Genetic Parameters and Trends for Growth Traits in Sardi Sheep

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Small Ruminant Research

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / s m a l l r u m r e s

Estimates of genetic parameters and genetic trends for pre-weaning growth traits in Sardi sheep

I. Boujenane

^a,∗

, I.T. Diallo

^b

aDepartmentofAnimalProductionandBiotechnology,InstitutAgronomiqueetVétérinaireHassanII,P.O.Box6202Rabat-Instituts,10101Rabat,Morocco

bPROGEBE,Guinea

a r t i c l e i n f o

Articlehistory:

Received6May2016 Receivedinrevisedform 28November2016 Accepted1December2016 Availableonline5December2016

Keywords:

Sheep Sardibreed Growthtrait Maternaleffects Heritability Genetictrend

a b s t r a c t

Theaimofthepresentstudywastoestimate(co)variancecomponentsforweightsatbirth(BW),30days (W30d)and90days(W90d)ofage,aswellasaveragedailygainfrombirthto30days(ADG1)andfrom 30to90days(ADG2)ofSardisheep.Recordsof3709lambsthatdescendedfrom1170damsand85 sireswerecollectedoveraperiodof21yearsintheKraKrasheepfarm.(Co)variancecomponentsand thecorrespondinggeneticparameterswereestimatedunderRestrictedMaximumLikelihood(REML) methodbyfittingofsixdifferentanimalmodelswithignoringorincludingmaternaleffectsincluding maternalgeneticormaternalpermanentenvironmentaleffects.Thebestmodelwaschosenbasedon Akaike’sInformationCriterion(AIC).Bivariateanalysiswasperformedusingthemostappropriatemodels obtainedinunivariateanalysis.Ageofdam,typeofbirth,sexoflambandyearofbirthshowedsignificant effectsonstudiedtraits,sotheywereconsideredasfixedeffectsinanalyzingmodels.Directheritability estimatesforBW,W30d,W90d,ADG1andADG2were0.07±0.02,0.09±0.03,0.05±0.02,0.01±0.01 and0.05±0.02,respectively.Maternalheritabilityestimateswere0.13±0.03and0.21±0.04forBWand W30d,respectively.TheseresultsindicatethatmaternaleffectsonBWandW30dofSardilambsneedto beconsideredinanyselectionprogrammeundertakeninthisbreed.Maternalpermanentenvironmental effectsaccountedfor11%,8%and8%ofphenotypicvarianceforW90d,ADG1andADG2,respectively.

Thedirectgeneticcorrelationsbetweenstudiedtraitswerepositiveinallcasesandrangedfrom0.23 forBW-ADG1to0.98forW90d-ADG2.Thecorrespondingphenotypiccorrelationsvariedfrom−0.01for BW-ADG1to0.92forW30d-ADG1.ThematernaladditivegeneticcorrelationbetweenBWandW30d was0.66.Also,theenvironmentalcorrelationsamongstudiedtraitsvariedfrom−0.04forBW-ADG1to 0.93forW30d-ADG1.Annualgeneticgainsfordirectandmaternaladditivegeneticeffectsofstudied traitswereclosetozero.Thustorealizeageneticprogress,selectionshouldbebasedonadditivegenetic valuesinsteadofphenotypiccharacteristics.

©2016ElsevierB.V.Allrightsreserved.

1. Introduction

TheSardiisthemostimportantsheepbreedinMorocco.Itis anexcellentmeatbreedanditishighlydesiredforthecelebra- tionofthereligiousAidAl-Adha.Thismayexplaintheincreaseof itsnumber,reachingabout2.15millionhead,andtheextension ofitsgeographicalarea,goingfromthecentretothenorthofthe country.Theheightatwithersofadultramsandewesaveraged 80–90cmand55–65cm,respectively.Thishighsize,however,is notaccompaniedbyaheavyweight,sinceadultweightsoframs andewesaveragedonly65kgand45kg,respectively(Boujenane,

∗Correspondingauthor.

E-mailaddresses:i.boujenane@iav.ac.ma,ismail.boujenane@gmail.com (I.Boujenane),diatelly@yahoo.fr(I.T.Diallo).

2005),indicatingthatthereisenoughpotentialforincreasingof bodyweightandmeatproduction.Therefore,thereisacrucialneed fortheimprovementofgrowthpre-weaningtraitsofSardisheep.

However,inordertodesignbreedingstrategiesforSardisheep, geneticandnon-geneticparametersforgrowthtraitsshouldbe estimated.Boujenaneetal.(2001)havealreadyestimatedgenetic parametersforgrowthtraitsofSardisheepusingasiremodel.How- ever,numerousstudies(Mohammadietal.,2010;Savar-Soflaetal., 2011;Boujenaneetal.,2015)havedemonstratedthattheinclusion of maternal effects in animal models for growth traits, partic- ularly duringthepre-weaning period,hasanimportant impact ontheestimatesofdirectheritability.However,ignoringmater- naleffectsmightresultinoverestimationofgeneticparameters and mightincrease thebiasinpredictedresponsestoselection (Ghafouri-KesbiandEskandarinasab,2008).Thus,toachieveopti-

http://dx.doi.org/10.1016/j.smallrumres.2016.12.002 0921-4488/©2016ElsevierB.V.Allrightsreserved.

mumprogressinaselectionprogramme,bothdirectandmaternal componentsshouldbetakenintoaccount,especiallyinthepres- enceofanantagonisticrelationshipbetweenthem(Ekizetal.,2004;

Rashidietal.,2008).

Therefore,themainobjectiveofthecurrentstudywastoesti- matethegeneticparameters fordirect andmaternal effectson growthtraitsinSardi sheepbyfittingdifferentmodels,includ- ingdirectadditivegenetic,maternaladditivegeneticandmaternal permanentenvironmentaleffects.

2. Materialandmethods 2.1. Animalsandmanagement

ThestudywascarriedoutattheKraKrasheepselectionfarm locatedinthecentreofMorocco.Thefarmhasasemi-aridclimate, withanaverageannualrainof200mm.Growthdatawerecollected overaperiodof21years(1983–2004,except1995).Eachyearabout 200eweswerejoinedto6ramsforaperiodof75days,frommid- JunetotheendofAugust.Anocturnesinglesirematingsystemwas practisedwitharatioofabout40ewesperram.Atmating,aram offamilywasnotallowedtomateewesofitsownfamily.Lambing periodoccurredfromNovembertoFebruary.Ewelambswerebred at18monthsofage.Throughoutthestudy,replacementanimals wereselectedbasedonweaningweightandphysicalconformation.

Animalsweregrazed onnatural pasturesduringthe day.In general,grazingpasturegrassescoveredabout70%ofbreedingani- malsfeedrequirements.Theremainingof30%wascoveredbya supplementationprovidedduringthemating(600g/day),thelast monthof pregnancyandthebeginning oflactation (750g/day).

Ramsreceivedabout1kgperdayofconcentratemixture.Allani- malswereprovidedwithminerallicksandwateringwasallowed twiceaday;beforeandaftergrazing.

Sheepwerevaccinatedagainstenterotoxaemiaandsheep-pox andtreatedagainstinternalandexternalparasites.Also,sickani- malswereusuallytreated.

2.2. Datarecordingandstudiedtraits

Atbirth,lambs wereear-taggedand informationconcerning theirdam,birthdate,sexandtypeofbirthwereregistered.Lambs ranwiththedamsupto3monthswhentheyareweaned.Lambs wereweighedatbirthandevery3weeksuntilweaning.Investi- gatedtraitswereweightsatbirth(BW),30days(W30d)and90days (W90d).Thelasttwoweightswerecomputedbyapplyingthelinear interpolationtoweightsobtainedatweightingsrealizedjustbefore andaftertheconsideredage(30or90days).Averagedailygains frombirthto30days(ADG1)andfrom30to90days(ADG2)were calculatedassuminglineargrowthratebetweentheappropriate weights.

2.3. Statisticalanalyses

Growth data were collected over a period of 21 years (1983–2004,except1995) on3709Sardilambsbornfrom1170 damsand85siresregisteredintheflockbookofSardibreed.Ini- tially,the datasetincluded4323 records.However,lambs with implausible birth dates, unknown sex, unknown type of birth or born from unknown parents were discarded. The structure ofdata set,whichis used inthecurrent study,is presented in Table1. Toidentify fixedeffects tobe includedin themodels, least-squaresanalyseswereconductedusingtheGLMprocedure (SAS,2002).Thiswasperformedonamodelincludingfixedeffects of age of dam (≤30months,>30–≤42months,>42–≤54months and>54months),typeofbirth(singleandtwin),sexoflamb(male

andfemale)andperiodofbirth(from1982/83to2003/04).Inter- actionsbetween fixedeffects werenot tested, and hencewere assumedtobenegligible.Whenaneffectwasdeterminedtobe significant(P<0.05),differencesamongleast-squaresmeanswere examinedbytheTukeymethodformultiplecomparisons.Allthe effectsweresignificantforalltraits,and thereforetheseeffects wereincludedinthemodel.

Variancecomponentswereestimatedforeachtraitseparately withderivative-freerestrictedmaximumlikelihood(REML)pro- ceduresusingtheMTDFREMLprogramme(Boldmanetal.,1995).

Six different models were fitted for each trait, by ignoring or including maternal additive genetic effect, covariance between direct-maternaladditivegeneticeffectandmaternalpermanent environmentaleffect.Thesemodelsare:

y= Xb+Zaa+e (1)

y= Xb+Zaa+Wpepe+e (2)

y= Xb+Zaa+Zmm+e Cov(a,m) =0 (3) y= Xb+Zaa+Zmm+e Cov(a,m) =A␴am (4) y= Xb+Zaa+Zmm+Wpepe+e Cov(a,m) =0 (5) y= Xb+Z_aa+Z_mm+W_pepe+e Cov(a,m) =A␴am (6) whereyisavectorofobservations,bisavectoroffixedeffectswith incidencematrixX,a∼N(0,A_a²)andm∼N(0,A_m²)arevectorsof directandmaternaladditivegeneticeffectswithincidencematri- cesZaandZm,respectively,pe∼N(0,I_dpe²)isavectorofrandom maternalpermanentenvironmentaleffectswithincidencematrix Wpe,ande∼N(0,In_e²)isavectorofrandomresidualeffects.Also,_a² isthedirectadditivegeneticvariance,_m² isthematernaladditive geneticvariance,amisthecovariancebetweendirectandmaternal additivegeneticeffects, _pe² isthematernalpermanentenviron- mentalvariance,_e²istheresidualvariance,Aistheadditivegenetic relationshipmatrix,andI_d andIn areidentitymatricesoforder equaltothenumberofdamsandthenumberofrecords,respec- tively.Convergencewasconsideredreachedwhenthevarianceof functionvalues(−2logL)inthesimplex wasless than10⁻⁸.To ensurethataglobalmaximumwasreached,severalotherroundsof iterationswereusedusingresultsfromthepreviousroundasstart- ingvalues.Whenestimatesdidnotchangeattheseconddecimal, convergencewasconfirmed.

Inunivariateanalysis,theAkaike’sinformationcriterion(AIC) wasusedtodeterminethemostappropriatemodelforestimating (co)variancecomponentsforeachtraitasfollows(Akaike,1973):

AIC_i= −2logL_i+2p_i

wherelogL_iisthemaximizedloglikelihoodofmodeliatconver- genceandp_iisthenumberofindependentlyestimatedparameters ofmodeli.ThemodelwiththesmallestAICwasconsideredasthe mostappropriatemodel.

Total heritability estimates were calculated according to Willham(1972)as:

h²_t = _a²+0.5_m²+1.5am

_p²

Similarderivative-freeREMLprocedureswereusedforbivariate analysestoestimatecovariancesbetweeneachpairoftraits.The modelsappliedinbivariateanalyseswerethosefittedforeachof theunderlyingtraitsintheunivariateanalyses.Firstly,weusethe simplexvarianceoflessthan10⁻³toreachconvergence.Thenthe (co)varianceestimatesobtainedfromthepreviousrunwereused asstartingvaluestoreachconvergencewiththesimplexvariance oflessthan10⁻⁵.Finally,(co)variancecomponentsconsideredin thepresentstudywerethoseobtainedatthelastiteration.

Table1

ThecharacteristicsofthedatastructureforgrowthtraitsofSardisheep.

Item Trait^a

BW(kg) W30d(kg) W90d(kg) ADG1(g/day) ADG2(g/day)

Numberofrecords 3709 3537 3378 3537 3372

Numberofanimals 4323 4195 4059 4195 4053

Numberofsires 85 85 85 85 85

Numberofdams 1170 1157 1147 1157 1146

Averagenumberofprogenypersire 35.5 35.0 35.0 35.0 34.9

Averagenumberofprogenyperdam 3.14 3.03 2.92 3.03 2.92

Arithmeticmean 3.71 10.3 22.2 218 198

Standarddeviation 0.71 2.05 5.13 59.7 60.4

Coefficientofvariation,% 14.3 13.4 12.5 19.6 17.1

aBW=birthweight;W30d=weightat30days;W90d=weightat90days;ADG1=averagedailygainfrombirthto30days;ADG2=averagedailygainfrom30to90days.

Using thebest model for each trait, breedingvalues ofani- malswerepredictedwithBestLinearUnbiasedPrediction(BLUP) methodology.Theannualgeneticgainwasestimatedbycalculat- ingtheregressioncoefficientofadditivegeneticvalues(directand maternal)ofeachanimalonitsbirthyearusingtheREGprocedure (SAS,2002).

3. Resultsanddiscussion 3.1. Non-geneticeffects

Least-squaresmeansandrespectivestandarderrorsforstud- iedgrowthtraitsarepresentedinTable2.Allnon-geneticfactors hadsignificanteffectsonthestudiedtraits(P<0.001).Lambsborn fromyoungdamshadlowergrowthperformancesthanthoseborn fromadultewes.Theincreaseinlambgrowthwiththeincrease indam’sageisexplainedbytheimprovementofmotheringabil- ityandmilkproductionofmatureewes.Also,BW,W30d,W90d, ADG1andADG2 oflambsbornas singleswere1.03kg,1.92kg, 3.3kg,28g/dand24g/d,respectivelyhigherthanthoseoflambs born as twins. The increase in litter size frequentlyleads to a decreaseingrowthperformancesbecauseofcompetitionbetween twinsformilkproduction.Maleswereheavierandgrewfasterthan females.Differencesbetweenthetwosexeswere0.17kg,0.55kg, 1.7kg,12g/dand17g/dforBW,W30d,W90d,ADG1andADG2, respectively.GrowthtraitsofSardilambsvariedamongbirthyears.

Thoseborn in 1996/97and 2003/2004 had the highest perfor- mances,whereasthosebornin1982/83and1992/93hadthelowest performances.Thesedifferencesbetweenbirthperiodsoccurred probablybecauseofchangesthataffecttheprevailingclimaticcon- ditions,feedingandgeneralmanagementpractices.Non-genetic effectsongrowthtraitsareinagreementwiththosefoundinthe literature(Eskandarinasabetal.,2010;Mohammadietal.,2013;

Boujenaneetal.,2015).

3.2. Modelscomparison

Estimatesof(co)variancecomponentsandgeneticparameters forgrowthtraitsandAICvaluesforeachofthesixmodelsaresum- marizedinTables3and4.Model1,whichignoredmaternaleffects, resultedinhigherestimates fordirectadditivegeneticvariance (_a²)anddirectheritability(h²_d)thandidtheothermodelsforall traits,exceptforW30dandADG1.Introducingeitheramaternal permanentenvironmentaleffect(Model2)oramaternalgenetic effect(Model3)increasedtheloglikelihoodvaluesanddecreased estimatesfor_a² andh²_d.Theadditionofdirect-maternalgenetic covarianceincreased_a²andh²_d(Model4)forW30dandADG1,but notforBW,W90dandADG2.Whenmaternalgeneticandmaternal permanentenvironmentaleffects,withorwithoutdirect-maternal geneticcovariance,wereincludedinthemodel(Models5–6),_a² andh²_dweresimilartothoseinModels2–4forBW,W90dandADG2.

OnthebasisoftheAICtestresultsandthenumberofparameters usedinthepresentstudy,Model4,whichincludeddirectadditive geneticandmaternaladditivegeneticeffects,anddirect-maternal geneticcovariance, wasthemostappropriatemodeltodescribe BW and W30d, whereas Model 2, which included direct addi- tivegeneticand maternalpermanentenvironmentaleffect,was themostappropriatemodeltodescribeW90d,ADG1andADG2.

Thusforthelattertraits,thepermanentenvironmentaleffectof thedamwasdeterminedtobemoreimportantthanthematernal geneticeffect.Ghafouri-KesbiandBaneh(2012)reportedthatfor IranianMakooeisheep,Model2withthedirectadditivegenetic andmaternalpermanentenvironmentaleffectsandModel5with thedirectadditivegenetic,maternaladditivegeneticandmater- nalpermanentenvironmentaleffectswerethemostappropriate modelsforbirthweightandweaningweight,respectively,whereas Jafaroghlietal.(2010)showedthatmodels2–6werenotsignifi- cantlydifferentintermsofloglikelihoodwhenestimatingvariance componentsandgeneticparametersforgrowthtraitsinMoghani sheep.

Themagnitudeofmaternalpermanentenvironmentalvariance asaproportionofphenotypicvariance(pe²)obtainedbydiffer- entmodels(2,5and6)waschangedfrom3to9%forBW,0–11%

forW30d,6–11%forW90d,0–8%forADG1and0–8%forADG2, respectively.Moreover,theinclusionofmaternalpermanentenvi- ronmentaleffectsinmodel2,5and6reducedmaternaladditive geneticvarianceforalltraitsanddirectadditivegeneticvariance for BW, W30d and ADG1.As inbreeding mayreduce theaddi- tivegeneticvarianceandtheheritability(Kristensenetal.,2005), thereductionobservedinthecurrentstudycouldnotbedueto inbreedingsinceitsmeanwaszerointhestudiedflock.Thus if significant,pe² shouldbeconsideredinordertoobtainaccurate estimatesofdirectandmaternalheritabilities.

3.3. Geneticparameters

Basedonthemostappropriatemodels,directheritabilityesti- matesforBW,W30d,W90d,ADG1andADG2were0.07,0.09,0.05, 0.01and0.05,respectively.Theseestimatesforbodyweightswere intherangeofthosereportedinseveralstudies(Boujenaneand Kansari,2002;Jafaroghlietal.,2010;Rashidietal.,2008;Boujenane etal.,2015).However,theywerelower thanthoseobtainedby Miraei-Ashtianietal.(2007)inSangsarisheep(0.33),Kushwaha etal.(2009)inChoklasheep(0.20)andGamasaeeetal.(2010)in Mehrabansheep(0.30).DirectheritabilityestimatesforADG1and ADG2inthecurrentstudywereclosetothosereportedbyMiraei- Ashtianietal.(2007)foraveragedailygainfrombirthtoweaning inSangsarisheep,butlowerthanthosefoundbyJafaroghlietal.

(2010),Abegazetal.(2005)andRashidietal.(2008)foraverage dailygainfrombirthtoweaninginMoghani,HorroandKermani sheepbreeds,respectively.Thelowdirectheritabilityestimatesfor growthtraitsobtainedinthecurrentresearchmaybeattributed

Table2

Numberofobservations(N),least-squaresmeans(LSM)±standarderrors(SE)forgrowthtraitsinSardilambs^a.

Factorsofvariation BW(kg) W30d(kg) W90d(kg) ADG1(g/day) ADG2(g/day)

N LSM±SE N LSM±SE N LSM±SE N LSM±SE N LSM±,SE

Ageofdam(years) ^{*** *** *** * ***}

Age≤2.5 1145 3.23±0.02^c 1051 9.38±0.08^b 1028 20.9±0.22^c 1051 206±2,62^b 1028 191±2,64^c 2.5<Age≤3.5 936 3.40±0.02^b 908 9.67±0.09^a 845 21.3±0.22^b 908 210±2,73^a 844 193±2,73^bc 3.5<Age≤4.5 928 3.46±0.02^a 905 9.79±0.09^a 839 21.70.22^a 905 212±2,77^a 838 198±2,73^a Age>4.5 700 3.45±0.02^ab 673 9.65±0.09^a 666 21.5±0.23^ab 673 208±2,89^ab 662 196±2,81^ab

Typeofbirth ^{*** *** *** *** ***}

Single 3135 3.90±0.01^a 2994 10.6±0.07^a 2875 23.0±0.20^a 2994 223±2,32^a 2869 207±2,44^a

Twin 574 2.87±0.02^b 543 8.68±0.09^b 503 19.7±0.23^b 543 195±2,89^b 503 183±2,84^b

Sexoflamb ^{*** *** *** *** ***}

Female 1905 3.30±0.02^b 1825 9.35±0.08^b 1756 20.6±0.21^b 1825 203±2,52^b 1754 186±2,59^b

Male 1804 3.47±0.02^a 1712 9.90±0.08^a 1622 22.2±0.21^a 1712 215±2,51^a 1618 203±2,57^a

Periodofbirth ^{*** *** *** *** ***}

aLeast-squaresmeanswithinacolumnthatdonothaveacommonsuperscript(a–c)aresignificantlydifferent(P<0.05).Fortraitsabbreviations,seefootnoteofTable1.

* P<0.05.

***P<0.001.

Table3

Estimatesof(co)variancecomponentsandgeneticparameterestimatesforgrowthtraitsinSardisheepfromunivariateanalyses.^a

Trait Model _a² _m² am ²_pe ²_e ²_p h²_d±SE h²_m±SE ram pe² h²_t AIC

BW 1 0.03 0.26 0.28 0.09±0.02 0.09 −864

2 0.02 0.02 0.24 0.28 0.06±0.02 0.09 0.06 −898

3 0.02 0.02 0.24 0.28 0.06±0.02 0.09±0.02 0.11 −898

4 0.02 0.04 −0.02 0.24 0.28 0.07±0.02 0.13±0.03 −0.60 0.04 −899

5 0.02 0.01 0.02 0.24 0.28 0.06±0.02 0.03±0.02 0.06 0.09 −896

6 0.02 0.03 −0.01 0.01 0.24 0.28 0.07±0.02 0.10±0.03 −0.67 0.03 0.02 −897

W30d 1 0.12 1.80 1.92 0.06±0.02 0.06 5905

2 0.05 0.20 1.66 1.92 0.03±0.02 0.11 0.03 5865

3 0.05 0.20 1.66 1.92 0.03±0.02 0.11±0.02 0.03 5865

4 0.17 0.41 −0.27 1.60 1.91 0.09±0.03 0.21±0.04 −1.00 0.02 5832

5 0.05 0.13 0.08 1.66 1.92 0.03±0.02 0.07±0.01 0.04 0.06 5867

6 0.14 0.19 0.16 0.00 1.58 2.07 0.07±0.03 0.09±0.01 1.00 0.00 0.23 5903

W90d 1 0.92 6.93 7.85 0.12±0.03 0.12 10321

2 0.36 0.89 6.52 7.77 0.05±0.02 0.11 0.05 10272

3 0.36 0.89 6.52 7.76 0.05±0.02 0.11±0.02 0.10 10272

4 0.34 0.80 0.10 6.53 7.78 0.04±0.02 0.10±0.03 0.20 0.11 10274

5 0.36 0.11 0.77 6.52 7.77 0.05±0.02 0.01±0.02 0.10 0.05 10274

6 0.34 0.33 0.11 0.47 6.53 7.78 0.04±0.02 0.04±0.03 0.32 0.06 0.09 10276

²_a=directadditivegeneticvariance;²_m=maternaladditivegeneticvariance;am=direct-maternalgeneticcovariance;_pe²=maternalpermanentenvironmentalvariance;

²e=residualvariance;p²=phenotypicvariance;h²_d= ^a22 p

=heritabilityofdirectgeneticeffects;h²m= ^m22 p

=heritabilityofmaternalgeneticeffects;ram= _a^am_m=correlation betweendirectandmaternalgeneticeffects;pe²=^2c2

p

=maternalpermanentenvironmentalvarianceasaproportionofphenotypicvariance;h²_t= ^2a+0.5m22^+1.5am p

.,Fortraits abbreviations,seefootnoteofTable1.

Boldvaluesindicateestimatesfromthemostappropriatemodel.

aVariancecomponentsofweightsareinkg².

tothelowqualityofpasturesonwhichtheflockwasmaintained, resultinginahighenvironmentalvariance.Furthermore,according totheobtaineddirectheritabilityestimatesinthepresentstudy, growthtraitsinSardisheepareclassifiedaslowheritabletraits, indicatingthatperformanceofanimalsareless usefulinidenti- fyingtheindividualswiththehighgeneticmerit,andtherefore, lowgeneticprogresswouldbeexpectedthroughphenotypicselec- tionprogrammes.Thus,todetectthebestanimalsandtorealizea geneticprogress,selectionshouldbebasedonestimatesofbreed- ingvaluesandnotonrawperformanceoflambs.

AsAICvaluesclearlyindicate,maternaladditivegeneticeffects are important for BW and W30d since when using the most appropriatemodel(Model4),h²_mestimateswere0.13and 0.21, respectively,buttheseeffectswerenotessentialforW90d,ADG1 andADG2.Itisnotablethatdatastructurehasagreatimpacton theaccuracyofmaternaleffectsestimation.ManiatisandPollott (2003)reportedthat foraccurately separatingmaternalgenetic componentsandmaternalpermanentenvironmentaleffectsfrom

combinedanddirecteffects, alargedataset,severalwell-linked generationsofrecordsandmanyrelationshipsbetweenrelatives relatedtothemotherareneeded.Thesmallimpactofmaternal effectsonW90dandADG2inthepresentstudymaybeduetothe factthatsucklinglambsrelymainlyontheirmother’smilkfrom birthto30days,whereasafterthisage,theimportanceofmilkyield ofdamsdecreasesmorerapidlyandlambsdependmoreonthem- selves.Sotheimpactofmaternalgeneticpotentialonbodyweight gainoflambs,asreportedbyGizawetal.(2007),wasdecreasedand thelamb’sphenotypeforgrowthwasmorerelatedtolamb’saddi- tivegeneticeffects.Theh²_mestimateforBWinthecurrentstudy wasinagreementwiththosereportedbyothers(Ekizetal.,2004;

Mandaletal.,2006b;Kushwahaetal.,2009),butlowerthanesti- matesofRashidietal.(2008),Gamasaeeetal.(2010)andJafaroghli etal.(2010)indifferentsheepbreeds.Moreover,thelowmaternal geneticinfluenceongrwthtraitsmaybeduetothelackofadequate pasturethatcouldbemaskingtheexpressionofthematernalability oftheewe(Ghafouri-Kesbietal.,2008).

Table4

Estimatesof(co)variancecomponentsandgeneticparameterestimatesforgrowthtraitsinSardisheepfromunivariateanalyses.^a

Trait Model ²a ²m am pe² e² p² h²_d±SE h²m±SE ram pe² h²t AIC

ADG1 1 56 1772 1829 0.03±0.02 0.03 30006

2 14 145 1668 1827 0.01±0.01 0.08 0.01 29983

3 14 145 1668 1827 0.01±0.01 0.08±0.02 0.05 29983

4 158 143 150 1539 1990 0.08±0.04 0.07±0.03 1.00 0.23 30030

5 14 6 139 1667 1827 0.01±0.01 0.00±0.00 0.08 0.01 29985

6 119 128 124 0 1570 1941 0.06±0.03 0.07±0.03 1.00 0.00 0.19 30017

ADG2 1 114 1045 1159 0.10±0.03 0.10 27029

2 58 90 1003 1150 0.05±0.02 0.08 0.05 27005

3 58 90 1002 1151 0.05±0.02 0.08±0.02 0.09 27005

4 49 52 50 1007 1158 0.04±0.02 0.04±0.03 1.00 0.13 27005

5 58 0 90 1003 1151 0.05±0.02 0.00±0.00 0.08 0.05 27005

6 49 52 50 0 1008 1160 0.04±0.02 0.05±0.01 1.00 0.00 0.13 27007

_a²=directadditivegeneticvariance;_m²=maternaladditivegeneticvariance;am=direct-maternalgeneticcovariance;_pe²=maternalpermanentenvironmentalvariance;

_e²=residualvariance;²_p=phenotypicvariance;h²_d= ^2a₂

p

=heritabilityofdirectgeneticeffects;h²_m= ^2m₂

p

=heritabilityofmaternalgeneticeffects;ram= _a^am_m=correlation betweendirectandmaternalgeneticeffects;pe²=^c22

p

=maternalpermanentenvironmentalvarianceasaproportionofphenotypicvariance,h²_t= ^2a+0.52m2^+1.5am p

.Fortraits abbreviations,seefootnoteofTable1.

Boldvaluesindicateestimatesfromthemostappropriatemodel.

aVariancecomponentsofADGs’areing².

Table5

Phenotypic,directgenetic,maternalgenetic,maternalpermanentenvironmentalandenvironmentalcorrelationestimatesamongstudiedtraitsunderbivariateanimal models.^a

Traits¹ rp ra rm re

BW–W30d 0.37 0.72±0.18 0.66±0.14 0.32±0.02

BW–W90d 0.27 0.68±0.18 – 0.22±0.02

BW–ADG1 −0.01 0.23±0.34 – −0.04±0.02

BW–ADG2 0.12 0.54±0.23 – 0.07±0.02

W30d–W90d 0.72 0.55±0.23 – 0.73±0.01

W30d–ADG1 0.92 0.86±0.08 – 0.93±0.01

W30d–ADG2 0.30 0.28±0.31 – 0.29±0.02

W90d–ADG1 0.66 0.79±0.14 – 0.67±0.01

W90d–ADG2 0.88 0.98±0.02 – 0.88±0.01

ADG1–ADG2 0.28 0.62±0.23 – 0.26±0.02

1Fortraitsabbreviations,seefootnoteofTable1.

arp:phenotypiccorrelation;ra:directgeneticcorrelation;rm:maternalgeneticcorrelation;re:environmentalcorrelation.

Table6

AnnualgeneticprogressforgrowthperformanceofSardilambsfrom1983to2004^a. Trait^b Directadditivegeneticeffects Maternaladditivegeneticeffects BW(g) −1.12±0.18^*** 1.08±0.19^***

W30d(g) 0.17±0.40NS −0.26±0.62NS W90d(g) −2.65±0.88^** –

ADG1(g) −0.01±0.004^** – ADG2(g) −0.03±0.01^** –

aNS:P>0.05.

bFortraitsabbreviations,seefootnoteofTable1.

**P<0.01.

***P<0.001.

Atbirthand30days,h²_mwasgreaterthanh²_d.Estimateswere 0.13vs.0.07forBWand0.21vs.0.09forW30d.Similartendency wasobservedbyBahreiniBehzadietal.(2007)andGhafouri-Kesbi andEskandarinasab(2008)indicatingthattheinclusionofmater- nalgeneticeffectsinthemodelledtoagreaterdecreaseindirect heritability.Therefore,ifmaternaleffectsarepresent,theyshould befittedinthemodel,otherwise,estimatesofdirectheritability willbeoverestimatedandthetruegeneticpotentialoflambsis maskedbymaternalperformancemakingtheselectionofsuperior animalsdifficult(Ghafouri-KesbiandBaneh,2012).

Fromappropriatemodels,fractionsofvarianceduetomaternal permanentenvironmentaleffectsonphenotypicvariancewere11%

forW90dand8%forADG1andADG2.Ghafouri-Kesbietal.(2008) reportedthatestimatesofc²were4%forWWandJafaroghlietal.

(2010)foundthecorrespondingestimateofabout0.08foraverage dailygainfrombirthtoweaning.

The best model (Model 4) showed that fitting the direct- maternal genetic covariance resulted to estimates of direct- maternalgeneticcorrelation(ram)of−0.60and−1.00forBWand W30d,respectively.ManiatisandPollot(2003)reportedthatthe highernegativeestimatesmaybeattributedtothedatastructure.

Nevertheless, moderate to high negative estimates of correla- tionsbetweendirectandmaternalgeneticeffectsforpre-weaning growthtraitswerecommonlyreportedintheliterature(Abegaz et al.,2002; Boujenaneand Kansari,2002; Rashidiet al.,2008;

Kariukietal.,2010).Likewise,verylargenegativeestimateswere alsoreportedinanumberofotherstudies(Ekizetal.,2004;Mandal etal.,2006b).Cundiff(1972)postulatedthatfromanevolutionary pointofview,thenegativecovariancebetweendirectandmaternal geneticeffectspreventsspeciesfrombecomingincreasinglylarger.

Asaconsequenceofthenegativecorrelationbetweendirectand maternalgeneticeffect,differentrankingsofindividualsmaybe obtainedwhenthematernalcontributionisignoredintheevalua- tionprocedure(Szwaczkowskietal.,2006).

Estimatesoftotalheritability(h²_t)forBW,W30d,W90d,ADG1 andADG2were0.04,0.02,0.05,0.01and0.05,respectively.These estimatesareverylow,suggestingthatmassselectionwouldnot beveryeffectiveinimprovingthesetraits.Withregard topub- lishedestimates,thetotalheritabilityforgrowthtraitsobserved inthisstudywerecomparablewiththeresultsofseveralstudies (BoujenaneandKansari,2002inTimahditesheep;Ekizetal.,2004 inTurkishMerinosheep;Mandaletal.,2006ainMuzaffarnagari

-200 -150 -100 -50 0 50 100 150

1983 1985

1987 1989

1991 1993 1996

1998 2000 2002

2004

Year of birth

Average direct genetic value (g)

BW W30d W90d ADG1 ADG2

Fig.1.DirectgenetictrendsforBW,W30d,W90d,ADG1andADG2intheSardibreed.

-150 -100 -50 0 50 100

1983 1985

1987 1989

1991 1993

1996 1998 2000

2002 2004

Year of birth Average maternal genetic value (g)

BW W30d

Fig.2.MaternalgenetictrendsforBWandW30dintheSardibreed.

sheep),butlowerthantheresultsofseveralstudies(Neseretal., 2001;Safari etal.,2005;Matikaetal.,2003)indifferentbreeds ofsheep.Willham(1972)advocatedthatwherematernalgenetic effectsarepresent,thepotentialresponsetoselectionmightbet- terbeexpressedbyh²_t.However,ifthereisnegativecovariance betweendirectandmaternalgeneticeffects,whichisthecaseinthe presentstudyforBWandW30d,phenotypicresponsetoselection maybediminished(Wolfetal.,1998).

Theresultsobtainedfrombivariateanalysesarepresentedin Table5.Estimatesofdirectgeneticcorrelationsbetweengrowth traitswerepositiveandmediumtohigh,varyingfrom0.23between BW and ADG1 to0.98 betweenW90d and ADG2. The positive directgeneticcorrelationsamongthestudiedtraitssuggestthat geneticfactorswhichinfluencethesetraitswereinsimilardirec- tionandthatselectionforanyofthesetraitswillbringoutapositive responsetoselectionforothers.Moreover,thedecreaseofdirect geneticcorrelationestimatesofBWwithW30dandW90dwith ageisinagreementwiththepublishedvaluesinotherliteratures (Mohammadietal.,2010;Abbasietal.,2012;Mohammadietal., 2013).Similartoourestimates,highandpositivegeneticcorrela- tionsbetweengrowthtraitshavebeenreportedbyseveralauthors

invarioussheepbreeds(Miraei-Ashtianietal.,2007;Rashidietal., 2008;Jafaroghlietal.,2010).Consequently,itislogicaltosuggest thatthetraitstobeincludedinthesheeprecordingsystemcould mainlybethose measuredearlyinlifeofthelambs, e.g.before weaning.

ExceptthematernalgeneticcorrelationbetweenBWandW30d thatwasestimatedto0.66,thosebetweentheothertraitswerenot estimatedbecausetheirbestmodelsdidnotincludethematernal geneticeffects.Thiscorrelationindicatesthatmaternaleffectsare partlyoriginatingfromtheprenatalperiodandcouldhavesome favourableeffectsonpost-natalgrowthtraitsofSardilambs.This estimateisinagreementwiththatofBahreiniBehzadietal.(2007) (0.68inKermanilambs)buthigherthanthatofMohammadietal.

(2010)(0.30inShallambs).

Phenotypiccorrelationsvariedfrom0.27and0.72amongbody weightsandvariedfrom−0.01and0.92betweenweightsandADG.

Thehighestphenotypiccorrelation(0.92)wasrecordedbetween W30dandADG1andthelowest(−0.01)wasfoundbetweenBW and ADG1.Theseestimates werecomparable tothosereported intheliterature(BahreiniBehzadi etal.,2007;Gamasaeeetal., 2010).Further,exceptcorrelationsbetweenW30dandADG1and

betweenW30dandADG2,theotherestimateswerelowerthan theircorrespondinggeneticcorrelationestimates.

Environmentalcorrelations amonggrowthtraitsvariedfrom

−0.04betweenBWandADG1to0.93betweenW30dandADG1.The estimatesofenvironmentalcorrelationsamonginvestigatedtraits werepositive(excepttheonebetweenBWandADG1)andgener- allylowerthanthoseofgeneticcorrelations.BahreiniBehzadietal.

(2007)andAbbasietal.(2012)reportedthattheenvironmental correlationsamonggrowthtraitswereallpositive.

3.4. Estimationofgeneticgain

Theannualgenetictrendfluctuatedthroughoutthestudyperiod (Figs. 1and2).Estimatesofdirectandmaternalannualgenetic trend obtained under univariate analyses for growth traits are showninTable6.Theannualdirectgenetictrendswerenegative andclosetozeroforallgrowthtraits.Estimateofdirectgenetic trendforBW (−1.12g/year)wasgenerallysimilartothelowest valuesreportedbyZishirietal.(2010)inSouthAfricanDormer breed(−2.1g/year)andbyDietal.(2014)inChinesesuperfine Merinosheep(−1g/year).However,itwaslowerthanestimates reportedinvariousothersheepbreeds(0g/yearinIranianBaluchi breed,GholizadehandGhafouri-Kesbi, 2015; 2g/yearin Iranian Kermani,Mokhtariand Rashidi,2010; 5g/yearin SouthAfrican DohneMerino,KlerkandHeydenrych,1990;21g/yearinIranian Afsharisheep,Ghafouri-Kesbietal.,2009).

TheestimateofdirectgenetictrendforW30dwas0.17g/year (P>0.05).Kariukietal.(2010)reportedanestimateof107g/year inDorpersheep.ForW90d,ADG1andADG2,directgenetictrends werenegativeandsignificant(P<0.01)and estimatedto−2.65,

−0.01 and −0.03g/year, repectively. While there are very few reportsdealingwithgenetictrendestimatesforADGinliterature, estimatesofdirect genetictrendforweaning weightinvarious sheep breeds are numerous. Bosso et al. (2007) and Ghafouri- Kesbietal.(2009)reportedthatthegenetictrendforpre-weaning growthrate(0–4months)increasedby0.01and0.001kgperyear, respectively.Thegenetictrendestimatesforweaningweightwere 7g/yearinBaluchibreed(GholizadehandGhafouri-Kesbi,2015), 40.6g/yearinDormerbreed(Zishirietal.,2010),96g/yearinDor- persheep(Kariukietal.,2010),128g/yearinKurdisheep(Rashidi andAkheshi,2007)and167ginAfsharisheep(Ghafouri-Kesbietal., 2009).

Further,maternalgenetictrendwaspositiveandhighlysignif- icant(P<0.001)andwas1.08g/yearforBWandnegativeandnot significant(P>0.05)andwas−0.26g/yearforW30d.Ahighervalue formaternalgeneticresponse(3g/year)wasreportedbyMokhtari andRashidi(2010)inIranianKermanibreed.Also,maternalgenetic trendestimateforBWwashigherthanthedirectone.Thismaybe duetomaternaleffectsinSardisheepthathavemorepronounced effectonBWthanthedirectadditivegeneticeffects.

Thelowandirrelevantrealizedannualgenetictrendsforgrowth traitsinthepresentstudycouldbeattributedprincipallytothe lowheritabilityestimatesandtothepoorselectionpractisedthat largelyreliesonphenotypiccharacteristicsinsteadofonanimals’

additivegeneticvalues.Inthisfarm,siresareselectedeveryyear atabout18month’sagebasedonconformation,weightandcon- formitytoexternalappearanceofbreed,butnotontheirgenetic merit,andyetselectionofanimalswithhigherphenotypicvalues doesn’tguaranteetheselectionofanimalswithhigherbreeding values.Thelowannualgeneticgainsobtainedinthecurrentstudy mightbealsoduetotheharshrearingenvironmentofSardisheep thatisnotverysuitablefortheexteriorisationoflambs’genetic potential.Infact,Sardisheepareraisedinasemi-aridzonechar- acterizedbyahightemperatureinsummerandpoorqualityand quantitypastures.

4. Conclusion

Thepresentstudycontributestothecomparison ofdifferent models for estimation of (co)variancecomponents and genetic parameters in Sardi sheep.Although only BW and W30d were affectedbymaternaladditivegeneticeffects,theresultsconfirmed thenecessityofincludingmaternaleffectsinmodelsappliedfor geneticevaluationofearlygrowthtraitsofSardisheep.Thelowher- itabilityestimatesobtainedforinvestigatedtraitsinthisresearch suggestthatmassselectionbasedonthesetraitsmayresultinslow geneticprogressforgrowthtraits.Thedifferenceinestimatesof geneticparametersdeterminedindifferentmodelsshowedthat modelchoiceisanimportantaspectforobtainingaccuratepredic- tionsofbreedingvaluesthataregoingtobeusedwhendeciding onabreedingprogramme.Finally,duetotheexistenceofgenetic variationforgrowthtraitsandgenerallypositiveandmediumto highgeneticcorrelationsamongtheinvestigatedtraits,itcanbe concludedthatimprovementofgrowthtraitsofSardisheepseems feasibleinselectionprogrammes.Nevertheless,itisrecommended toimprovethemanagementofflockinordertoreducetheenviron- mentalvarianceandtoincreaseheritabilityestimates,andalsoto selectreplacementanimalsbasedontheirgeneticmeritbutnoton theirrawperformancesbecausewithoutaselectiononthegenetic merit,nogeneticimprovementwillbeexpected.

Acknowledgment

Theco-authorwouldliketothankthePROGEBE-GUINEAProject forfinancinghisMasterstudies.

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