Thesis (PhD (Animal Sciences))--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The Dorper sheep breeders developed their own linear type scoring system based on a 5-point scale
which assesses Conformation, Size, Type, Fat distribution and Colour. For many decades Dorper sheep
breeders have been so consistent with adherence to these breed standards without paying much
attention to performance testing of their stud animals. However, there is a paucity of information
pertaining to the genetic relationships between visually assessed traits and objectively measured growth,
reproduction and fitness traits in the breed. Slow genetic gains in Dorper production traits are assumed to
be caused by over-accentuation of type traits but those assumptions needed to be scientifically validated.
It was therefore vital to derive these relationships as they could have a negative impact on genetic
progress in the event that some antagonisms existed. Against this background, the major objectives of
this study were to estimate genetic parameters and trends for production, reproduction, fitness and
subjective traits using data extracted from National Small Stock Improvement Scheme (NSIS).
Furthermore, the study correlated performance data with subjectively assessed traits to derive genetic
relationships between them to establish the effect of selecting Dorper sheep on breed standards has on
objective traits of economic importance.
Genetic parameters and relationships were estimated for subjectively assessed and objectively measured
traits using linear and threshold methods. Linear methods were applied via the implementation of
Residual Maximum Likelihood (REML) procedures and Bayesian methods were implemented through
Gibbs sampling. It was established through the implementation of single-trait and multi-trait analyses that
live weight and growth traits were moderately to highly heritable. Maternal effects were also significant for
such traits. Subjectively assessed traits were demonstrated to be lowly to moderately heritable using both
linear and threshold methods. There were positive genetic and environmental correlations between live
weight, growth and subjectively assessed traits with the exception of Colour. There was favourable
selection response to live weight and growth traits in a Dorper flock, with the exception of average daily
gain during the post weaning phase where there was a slight negative trend. Subjectively assessed traits
with the exception of Size responded favourably to selection. It was concluded that breeders should
consider removing Colour from their breeding objectives, and focus more on selecting animals based on
BLUP breeding values of objectively measured traits. The across flock genetic evaluation of all Dorper
records demonstrated through the implementation of both linear and threshold methods that reproduction
and fitness traits were lowly to moderately heritable and exhibited favourable genetic correlations
amongst themselves. It was further established that ewe rearing ability, ewe stayability and ewe
productive life are lowly heritable and have some favourable correlations with component traits of
reproduction. There was little genetic change in reproduction and fitness traits, but traits generally
deteriorated where significant trends were found. It was concluded that breeders should select their
animals on objectively measured production and reproduction traits and not put as much emphasis on
breeding standards. / AFRIKAANSE OPSOMMING: Die Dorper skaaptelers het hul eie liniêre puntestelsel ontwikkel wat op ‘n 5-punt skaal bouvorm, grootte,
tipe, vetverspreiding en kleur beoordeel. Dorper skaaptelers se fokus was vir baie dekades om hierdie
rasstandaarde na te kom, sonder om aandag te gee aan die prestasietoetsing van hul stoetdiere.
Rasverbetering in die Dorperskaapras is gebaseer op subjektiewe beoordeling van eienskappe soos dit in
die skouring bepaal word. Daar is egter ‘n gebrek aan inligting aangaande genetiese verwantskappe
tussen visueel beoordeelde eienskappe en objektiewe eienskappe soos groei, reproduksie en fiksheid.
Dit word aangeneem dat stadige genetiese vordering in produksie-eienskappe van Dorpers deur ‘n
oorbeklemtoning van tipe eienskappe veroorsaak word, maar hierdie aannames moet wetenskaplik
bewys word. Daarom is dit uiters belangrik om die verwantskappe tussen subjektiewe en objektiewe
eienskappe te bepaal, aangesien hulle ‘n moontlike negatiewe effek op genetiese vordering mag uitoefen
as daar wel antagonismes bestaan. Teen hierdie agtergrond is die hoofdoelwitte van hierdie studie om
prestasiedata vanuit die Nasionale Kleinveeverbeteringskema (NSIS) te onttrek en die beraming van
genetiese parameters en tendense vir produksie, reproduksie, fiksheid en subjektiewe eienskappe.
Verder het hierdie studie prestasiedata met subjektiewe beoordeelde eienskappe gekorreleer om
genetiese verwantskappe tussen subjektiewe en objektiewe eienskappe te bepaal.
Genetiese parameters en -verhoudings was beraam vir subjektief beoordeelde en objektiewe gemete
eienskappe met die gebruik van lineêre- en drumpelwaardemetodes. Lineêre metodes is toegepas d.m.v
die implementering van Residuele Maksimum Waarskynlikheid (REML) prosedures en die Bayesiaanse
metodes deur Gibbs steekproefneming. Dit is bevestig dat dat liggaamsgewig en groei-eienskappe matig
tot hoog oorerflik is. Maternale-effekte het ook ‘n beduidende invloed op hierdie eienskappe gehad.
Subjektiewe eienskappe is laag tot matig oorerflik, volgens beide lineêre en drempelwaarde metodes.
Daar was positiewe genetiese- en omgewingskorrelasies tussen liggaamsgewig, groei en subjektiewe
eienskappe, met die uitsondering van kleur. Daar was ‘n gunstige seleksie respons vir liggaamsgewig en
groei-eienskappe met die uitsondering van gemiddelde daaglikse toename gedurende die na-speense
fase wat ‘n afname in die gemiddelde voorspelde teelwaardes getoon het. Subjektiewe eienskappe, met
die uitsondering van grootte, het in die studietydperk geneties verbeter. Die gevolgtrekking is dat telers dit
moet oorweeg om kleur (subjektiewe eienskap) van hul teeldoelwitte te verwyder en om diere op BLUP
teelwaardes van objektiewe eienskappe moet selekteer. Die genetiese evaluasie van die nasionale kudde
het getoon dat reproduksie- en fiksheidseienskappe laag tot matig oorerflik is en gunstige korrelasies
onderlings toon. Dit is verder bevestig dat grootmaakvermoë, terughouvermoë en produktiewe leeftyd
laag oorerflik is, en sekere gunstige korrelasies met die komponente van reproduksie toon. Daar was
geen genetiese verandering in reproduksie en fiksheid eienskappe in die nasionale kudde nie, moontlik
omdat geen seleksie toegepas is nie, a.g.v ‘n oorbeklemtoning van rasstandaarde. Die gevolgtrekking is dat telers diere moet selekteer gebaseer op produksie en reproduksie eienskappe, en minder klem lê op
rasstandaarde.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/18105 |
| Date | 12 1900 |
| Creators | Zishiri, Oliver T. |
| Contributors | Cloete, S. W. P., Dzama, K., Oliver, J. J., Stellenbosch University. Faculty of AgriSciences. Department of Animal Sciences. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | ix, 193 p. : ill. (some col.) |
| Rights | Stellenbosch University |
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