Genetics of ovine resistance to gastrointestinal nematodes

Mpetile, Ziyanda (2019-04)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: This study consists of various papers on the genetic evaluation of sheep for resistance to gastrointestinal nematodes in South Africa using faecal worm egg count (FWEC) as a proxy. Outcomes from the study suggested that Merino lines divergently selected for reproduction at Elsenburg research farm did not differ for log transformed FWEC. Heritability estimates for FWEC at Tygerhoek research farm were low in autumn (0.07 ± 0.05), but moderate in winter (0.13 ± 0.05) and spring (0.19 ± 0.05), suggesting that the best time to sample FWEC for genetic improvement in parasite resistance in Mediterranean South Africa is after the break of season. Genetic and phenotypic parameters for FWEC, live-weight traits, wool traits, subjective traits and testis dimensions were estimated in the Elsenburg Merino flock. The direct heritability estimates of these traits ranged from 0.09 ± 0.04 for FWEC to 0.60 ± 0.03 for clean yield (CY). Genetic correlations of FWEC with the other traits of economic importance were not significant, ranging from -0.24 ± 0.18 with crimps per 25 mm to 0.30 ± 0.23 for weaning weight, suggesting that selection for parasite resistance is unlikely to hamper genetic progress in other trait complexes. Genetic variation of FWEC was estimated in mature reproducing Elsenburg Merino ewes and correlated with number of lambs born and weaned. The heritability of FWEC was low at 0.03 ± 0.03, implying genetic change in FWEC of mature ewes would be difficult. Heritability of number of lams born and weaned was respectively 0.19 ± 0.03 and 0.11 ± 0.03. FWEC and reproduction traits were favourably correlated. An across-flock genetic evaluation of FWEC in South African Merinos at both Elsenburg and Tygerhoek yielded a heritability estimate of 0.11 ± 0.02 for FWEC. The sire x site/season variance ratio was low at <2%, suggesting minimal reranking of sires across flocks. Further studies suggested an overall breed difference for FWEC between Dormer and SA Mutton Merino yearlings and breeding ewes although the results were not consistent across years. Genetic parameters for FWEC and the Famacha© traits were investigated in Merinos at Tygerhoek and Dormers and SA Merinos at Elsenburg. All traits were heritable, suggesting that genetic improvement would be feasible should it be desired. Genetic correlations suggested that selection for lower eye scores would result in a change in FWEC at Elsenburg. Genetic correlations also suggested that animals with anaemic eye scores of 4 would be more likely to have high FWECs at Elsenburg, where haematophageous helminths were more likely because of irrigated pastures. The study provided useful information on genetic parameters for FWEC as indicator of host resistance to gastrointestinal nematodes and provided the background needed for planning further research on this important input trait.

AFRIKAANSE OPSOMMING: Die studie bestaan uit artikels aangaande die genetiese evaluasie van skape vir weerstand teen inwendige rondewurmparasiete deur wurmeiertellings (WET) as aanduiding te gebruik. Resultate het daarop gedui dat Merinolyne op Elsenburg navorsingsplaas wat uiteenlopend vir reproduksie of geselekteer is nie vir WET verskil het nie. Die oorerflikheid van WET op die Tygerhoek navorsingsplaas was laag in die herfs (0.07 ± 0.05) en matig in die winter (0.13 ± 0.05) en lente (0.19 ± 0.05). Die resultaat dui daarop dat monsterneming vir die bepaling van WET verkieslik na die eerste winterreëns in Meditereense Suid-Afrika gedoen moet word. Genetiese en fenotypiese parameters vir WET, liggaamsgewig-eienskappe, woleienskappe, subjektiewe eienskappe en testiseienskappe is in die Elsenburgkudde beraam. Direkte oorerflikheidsberamings vir die eienskappe het tussen 0.09 ± 0.04 vir WET en 0.60 ± 0.03 vir skoonopbrengs gevarieer. Genetiese korrelasies van WET met die ander ekonomies belangrike eienskappe was nie betekenisvol nie en het tussen -0.24 ± 0.18 vir kartels per 25 mm en 0.30 ± 0.23 vir speengewig gevarieer, wat daarop dui dat seleksie vir parasietweerstand waarskynlik nie genetiese vordering in ander eienskapkomplekse sal benadeel nie. Genetiese variasie in WET in volwasse Elsenburg Merino-ooie sowel as genetiese korrelasies met getal lammers gebore en speen is vervolgens beraam. Die oorerflikhed van WET was laag op 0.03 ± 0.03, aanduidend dat dit moeilik sal wees om WET in volwasse ooie geneties te verander. Die oorerflikhede van getal lammers gebore en gespeen was onderskeidelik 0.19 ± 0.03 en 0.11 ± 0.03. WET en reproduksie-eienskappe was gunstig gekorreleer. ‘n Oorkudde genetiese evaluasie vir WET met Merinos van beide Elsenburg en Tygerhoek het ‘n oorerflikheidsberaming van 0.11 ± 0.02 vir WET opgelewer. Die vaar x lokaliteit/seisoen variansieproporsie was laag teen <2%, wat daarop dui dat vaars se rangorde redelik konstant oor lokaliteite behoort te wees. Verdere studies het op ‘n algemene rasverskil vir WET tussen Dormer- en SA Vleismerino-jongskape sowel as volwasse ooie gedui, alhoewel die rasverskil nie konsekwent oor jare was nie. Genetiese parameters is vir WET en die Famacha©-eienskappe op Tygerhoek en Elsenburg beraam. Alle eienskappe was oorerflik, sodat genetiese vordering haalbaar behoort te wees indien dit verlang sou word. Genetiese korrelasies het daarop gedui dat diere met anemiese oogpunte van 4 op Elsenburg, met ‘n groter waarskynlikheid van bloedsuiende parasiete as gevolg van besproeide weidings, waarskynlik ook hoër WETs sou hê. Die studie het nuttige inligting verskaf oor WET as ‘n indikator-eienskap vir die weerstand van skape as gashere teen inwendige parasiete sowel as agtergrond vir verdere navorsing op hierdie belangrike inset-eienskap verskaf.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105886
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