Abstract:
Although fetomaternal disproportion is
the major cause of dystocia in heifers, pelvis area (PA)
is not recommended as a culling tool due to its relatively
low importance and genetic correlation with calf birth
weight (BWT), the most important factor associated
with dystocia. The objective of this observational study
of 484 limited bred yearling beef heifers was to compare
the effects of different methods of adjustment of PA data
for culling to select against dystocia. Multivariable analyses
were used to determine predictors of PA, calf BWT,
and dystocia. Hypothetical culling rates of 10 and 20%
were then applied after ranking heifers by each of the
following: unadjusted PA, PA adjusted to 365 d of age by
subtracting 0.27 cm2 per day of age difference between
each heifer’s age and 365 d (APA), PA:prebreeding
BW ratio (PA:BW), PA adjusted to the median BW of
the group using the regression coefficient of PA on BW
within age group (BWPA), and PA similarly adjusted to
the median lean BW (LBWPA). Dam parity, sire, prebreeding
age, prebreeding BW, and prebreeding BCS
were associated with PA whereas dam parity, sire, own BWT, PA, AI bull, and calf gender were associated with
calf BWT (P < 0.05). Dam parity, calf BWT, and either
BWPA or LBWPA were the only independent predictors
of dystocia (P < 0.05). Adjusting PA to BW or lean BW
(LBW) improved the sensitivity and specificity to predict
dystocia. After hypothetical culling by PA, retained
heifers were heavier and had a higher calving rate and
calves tended to be heavier at birth compared to culled
heifers, but dystocia rates were not different. Culling by
APA resulted in similar effects, except that dystocia rate
tended to be lower in retained heifers. Culling by PA:BW
resulted in lower dystocia rate in retained than in culled
heifers, but retained heifers had lower prebreeding BW
than culls. Culling by BWPA and LBWPA resulted in
lower proportions with dystocia and a tendency towards
higher calving rates in the retained heifers, without affecting
the prebreeding BW or calf BWT. It is concluded that
pelvimetry is a useful culling tool to aid in the management
of dystocia in yearling heifers and that adjustment
of PA to median BW or LBW within age group improves
its accuracy and avoids the undesirable side effects.
