Genetic evaluation systems require accurate estimates of genetic parameters. The genetic,
phenotypic and environmental parameters for objectively measured wool, subjectively
assessed wool and conformation traits as well as 16-month live weight were estimated for
South African Merino sheep. Records of the Tygerhoek Merino resource flock were used
to estimate these parameters. The database consisted of records of 4 495 animals, the
progeny of 449 sires and 1 831 dams born in the period 1989 to 2004. The pedigree
records used have been collected between 1969 and 2004. In this data, four lines were
represented, namely a line selected on clean fleece weight, a line selected against rearing
failure, a fine wool line, as well as an unselected control line. Single trait linear mixed
models were used. Fixed effects that were assessed included birth status
(single/multiple), sex (male/female), age of the dam in years (2-6+years), year of birth
(1989-2004) selection line (1-4) and the sex*birth year interaction. Preliminary analysis
showed that all the fixed effects had significant (P<0.05) effects on all the traits studied.
The random effects included a combination of direct additive, maternal additive and
maternal permanent environmental effects as well as the covariance between direct and
maternal additive effects.
The direct heritability estimates (h²a) for objectively measured traits were 0.38 for 16-
month live weight (LW), 0.36 for greasy fleece weight (GFW), 0.40 for clean fleece
weight (CFW), 0.65 for clean yield (CY), 0.37 for staple length (SL), 0.20 for staple
strength (SS), 0.68 for fibre diameter (FD), 0.61 for coefficient of variation of fibre
diameter (CVFD) and 0.61 for standard deviation of fibre diameter (SDFD). The
maternal heritability estimates were 0.05 for LW, 0.09 for GFW, 0.10 for CFW and 0.05
for FD. The proportion of the total phenotypic variance due to the maternal permanent
environment (c²pe) amounted to 5% for fleece weights. The correlation between direct and
maternal genetic effects for LW, GFW and CFW were -0.28, -0.65 and -0.70
respectively. These results suggested that worthwhile responses to selection for
objectively measured traits would result from directed selection. Estimates of h²a for subjectively assessed wool traits were 0.49 for wool quality (QUAL),
0.28 for regularity of crimp (ROC), 0.33 for wool colour (COL), 0.23 for wool oil (OIL),
0.21 staple formation (STAPL), 0.22 belly and points (BANDP), 0.50 for woolly face
score (WFS), 0.15 for face cover score (FCS), and 0.45 for pigmentation (PIGM).
Maternal genetic effects were significant for QUAL, ROC and COL amounting to 0.06,
0.03 and 0.07 respectively. The correlation between direct and maternal genetic effects
for QUAL, ROC and COL were -0.62, -0.70 and -0.45 respectively. The h²a estimates for
subjectively assessed conformation traits were 0.13 for topline (TOPL), 0.39 for total fold
score (TOT), 0.15 for front quarters (FQ), 0.16 for pastern score (PS), 0.32 for hocks
(HOCKS) and 0.37 for the general conformation score of the head (GEN). Among the
subjectively assessed conformation traits the maternal genetic effects were significant
only for PS at 0.05 while GEN was affected by c²pe at 5%. The correlation between direct
and maternal genetic effects was -0.71 for PS. The improvement of subjectively assessed
wool and conformation traits by selection seems possible from the results of present
study.
Genetic correlations (rg) among objectively assessed traits general agreed with literature
values. Among the subjectively assessed traits favourable rg estimates were estimated
between ROC and COL (0.31), for QUAL with ROC (0.49) and COL (0.26) and between
STAPL and BANDP (0.58). The relationships between ROC and STAPL (-0.49) and for
QUAL with STAPL (-0.45) and BANDP (-0.20) were unfavourable. Estimates of rg
among subjective conformation traits were generally low, variable in sign and not
significant. The only exceptions were rgâs between HOCKS and FQ (0.71) and of GEN
and TOPL with TOT (-0.31 and -0.47 respectively). Phenotypic and environmental
correlations among objective, subjective wool and conformation traits were low to
moderate. Maternal relationships of respectively 0.86 and 0.19 were found between GFW
and CFW and between CFW and FD.
The rg of LW with subjective wool traits were not significant, with the exception of
moderate and favourable rg for LW with ROC (0.20) and for LW with WFS (0.20). These
results suggested that heavier sheep are likely to have more even fleeces and higher scores for WFS. The rg were favourable with COL (-0.19), STAPL (0.35) and BANDP
(0.42). CFW and CY were also favourably related to COL, STAPL, BANDP and QUAL
where sheep with higher fleece weights and yield had wool with higher quality and better
scores for COL, STAPL as well as BANDP. SL was favourably related to COL, BANDP
and WFS and negatively related to OIL. These results indicated that sheep with longer
staples had better scores for COL, BANDP and WFS as well as lower scores for OIL.
Among the subjective wool traits only ROC was significantly related to SS at 0.33.
Moderate and unfavourable genetic correlations were estimated for FD with QUAL,
ROC and FCS, where sheep with a good quality wool that have more even fleeces and
softer wool covering the face had reduced FD. The corresponding relationship with
STAPL and BAND were unfavourable. Genetic correlations of CVFD and SDFD with
QUAL and ROC were favourable and while these traits were unfavourably related to a
number with STAPL. Phenotypic and environmental relationships between objectively
measured traits and subjectively assessed wool traits were low to high and variable in
sign.
On the genetic level LW was favourably related to all the subjectively assessed
conformation traits studied. Heavier sheep were generally plainer, had better scores for
GEN, HOCKS, and FQ as well as for TOPL. Estimates of rg were favourable for CY with
GEN (0.25), HOCKS (0.19), TOT (-0.26) and FQ (0.18), for FD with PS (-0.16) and
TOPL (-0.18), for CVFD with HOCKS (-0.17), FQ (-0.33) and TOPL (-0.25), between
CFW and GEN (0.23), and between SS and FQ (0.39). The relationships of SDFD with
FQ and TOPL were negative. Unfavourable correlations occurred between SS and TOT
(0.25), between FD and HOCKS and FD and TOT at 0.13, for fleece weights with TOT
(GFW 0.48 and CFW 0.28) and TOPL (GFW 0.34 and CFW 0.29) and between CY and
TOT at -0.26. Phenotypic and environmental relationships between objectively measured
traits and subjectively assessed wool traits were low to high in magnitude and variable in
sign.
The results showed that selection for LW and objective wool traits will thus not seriously
compromise subjective wool and conformation traits. The exception is FD, CVFD and SDFD, where unfavourable genetic relationships with STAPL and BANDP were found.
Furthermore, from these results, it was clear that unrestrained discrimination against
wrinkles in Merino sheep would adversely affect economically important objectively
measured wool traits such as SS, FD and wool weight. Selection against wrinkles should
be carefully monitored to minimise the risk of animals producing reduced fleece weights
with broader fibres. It is therefore, proposed that animals with excessive wrinkles be
culled from the breeding stock.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-08222008-082852 |
| Date | 22 August 2008 |
| Creators | Matebesi-Ranthimo, Puleng Agatham |
| Contributors | Prof SWP Cloete, Prof JB van Wyk |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-08222008-082852/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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