Genetic analyses of South African terminal sire sheep breeds

[摘要] ENGLISH ABSTRACT: Fluctuations and a general decline in the ratio between wool and meat prices resulted in marked changes in theSouth African sheep industry. Commercial producers now exploit other mechanisms such as terminalcrossbreeding of Merino-type with meat type breeds or dual-purpose breeds to attain short-term benefitsresulting from price fluctuations between wool and mutton without compromising the wool-producing capacitiesof ewe flocks. Most components of lamb production have low heritability. However, heterosis can be achieved bymating wool-type breeds with specialist meat breed rams. Genetic improvement of livestock depends on definingbreeding objectives, estimation of genetic parameters and accurately identifying the right animals to be used forfuture breeding. Genetic parameters for traits of economic importance in terminal sire sheep breeds that couldbe used on Merino-type ewes in commercial operations in South Africa had not been published for the nationalflock apart from a preliminary study having been conducted by Olivier et al. (2004). Selection objectives werepoorly defined due to lack of parameter estimates for variance and covariance components. Against thisbackground, this study obtained pedigree information and live weight data from the National Small StockImprovement Scheme for the Dormer, Ile de France and Merino Landsheep and estimated non-genetic factorsand genetic parameters influencing early growth traits. Genetic and phenotypic trends for early growth traitswere constructed for the three breeds to monitor genetic progress.Non-genetic factors influencing early growth traits in the Dormer, Ile de France and Merino Landsheep wereestimated using data obtained from the National Small Stock Improvement Scheme of South Africa. The originaldata sets for the Dormer, Ile de France and Merino Landsheep consisted of the following number of recordsrespectively: 52 202, 35 553 and 7 772. However, pre-weaning weights were available for the Ile de France andMerino Landsheep breeds only and post-weaning weights were available only for the Dormer breed. The datasets were complicated to such an extent that smaller data sets had to be generated to analyse for fixed effects.The traits that were analysed were birth weight, pre-weaning weight, weaning weight and post-weaning weight.The fixed effects, identified as having a significant effect (P < 0.01) on early growth traits were sex of lamb, birthtype, age of dam, contemporary groups, age at which the trait was recorded and month of birth and year of birth in the Merino Landsheep breed. Although some significant interactions were found, they were subsequentlyignored owing to their very small effects. In all three breeds, male lambs were significantly (P < 0.001) heavierthan female lambs and single-borne lambs were significantly heavier at birth than multiple borne lambs. The ageof dam had a significant curvilinear regression on all early growth traits in all three terminal sire sheep breeds. Itwas concluded from the study that the influence of non-genetic factors on early growth traits should be adjustedfor or eliminated statistically in genetic evaluations to get accurate genetic parameter estimations.(Co)variance estimates for birth weight, weaning weight and post-weaning weight were obtained for the Dormerbreed using restricted maximum likelihood procedures (REML). Direct heritabilities (h2) in single-trait analyseswere 0.21 ± 0.03, 0.23 ± 0.02 and 0.29 ± 0.05 for birth weight, weaning weight and post-weaning weight,respectively. Direct heritabilities of 0.28 ± 0.04, 0.55 ± 0.06 and 0.32 ± 0.02 for birth weight, weaning weight andpost-weaning weight respectively were obtained using three-trait analysis. Direct maternal genetic effects (m2)were excluded from the analyses because of the failure to partition maternal effects into maternal genetic andmaternal permanent environmental effects (m2 and c2). This culminated as a consequence of poor data andpopulation structures emanating from the loss of genetic links across flocks due to the random entrance and exitof flocks from the recording scheme. Maternal permanent environment was estimated at 0.15 ± 0.02, 0.13 ± 0.02and 0.20 ± 0.03 for birth weight, weaning weight and post-weaning weight respectively using single-trait analysis.The correlation between direct effects and maternal effects (ram) was excluded from the analyses due thestructure of the data. Genetic, phenotypic and environmental correlations between early growth traits were low tomoderate. The medium to high heritability estimates for early growth traits obtained in the study led to theconclusion that Dormer sheep can successfully be used in terminal crossbreeding programs to improve meatproduction characteristics.Direct heritability estimates were 0.31 ± 0.14, 0.09 ± 0.02 and 0.14 ± 0.003 for birth weight, pre-weaning weightand weaning weight respectively using single-trait analysis for the Ile de France breed. Maternal effects weresignificant for all the traits studied despite the failure to properly partition them into their components due to theloss of genetic linkages across generations emanating from poor data structure. Genetic, phenotypic andenvironmental correlations were estimated using three-trait analysis and were found to be low to moderate forearly growth traits. Direct genetic and maternal permanent environmental ratios were also computed and theydid not differ much from the results obtained using single-trait analyses. The reasonable genetic parameterestimates obtained in the study led to the conclusion that the Ile de France can be selected to use as sires incrossbreeding programs.Genetic parameters were estimated for early growth traits in the Merino Landsheep breed. REML estimates ofbirth weight, pre-weaning weight and weaning weight were obtained using animal models in single-traitanalyses. The direct heritability estimate for birth weight was 0.23 ± 0.13 using an animal model with additivedirect genetic effects and dam permanent environmental effects as the only random factors. The dam permanentenvironmental effect for birth weight amounted to 0.10 ± 0.07. Direct heritability for pre-weaning weight was 0.36± 0.05 and the dam permanent environmental effect 0.56 ± 0.03. Weaning weight was estimated using an animal model that contained direct additive effects and dam permanent environmental effects. The direct heritabilityestimate for weaning weight was 0.17 ± 0.03. Maternal genetic effects were estimated to be 0.02 ± 0.01.Genetic and phenotypic trends were constructed for early growth traits in the Dormer, Ile de France and MerinoLandsheep breeds. The traits that were considered were birth weight, pre-weaning weight, weaning weight andpost-weaning weight. However, pre-weaning weights were available for the Ile de France and Merino Landsheepbreeds only and post-weaning weights were available only for the Dormer breed. The Dormer exhibitedsignificant improvement in the phenotypic and genetic aspects of early growth traits during the 17 years ofevaluation (1990-2007). The average predicted direct breeding values of birth weight decreased by 0.055 %during the evaluation period. The predicted direct breeding value for weaning weight increased by 0.12 % duringthe 17 year period. Post-weaning weight improved by 0.32 % per annum. The Ile de France registered anincrease in the predicted breeding value of birth weight which amounted to 0.025 % per annum. Averaged directbreeding values for pre-weaning weight increased at an annual rate of 0.23 %. and that of weaning weightincreased by 1.21 %. In the Merino Landsheep the predicted direct breeding value for birth weights decreasedby 0.04 % per annum and pre-weaning and weaning weights increased by 0.36 % and 0.10 % respectively. Thetrends were obviously biased due to inconsistencies in data structure and very few records available for analysisin this breed.In conclusion, it was evident that the additive genetic variation was available for all the early growth traits in allthe three breeds. Although adequate genetic variation for substantial genetic progress was available, onlymodest rates of progress were attained for all the traits in all three breeds. The only possible exception wasweaning weight in the Ile de France breed, which improved at > 1 % per annum. At least all changes were in thedesired direction. Breeders should be encouraged to record data consistently, as one of the major shortcomingsin the data for all breeds were a lack of continuity in the submission of data to the NSIS. More informativeanalyses ought to be feasible if this requisite could be met.

[发布机构] Stellenbosch University