A genetic study of early growth traits and ewe productivity in merino sheep

Jaleta, Gemeda Duguma (2001-12)

Thesis (MScAgric)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: The objectives of this study were to investigate genetic parameters of early growth traits, lifetime ewe productivity (total number born, number weaned and weight weaned per ewe lifetime production) and testis measurements in the Merino flock maintained at the Tygerhoek Experimental Farm. Non-genetic fixed factors influencing these traits were evaluated by using appropriate models. The influence of non-genetic factors on preweaning lamb survival rate and the effect of scrotal circumference on ewe fertility was also studied. Fixed effect models were fitted to the data using either Procedure GLM or CATMOD in SAS (1996) depending on the parameters being estimated. Variance components were estimated by Restricted Maximum Likelihood (REML) procedures, fitting an animal model. Records of 8310 lambs born from 2538 ewes and sired by 681 rams covering the period 1970 to 1998 were used in the analysis of birth weight (BW), weaning weight (WW) and preweaning average daily gain (ADG). All three traits were affected (p < 0.001) by type of birth of lambs (single, multiple), sex, year of birth of lambs, group of animals (selection, control) and age of dam at lambing (2- to 6-yr old). Male lambs and singles were heavier both at birth and weaning and grew faster (p < 0.001) than females and multiples, respectively. Non-selected animals were lighter than selected animals at birth and weaning with an inferior growth rate. BW increased with increasing dam age at lambing until a maximum of 3.7 kg was reached at 6-yr of age. However, WW and ADG reached a maximum at 4.5- and 4.2-yr of age, respectively. Variance components for BW, WW and ADG were estimated using Restricted Maximum Likelihood (REML) procedures. Four different animal models were fitted, differentiated by including or excluding maternal effects. The direct heritability estimates (h") ranged from 0.19 to 0.38, 0.25 to 0.40 and 0.26 to 0.40 for BW, WW and ADG, respectively. Estimates of direct heritability (h") were substantially higher when maternal effects were ignored. The direct heritability (h") estimate for BW decreased from 0.38 to 0.19 when both or either of the maternal effects, genetic or environmental, were fitted in the model. The h2 estimate for WW was 0.36 when both maternal effects were ignored from the model and decreased to 0.29 and 0.25 when maternal effects were fitted. A similar trend was observed for ADG. The maternal heritability (rn-) for BW ranged from 0.38 to 0.41 when only maternal additive genetic effects were fitted in the model, and decreased to 0.25 and 0.27 when the maternal permanent environmental effect (c") was fitted. The m" for WW and ADG ranged from 0.02 to 0.11 and from 0.01 to 0.10, respectively. The respective c2 estimates ranged from 0.05 to 0.10 and from 0.02 to 0.09. Moderate negative genetic correlations (ram) between direct and maternal additive genetic effects were observed in BW, while close to zero estimates were obtained for WW and ADG. The direct additive genetic correlation estimates between BW and WW and BW and ADG were 0.16 and 0.04, respectively. The corresponding maternal additive genetic correlation estimates were 0.93 and 0.60. The direct and maternal additive genetic correlation estimates between WW and ADG were 0.99 and 0.85, respectively. Data of 3272 lambing records collected on 818 ewes born from 689 dams and sired by 371 rams were used in the evaluation of ewe lifetime productivity (total number born, number weaned and total weight weaned). These records were collected from ewes born from 1969 to 1994 and gave birth from 1971 to 1999. Year of birth of ewe and ewe two-tooth liveweight affected (p < 0.001) the ewe reproduction traits investigated. Multiple born ewes were superior both in total number of lambs born and weaned than singles. Heritability estimates of 0.23, 0.17 and 0.20 were obtained for total number of lambs born (TLB), total number of lambs weaned (TLW) and total weight of lambs weaned per ewe lambing over four lambing opportunities (TWW), respectively. For total weight weaned per ewe lambing over the first lambing opportunity (TWWl) a heritability estimate of 0.02 was computed. Genetic correlation estimates of -0.10, 0.57 and 1.00 were obtained between TWW1 and TLB, TWW1 and TLW and TWWl and TWW, respectively. High and positive genetic correlations ranging from 0.61 to 0.92 were estimated between ewe lifetime reproduction traits. Data from 1380 rams born from 1986 to 1998 were used to investigate genetic parameters for scrotal circumference (SC), testis diameter (TD) and two-tooth liveweight (LW). The other objective of this study was to estimate the magnitude of the effect of SC on ewe fertility. Year of birth, selection group and LW were significant (p < 0.001) sources of variation both for SC and TD. Heritability estimates ranged from 0.29 to 0.40, 0.25 to 0.38 and 0.49 to 0.52 for SC, TD and LW, respectively. Adjustment for LW decreased heritability estimates of SC and TD by 11.0 and 8.0 %, respectively. The genetic correlation between SC and TD was unity. Rams with larger SC had a significant effect on ewe fertility (ewes lambing per ewe mated). Average survival rate from birth to 100 days of age was 0.79 and was influenced by lamb birth weight, year of birth, age of dam at lambing, type of birth (single, multiple) and sex of lambs. Lambs with a liveweight between 4.0 to 4.9 kg at birth showed the highest survival rate while lambs with a liveweight of:s 2.0 kg showed the lowest survival rate.

AFRIKAANSE OPSOMMING: '0 Genetiese studie van vroeë groei-eienskappe en ooiproduktiwiteit by Merinoskape: Die doel van die studie was om genetiese parameters VIr vroeë groei-eienskappe, ooileeftydsproduksie (totale aantal gebore, aantal gespeen en totale gewig gespeen) en testismaatstawwe in die Merinokudde van die Tygerhoek Navorsingstasie te beraam. Niegenetiese vaste effekte wat hierdie eienskappe beïnvloed, is met behulp van gepaste modelle geëvalueer. Die invloed van nie-genetiese faktore op voorspeense lamoorlewing en die invloed van skrotumomvang op ooivrugbaarheid is ook ondersoek. Vaste-effek modelle is, afhangende van die parameters wat beraam is, deur middel van of die GLM of CATMOD prosedures van SAS (1996) op die data gepas. Variansiekomponente is met behulp van die Beperkte Maksimum Aanneemlikheidsprosedure (REML), deur die passing van 'n dieremodel, beraam. Rekords van 8310 lammers wat tussen 1970 en 1998 van 2538 ooie gebore is en die nageslag van 681 ramme was, is vir die ontleding van geboortegewig (BW), speengewig (WW) en voorspeense daaglikse toename (ADG) gebruik. Al drie eienskappe is deur geboortestatus (enkeling, meerling), geslag, jaar van geboorte, groep (seleksie, kontrole) en ouderdom van die moeder met lamming (2- tot 6-jr oud) beïnvloed (p<0.001). Ramlammers en enkelinge was met beide geboorte en speen swaarder en het vinniger (p<O.OOI) as onderskeidelik ooilammers en meerlinge gegroei. Nie-geselekteerde lammers was ligter as dié van die seleksiegroep met beide geboorte en speen en het ook stadiger gegroei. BW het met toename in moederouderdom tot 'n maksimum van 3.7 kg by 6-jr oud ooie gestyg. Maksimum WW en ADG is egter by 3- en 5 jr oud ooie onderskeidelik bereik. Variansiekomponente vir BW, WW en ADG is met behulp van Beperkte Maksimum Aanneemlikheidsprosedures (REML) beraam. Vier verskillende modelle is gepas, waarin mateme effekte óf in- óf uitgelaat is. Die direkte oorerflikhede (h2 ) het van 0.19 tot 0.38, 0.25 tot 0.40 en 0.26 tot 0.40 vir BW, WW en ADG onderskeidelik gevarieer. Beramings van die direkte oorerflikheid (h2 ) was aansienlik hoër waar die mateme effekte geïgnoreer is. Die direkte oorerflikheidsberaming (h2 ) vir BW het vanaf 0.38 tot 0.19 verlaag toe beide of een van die mateme effekte, geneties of omgewings, in die model ingesluit is. Die h2 beraming vir WW was 0.36 toe beide mateme effekte geïgnoreer is en het tot 0.29 en 0.25 gedaal toe beide in die model gepas is. 'n Soortegelyke tendens is vir ADG waargeneem. Die mateme oorerflikheid (rrr') van BW het vanaf 0.38 tot 0.41 gevarieer waar slegs die direkte mateme effek gepas is maar tot 0.25 en 0.27 gedaal toe die mateme permanente omgewingseffek (c2 ) additioneel gepas is. Die m2 vir WW en ADG het onderskeidelik van 0.02 tot 0.11 en van 0.0 I tot 0.10 gewissel. Die ooreenstemmende c2 beramings het vanaf 0.05 tot 0.10 en vanaf 0.02 tot 0.09 gewissel. Matige negatiewe genetiese korrelasies (ram) tussen die direkte en mateme genetiese effekte is vir BW gevind, terwyl bykans zero beramings vir beide WW en ADG gevind is. Die direkte genetiese korrelasies tussen BW en WW en tussen BW en ADG was onderskeidelik 0.16 en 0.04. Die ooreenstemmende mateme genetiese korrelasies was 0.93 en 0.60. Die genetiese korrelasies tussen die direkte en mateme genetiese effekte tussen WW en ADG was onderskeidelik 0.99 en 0.85. Data van 3272 lamrekords van 818 ooie vanaf689 moeders en 371 vaders is vir die evaluasie van ooileeftydsproduktiwiteit (totale aantal gebore, aantal gespeen en totale speengewig) gebruik. Hierdie rekords is van ooie wat tussen 1971 en 1999 gelam het, versamel. Jaar van geboorte van die ooi en ooi-tweetandgewig het die reproduksie-eienskappe ondersoek, beïnvloed (p<O.OOI). Meerlinggebore ooie was beter as enkelgebore ooie ten opsigte van aantal lammers gebore en gespeen. Oorerflikheidsberamings van onderskeidelik 0.23, 0.17 en 0.20 is vir aantal lammers gebore (TLB), totale aantal lammers gespeen (TLW) en totale gewig oor vier lamgeleenthede gespeen (TWW), gevind. Vir totale gewig per ooi met die eerste lamgeleentheid gespeen (TWWJ), is 'n oorerflikheidsberaming van 0.02 bereken. Genetiese korrelasies van -0.10, 0.57 en 1.00 is onderskeidelik tussen TWW, en TLB, TWW, en TLW en TWW, en TWW beraam. Hoë en positiewe genetiese korrelasies wat tussen 0.61 en 0.92 gevarieer het, is tussen ooileeftydsreproduksie-eienskappe beraam. Data van 1380 ramme wat tussen 1986 en 1998 gebore is, is gebruik om genetiese parameters van skrotumomvang (SC), testisdeursnee (TD) en tweetandgewig (LW) te beraam. 'n Ander doel van die ondersoek was om die effek van SC op ooivrugbaarheid te ondersoek. Jaar van geboorte, seleksiegroep en LW was betekenisvolle (p<0.001) bronne van variasie vir beide SC en TD. Oorerflikheidsberamings het van 0.29 tot 0.40, 0.25 tot 0.38 en 0.49 tot 0.52 vir onderskeidelik SC, TD en LW gevarieer. Korrigering vir LW het die oorerflikhede van SC en TD met onderskeidelik 11.0 en 8.0 % verlaag. Die genetiese korrelasie tussen SC en TD was een. Ramme met hoër SC het 'n betekenisvolle invloed op ooivrugbaarheid (ooie gelam per ooie gepaar) gehad. Die gemiddelde oorlewingstempo vanaf geboorte tot 100 dae was 0.79 en is deur geboortegewig van die lam, jaar van geboorte, ouderdom van die ooi, geboortestatus (enkeling, meerling) en geslag van die lam beïnvloed. Lammers met 'n geboortegewig van tussen 4.0 en 4.9 kg het die hoogste oorlewingstempo gehad, terwyl lammers met 'n geboortegewig van ~2.0 kg die laagste oorlewingstempo gehad het.

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