Enhancing genetic evaluations for sheep growth and carcass traits in Ireland and the UK
The use of genetic evaluations within sheep breeding improvement schemes haveproved to be valuable to both the Irish and UK industries, with high levels ofeconomic gain achieved through enhanced growth, live body weight and carcasscomposition traits. Sheep live body weight and carcass composition traits areessential components in determining the profitability of sheep production systems.Previous genetic studies in Ireland have predominantly focused on producing geneticparameters across a multi-breed population rather than on a breed by breed basis.Genetic evaluations have already proven to be of significant economic benefit to theUK sheep industry with over 30 years of genetic selection having taken place.Furthermore, great potential exists for the development of across-country sheepgenetic evaluations particularly with the high levels of breeding stock and germplasmbeing exchanged between Ireland and the UK. These across-country evaluationscould in turn lead to more judicious genetic selection and increased levels of geneticgain as well economic benefits for the global sheep industry.The overall aim of this thesis was to enhance the genetic evaluations forsheep live body weight and carcass composition traits in Ireland and the UK.Specific objectives were to: (i) update the existing within-country genetic evaluationsin the two countries, and (ii) examine the feasibility of and develop a joint across country genetic evaluation system. Data were obtained from Sheep Ireland (the Irish national database) andAHDB (the UK national sheep breeder database). After all data edits were complete137,402 records from 50,372 Irish lambs and 132,490 records from 55,155 UKlambs born between 2010 and 2017 spread across 416 and 374 Irish and UK flocks,respectively, remained for further analyses. Data pertained to purebred Texel,Suffolk and Charollais sheep.Genetic analyses were undertaken on three live body weight traitsmeasured at different growth stages and two carcass composition traits in purebredIrish Texel, Suffolk and Charollais lambs. Variance components were estimated foreach trait using mixed linear models separately for each breed. Significant (P<0.05)heritability estimates ranged from 0.14 to 0.30 for live body weight traits and from0.15 to 0.31 for carcass composition traits across the three breeds. Positive geneticcorrelations were estimated between all traits for each of the three breeds studied.These results showed that significant levels of genetic variation exist both amonganimals and between breeds which in turn warrants genetic evaluations in Irelandbeing produced on a within breed basis.Within breed genetic parameters were derived for the UK sheeppopulation today for Texel, Suffolk and Charollais purebred lambs. Significant(P<0.05) heritability estimates for the three aforementioned breeds ranged frombetween 0.12 to 0.30 for two live body weight traits and from between 0.18 to 0.42for the two carcass composition traits. As with the previous study on Irish data,strong positive genetic correlations were observed between all traits analysed. Theseresults demonstrated that even after numerous years of selection within the UKpopulation much variation still exists meaning there is still much potential for geneticimprovement. In addition to this, genetic parameters differed significantly betweenbreeds particularly for carcass composition traits indicating that within breed analysisshould be considered for future genetic evaluation systems in the UK.The concluding study of this thesis was to produce international geneticevaluations for sheep from both Ireland and the UK. This study firstly determined thelevel of connectedness between the two countries with common animals to bothcountries identified. A total of 8,392 Texel parents with progeny in both countrieswere identified before data edits were applied. Genetic correlations were thenestimated between corresponding traits in both countries and ranged from between0.82 and 0.88. A bivariate analysis was completed to produce EBVs for acrosscountrygenetic evaluations. Response to selection was estimated from sire EBVsand results indicated higher rates of genetic gain being achieved when selection wasbased on across-country evaluations in comparison to within-country evaluations.Rates of genetic gain improved from between 2.59% and 19.63% from selectinganimals based on across-country genetic evaluations with the greatest rate ofimprovement observed for the ultra-sound scan weight trait. Most of theimprovement in genetic gain was achieved from the higher selection intensity whenrecords from both Ireland and the UK were pooled together.In conclusion, current within-country parameters have been updated anddemonstrate the merit of conducting genetic evaluations on a breed by breed basisrather than across breed. Results from this thesis also reveal that across-countrygenetic evaluations will be of significant benefit to the sheep industry in both Irelandand the UK if implemented in the future. There is huge potential in terms of bothgenetic and economic gain from producing international evaluations. Although thepresent study focused on live body weight and carcass traits, there is scope to includea range of maternal and other important animal traits as well as incorporating morebreeds and countries into these international evaluations and this should beinvestigated in the future.