Direct and correlated responses to selection for high or low ultrasonic backfat depth in Southdown sheep : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University
Divergent selection was employed to establish high and low
lines for liveweight-adjusted backfat depth (LABF) assessed
ultrasonically in Southdown sheep. The selection lines were initially
constituted from several sources with stock brought-in during the first
three years of the experiment (1976-1978). These first years were used
to evaluate ultrasonic equipment for measuring backfat depth. The
lines were closed in 1979. Data analysed in this study were collected
over 8 years (1979-1986) representing, approximately 2.66 generations.
Selection was practised in two stages, with a preliminary
selection based on the first LABF on the rams and ewes, and a final
selection based on an average of all measurements assessed throughout
the year for the rams only.
Direct selection for high or low backfat depth resulted in
the 1986 born animals in the high line having about 1.69mm (59.6%DEV)
and 2.00mm (49.57%DEV) thicker backfat than the low line in the rams
and the ewes, respectively. The responses to selection per unit of
cumulated selection differential were in most cases high. Due to prior
selection and difficulties in assessing the selection pressure, it was
concluded that these regressions poorly represented the selection
Correlated responses to selection for and against backfat
depth were generally small. However, consistent positive correlated
response were observed in liveweight-adjusted height and length (LAH
and LAL) over the selection period. These responses imply a negative
genetic correlation between these traits and backfat depth . This
finding was in agreement with the published literature.
Phenotypic correlations were calculated within-trait
between-days and between-traits within-days. Correlations were pooled
within-trait following tests of homogeneity. The within trait values
were generally moderate to high and they were in agreement with the
values reported in the literature. The between-traits correlation
values were generally low, but were consistently negative for LABF-LAH
and LABF-LAL, and consistently positive for LAL-LAH . Repeatability
estimates, using the within-trait combinations, were also in agreement
with the literature and suggested a moderate to high repeatability for
LABF and LAH. Repeatability estimates for LAL were low to moderate and
they were slightly smaller than the values reported in the literature.
Estimates of the heritability of LABF varied with method
used. The paternal half-sib method resulted in low values (0.14 to
0.19) while dam/offspring method gave moderate values (0.29 to 0.43) .
Corresponding heritability estimates for LAH and LAL were about 0.31
and -0.14, respectively. These values were smaller than other results
quoted in the literature. It was concluded that truncation selection
on LABF reduced the genetic variability of these traits, although not
to the same extent as for LABF.
It was concluded that divergent selection for LABF was
effective, resulting in lines with significantly different backfat
depth at the same liveweight. Furthermore, selection for low LABF led
to significantly longer and taller animals.