Genetic Influences on the Behavior of Chickens Associated with Welfare and Productivity - Genetics and the Behavior of Domestic Animals - page 331

Dunnington, 1987 ). Zelenka et al. (1988) were able to induce egg production
in non-layers of this line by force feeding.
Walking Problems and Tibial Dyschondroplasia
Tibial dyschondroplasia (TD), involving abnormal cartilage development in
the proximal end of the tibia, occurs primarily in broiler chickens being fed
ad libitum and the incidence is higher in males than in females. TD seriously
impairs walking ability when present in advanced stages and may reduce
weight gains due to the inability of birds to move freely in obtaining feed
and water ( Yalcin et al., 1995; Whitehead, 1997; Farquharson and Jefferies,
2000; Leach and Monsonego-Ornan, 2007 ). It has been shown that suscepti-
bility is genetically influenced and that selection procedures are effective in
separating lines selected bidirectionally for incidence ( Leach and Nesheim,
1965, 1972; Ray et al., 2006; Sheridan et al., 1978; Shirley et al., 2003;
Wong-Valle et al., 1993a ). Wong-Valle et al. (1993a) included an unselected
control in their selection study and after seven generations of selection
incidences at 7 weeks of age were 92.0, 5.4, and 13.2% in the high selected,
low selected, and control lines, respectively.
Primary breeders are concerned about the presence of TD in their stocks
as indicated by their use of an instrument which quickly and accurately diag-
noses the disease in live broilers ( Bartels et al., 1989; Wong-Valle et al.,
1993a ). Deleterious effects of the disease among prospective broiler breeders
are minimized by weight-control management. Although results are not
always consistent among studies, it appears from the selection study that
body weight at 7 weeks did not differ between the high- and low-incidence
lines in three of the first four generations ( Wong-Valle et al., 1993b ).
Kuhlers and McDaniel (1996) and Zhang et al. (1998) , using data from seven
generations of selection in the same stocks, indicated that TD expression and
body weights at 4 and 7 weeks of age are genetically and phenotypically
independent traits.
Fear-Associated Behavior
Fear is an emotional internal state which can cause physiological indicators
of stress ( Bronson and Eleftheriou, 1965; Lang and McTeague, 2009;
Ohmura and Yoshioka, 2009; Selye, 1956 ). Reliable behavioral indicators of
fear are difficult to establish. For example, Duncan and Filshie (1979)
showed that genetic stocks that appeared to be less fearful as indicated by
escape and avoidance behavior, exhibited as much fearfulness as another
stock when heart rate was used as the criterion of fearfulness. Also, different
behavioral indicators may yield inconsistent results when genetic stocks are
compared. Murphy (1978) outlined problems in classifying genetic stocks as
“flighty” and “docile”. Difficulties in classification of relative fearfulness