Abstract :
Across flock genetic parameter estimates for growth, reproduction and fitness traits were
derived using residual maximum likelihood (REML) procedures for the South African Dorper
sheep breed. Data were retrieved from the National Small Stock Improvement Scheme
(NSIS) data base from breeders that were constantly submitting performance data for a
period spanning 30 years (1980–2009). Weaning weight and post-weaning weight were
moderately heritable with estimates being 0.21 ± 0.01 and 0.27 ± 0.02 respectively. Maternal
effects were significant, although of fairly low magnitude in live weight traits. It was
demonstrated that reproductive and fitness traits such as total weaning weight per ewe
lambing (TWW/L), number of lambs born per ewe lifetime (NLB/EL), number of lambs
weaned per ewe lifetime (NLW/EL), lamb survival to weaning and litter size were lowly
heritable with their estimates being 0.12 ± 0.02, 0.10 ± 0.01, 0.09 ± 0.01, 0.07 ± 0.01 and
0.07 ± 0.02 respectively. Selection of Dorper sheep with the objective of improving reproduction
and fitness is feasible although relatively low heritability estimates indicate that
annual genetic progress may be slow. Low heritabilities for ewe productive life (EPL) and
stay ability at two, three and four years of age were estimated to be 0.05 ± 0.01, 0.05 ± 0.02,
0.09 ± 0.03 and 0.11 ± 0.04 respectively. This indicated that these traits are, to some extent,
influenced by genetics. Lambing chances per ewe lifetime (LC/EL) and times lambed per ewe
lifetime were demonstrated to have a moderate heritability (0.23 ± 0.01 and 0.11 ± 0.04
respectively) and would also result in a correlated selection response in both reproduction
and longevity. No genetic basis was found for inter-lambing period (ILP). It was therefore
deduced that ILP can be improved through sound management. Live weight traits
were positively correlated with reproduction and fitness traits. TWW/L was genetically
and environmentally positively correlated with survival, litter size and EPL. It was concluded
that despite the low additive genetic variances for reproduction and fitness, genetic
improvement is feasible. Therefore, breeders should incorporate these traits in their selection
strategies to achieve maximum production.
