Studies on the environmental and genetic parameters for lamb survival, growth and wool traits of the Elsenburg Dormer and SA Mutton Merino flocks

Thesis (MScAgric)--Stellenbosch University, 2020.

ENGLISH ABSTRACT: Environmental and genetic parameters and trends of the Elsenburg South African Mutton Merino (SAMM) and Dormer resource flocks were estimated. The breeds were also compared for lamb survival, growth, wool, carcass and meat quality traits. Early growth and lamb survival data over a 64-year period (1955 - 2019) and yearling weight and wool data over a 36-year period (1983 - 2019) were used in the study for SAMM sheep. Single-trait heritability estimates of SAMM lambs amounted to 0.07 for birth weight (BW), 0.03 for weaning weight (WW), 0.02 for lamb survival (LS), 0.24 for yearling weight (YW), 0.58 for clean yield (CY), 0.37 for clean fleece weight (CFW), 0.34 for staple length (SL), 0.55 for coefficient of variation of fibre diameter (CVFD) and 0.68 for fibre diameter (FD). Maternal heritability estimates were 0.15 for BW, 0.06 for WW and 0.01 for LS. Multi-trait heritability estimates compared well with the single-trait estimates. Genetic correlations among weight traits ranged from 0.04 for BW and YW to 0.81 for WW and YW. Low to moderate correlations among yearling traits accorded with previous literature for wool breeds. Genetic trends showed favourable but extremely slow genetic responses. Data from 1943 to 2019 were used to investigate genetic and environmental parameters and trends for early growth traits and LS in the Dormer flock. Yearling live weight- and wool traits were recorded for a shorter period from 2008 to 2018. Direct single-trait heritability estimates were 0.15 for BW, 0.06 for WW, 0.02 for LS, 0.19 for YW, 0.63 for CY, 0.56 for CFW, 0.59 for SL, 0.59 for SS, 0.60 for CVFD and 0.80 for FD. Maternal genetic effects amounted to 0.21 for BW, 0.08 for WW and 0.06 for YW. Genetic correlations among live weight traits ranged from low (-0.11) between BW and YW to very high (0.93) for WW and YW. Genetic correlations among wool traits were mostly low. Genetic trends for all weight traits suggested significant positive trends with time. Expressed relative to the overall phenotypic mean, these trends amounted to 0.12% for BW, 0.16% for WW and 0.45% for YW. Selection from 2013 for direct-, maternal- and a combination of direct and maternal breeding values up to 2019 resulted in genetic trends amounting to 0.07% for BW, 0.79% for WW and 1.01% for YW in the line selected on direct breeding values for WW. Corresponding genetic trends amounted to respectively -0.51%, 0.17% and 0.27% in the line selected on maternal breeding values for WW and respectively -0.20%, 0.41% and 0.58% in the line selected for a combination of direct and maternal breeding values. Maternal genetic trends were markedly smaller in magnitude even in those lines were selection was based on maternal breeding values. Results indicated that SAMM lambs were heavier at birth compared to their Dormer contemporaries (4.19 vs. 4.13 kg). Dormers were heavier than SAMMs at weaning (27.3 vs. 25.0 kg) and also when weighed as yearlings (49.7 vs. 47.8 kg). Dormer lambs had a higher survival rate than SAMM lambs at 0.89 and 0.81, respectively. Clean fleece weight was the only wool trait not affected by breed. Dormer yearlings had a higher CY and SL than SAMM yearlings. In contrast, SAMM yearlings had a lower CVFD and finer wool than their Dormer contemporaries (respectively 22.4 vs. 28.6 μm). The two breeds did not differ for slaughter weight, carcass weight or dressing percentage. Dormers had a thicker subcutaneous fat cover at the rump than SAMM contemporaries (5.00 vs 4.18 mm). It was concluded that all traits considered were variable and heritable, although selection responses to traits such as lamb survival and weaning weight in SAMMs may be slow. Breed differences for economically important traits were consistent with the roles of the two breeds, with the Dormer as a coarse-woolled terminal sire breed and the SAMM as a dual-purpose breed.

AFRIKAANSE OPSOMMING: Omgewings- en genetiese parameters en -tendense van die Elsenburg Suid-Afrikaanse Vleismerino (SAVM)- en Dormerhulpbronkuddes is geëvalueer. Die rasse is ook vir lamoorlewing, sowel as groei-, wol-, karkas- en vleiseienskappe vergelyk. Data vir vroeë groei en lamoorlewing van SAVM’s oor ‘n 64-jaar periode (1955 - 2019) is gebruik, terwyl jaaroudgewig en woleienskappe oor 'n periode van 36 jaar (1983 - 2019) ontleed is. Beramings van enkel-eienskap oorerflikheid was 0.07 vir geboortegewig, 0.03 vir speengewig, 0.02 vir lamoorlewing, 0.24 vir jaaroudgewig, 0.58 vir skoonopbrengs, 0.37 vir skoonvaggewig, 0.34 vir stapellengte, 0.55 vir koëffisiënt van variasie (KV) van veseldikte en 0.68 vir veseldikte. Maternale oorerflikheidsberamings was 0.15 vir geboortegewig, 0.06 vir speengewig en 0.01 vir lamoorlewing. Beramings vir oorerflikheid met meer as een eienskap het goed vergelyk met die beramings vir enkel-eienskap oorerflikhede. Genetiese korrelasies tussen gewigseienskappe het gewissel van 0.04 vir geboortegewig en jaaroudgewig tot 0.81 vir speengewig en jaaroudgewig. Lae tot matige genetiese korrelasies tussen jaaroudeienskappe was in ooreenstemming met vorige literatuur vir wolrasse. Genetiese tendense het gunstige, maar uiters stadige genetiese vordering getoon. Genetiese- en omgewingsparameters en tendense vir vroeë groei eienskappe en lamoorlewing in die Dormerkudde is met data van 1943 tot 2019 ondersoek. Data vir jaaroudgewig en woleienskappe is oor 'n korter tydperk van 2008 tot 2018 aangeteken. Direkte enkel-eienskap oorerflikheidsberamings was 0.15 vir geboortegewig, 0.06 vir speengewig, 0.02 vir lamoorlewing, 0.19 vir jaaroudgewig, 0.63 vir skoonopbrengs, 0.56 vir skoonvaggewig, 0.59 vir stapellengte, 0.59 vir stapelsterkte, 0.60 vir KV van veseldikte en 0.80 vir veseldikte. Die maternale effek van die moeder was 0.21 vir geboortegewig, 0.08 vir speengewig en 0.06 vir jaaroudgewig. Genetiese korrelasies tussen gewigseienskappe was laag tussen geboortegewig en jaaroudgewig (0.11) en baie hoog vir speengewig en jaaroudgewig (0.93). Genetiese korrelasies tussen woleienskappe was meestal laag. Genetiese tendense vir alle gewigseienskappe dui op beduidende positiewe tendense met verloop van tyd. Relatief tot die algehele fenotipiese gemiddelde, het hierdie tendense 0.12% vir geboortegewig, 0.16% vir speengewig en 0.45% vir jaaroudgewig beloop. Seleksie vanaf 2013 vir direkte-, maternale- en 'n kombinasie van direkte en maternale teeltwaardes tot en met 2019 het gelei tot genetiese tendense van 0.07% vir geboortegewig, 0.79% vir speengewig en 1.01% vir jaaroudgewig in die lyn wat geselekteer is vir direkte teeltwaardes vir speengewig. Ooreenstemmende genetiese tendense was onderskeidelik -0.51%, 0.17% en 0.27% in die lyn wat geselekteer is vir maternale teeltwaardes vir speengewig en onderskeidelik -0.20%, 0.41% en 0.58% in die lyn wat gekies is vir 'n kombinasie van direkte- en maternale teeltwaardes. Maternale genetiese tendense was aansienlik kleiner, selfs in die lyne wat vir maternale teeltwaardes geselekteer is. Die rasvergelykingsresultate het gedui daarop dat SAVM-lammers swaarder was by geboorte in vergelyking met hul Dormer-tydgenote (4.19 vs. 4.13 kg). Dormers was swaarder as SAVMs by speen (27.3 vs. 25.0 kg) en ook by jaaroud (49.7 vs. 47.8 kg). Dormerlammers het beter oorleef as SAVM’s, met onderskeidelik 0.89 en 0.81 lammers gespeen per lam gebore. Skoonvaggewig was die enigste woleienskap wat nie deur die ras beïnvloed is nie. Jaaroud Dormers het hoër skoonopbrengs en stapellengte waardes as SAVM’s gehad. Daarenteen het jaaroud SAVM’s ‘n laer KV van veseldikte en fyner wol (onderskeidelik 22.4 vs. 28.6 μm) gehad as hul Dormer tydgenote. Die twee rasse het nie vir slaggewig, karkasgewig of uitslagpersentasie verskil nie. Dormers het 'n dikker onderhuidse vetbedekking op die kruis gehad as SAVM tydgenote (5.00 vs. 4.18 mm). Daar is tot die gevolgtrekking gekom dat alle eienskappe wat oorweeg is, veranderlik en oorerflik was, alhoewel response op seleksie by eienskappe soos lamoorlewing en speengewig by SAVMs stadig kan wees. Rasverskille vir ekonomies belangrike eienskappe stem ooreen met die rolle van die twee rasse, met die Dormer as 'n growwe wol terminale ramras en die SAVM as 'n dubbeldoelras.

Masters