio are
correlated in the desired direction ( 0.40 in the Pym and Nicholls study),

commercial breeders have chosen to select for increased gains in body
weight only. Obviously such selection has resulted in increased appetite
in broiler stocks. From the considerations above, it comes as no surprise
to learn ( Nir et al., 1978 ) that in comparisons of a light and heavy breed
to 18 days of age, heavy breed chicks consumed only 11% more feed
and failed to gain more weight than ad libitum fed controls of their own
breed when force-fed to gut capacity as compared to light-breed chicks
that consumed 43% more feed and gained 30% more body weight when
force-fed to capacity as compared to ad libitum fed controls of their
breed. Because of their huge appetites, meat-strain birds kept as broiler
breeders are severely restricted in feed intake to prevent obesity of adults
which would otherwise reduce health, longevity, and reproductive perfor-
mance, and increase the incidence of leg problems ( Decuypere et al.,
2010; Sandilands et al., 2005; Siegel, 1984; Su et al., 1999; Yu et al.,
1992 ).
Profound effects of selection for food intake and for body weight to a
given age in changing appetite and associated physiological phenomena have
been demonstrated by several investigators, and especially by Siegel and his
colleagues ( Siegel and Dunnington, 1990 ). The latter selected bidirectionally
for 8-week body weight over more than a quarter-century. Their high and
low lines diverged dramatically not only for body weight ( Dunnington and
Siegel, 1985 ) but also for specific aspects of food intake behavior
( Dunnington et al., 1987 ). Long-term selection for increased body weight
apparently resulted in “genetic lesions” of brain satiety centers whereas
selection for decreased body weight was offset by electrolytic lesioning of
the satiety centers ( Burkhart et al., 1983 ), respectively, involving the mela-
nocortin circuit of the brain ( Hen et al., 2006 ).
Because of poor appetite and associated low feed intake, minimal body
weights and fatness required for the onset of egg laying may not be met
( Bornstein et al., 1984; Zelenka et al., 1988 ). In the low-weight line
described above, some females are anorexic, eating so little feed ad libitum
that onset of lay is delayed or prevented ( Dunnington et al., 1984 ); thus over
50% of pullets did not mature in generations 25 and 26 ( Siegel and

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