Breeding success in the broiler chicken has been accompanied by gait problems
which are detrimental to productivity and welfare. Although these gait issues have
not been reported to the same extent in Pekin ducks, there is concern that such
problems will manifest if the duck continues on its current selection trajectory.
In order to understand how changes in morphology due to selection have affected
gait in both species, divergent lines were objectively assessed for gait using a
pressure platform (12 birds per line at three, five and seven weeks of age). The
broiler chicken was compared to the slower growing layer chicken and the Pekin
duck to its slower growing ancestor, the mallard. Two breeding lines of Pekin duck
were also assessed. After gait assessment, the leg bones (femur and tibiotarsus)
were scanned by computed tomography to measure morphological changes which
have occurred due to selection for high growth and meat yield. Results were
analysed by ANOVA, accounting for age and sex.
During walking, heavy lines walked at a slower velocity, displayed a wider stance
and spent more time supporting their mass on both feet than their lighter
conspecifics, strategies which are likely to improve balance. The foot angle while
walking differed between lines; all duck lines rotated their feet internally whereas the
layer chickens’ feet were aligned with the direction of travel. Conversely the broiler
chicken rotated its feet externally by seven weeks of age.
Morphologically, the main differences were between species. Duck lines reached
adult leg size earlier than chickens, which may be a response to differing adaptive
environments prior to domestication. This early cessation of bone growth in ducks
may provide more opportunity for the bones to remodel to handle the loads imposed
on them. Lower levels of porosity and a unique cortical architecture observed in
ducks endow relatively greater bone strength. Bone curvature also differed between
species; the tibiotarsus curved more laterally in ducks than in chickens and may be
a swimming adaptation that hinders locomotion on land in the modern production
bird.
In order to improve the objectivity of selection for better gait in poultry, the genetic
parameters of gait components selected on the basis of results in this thesis were
estimated using a linear mixed model in a population of Pekin ducks of known
pedigree. As they are a simpler measure, similar or improved heritability estimates
were estimated for these gait components when compared with the standard
commercial gait score which is based on a subjective view of walking ability.
Intense selection for economic traits has altered gait in similar ways in both species.
To improve gait in poultry, greater breeding success may be achieved by focussing
on those components of gait which have changed through selection, rather than
using a subjective overall visual gait score. Furthermore, in both species,
adaptations for pre-domesticated life may have affected the ability with which the
selected lines have accommodated their gait to other morphological changes
associated with increasing body mass