Genetic evaluation of the South African dorper sheep breed

[摘要] ENGLISH ABSTRACT: The Dorper sheep breeders developed their own linear type scoring system based on a 5-point scalewhich assesses Conformation, Size, Type, Fat distribution and Colour. For many decades Dorper sheepbreeders have been so consistent with adherence to these breed standards without paying muchattention to performance testing of their stud animals. However, there is a paucity of informationpertaining 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 tobe 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 geneticprogress in the event that some antagonisms existed. Against this background, the major objectives ofthis study were to estimate genetic parameters and trends for production, reproduction, fitness andsubjective traits using data extracted from National Small Stock Improvement Scheme (NSIS).Furthermore, the study correlated performance data with subjectively assessed traits to derive geneticrelationships between them to establish the effect of selecting Dorper sheep on breed standards has onobjective traits of economic importance.Genetic parameters and relationships were estimated for subjectively assessed and objectively measuredtraits using linear and threshold methods. Linear methods were applied via the implementation ofResidual Maximum Likelihood (REML) procedures and Bayesian methods were implemented throughGibbs sampling. It was established through the implementation of single-trait and multi-trait analyses thatlive weight and growth traits were moderately to highly heritable. Maternal effects were also significant forsuch traits. Subjectively assessed traits were demonstrated to be lowly to moderately heritable using bothlinear and threshold methods. There were positive genetic and environmental correlations between liveweight, growth and subjectively assessed traits with the exception of Colour. There was favourableselection response to live weight and growth traits in a Dorper flock, with the exception of average dailygain during the post weaning phase where there was a slight negative trend. Subjectively assessed traitswith the exception of Size responded favourably to selection. It was concluded that breeders shouldconsider removing Colour from their breeding objectives, and focus more on selecting animals based onBLUP breeding values of objectively measured traits. The across flock genetic evaluation of all Dorperrecords demonstrated through the implementation of both linear and threshold methods that reproductionand fitness traits were lowly to moderately heritable and exhibited favourable genetic correlationsamongst themselves. It was further established that ewe rearing ability, ewe stayability and eweproductive life are lowly heritable and have some favourable correlations with component traits ofreproduction. There was little genetic change in reproduction and fitness traits, but traits generallydeteriorated where significant trends were found. It was concluded that breeders should select theiranimals on objectively measured production and reproduction traits and not put as much emphasis onbreeding standards.

[发布机构] Stellenbosch University