Genetic study of gestation length in andalusian and arabian mares

22 Pages·2006·0.49 MB·English

AnimalReproductionScience95(2006)75–96
Genetic study of gestation length in andalusian
and arabian mares
M. Valeraa, F. Blesab, R. Dos Santosc,∗, A. Molinab
aDepartamentodiCienciasAgroforestales,EscuelaUniversitariadeIngenier´ıaTe´cnicaAgr´ıcola,
UniversidaddeSevilla,Ctra.DeUtrera,km.1,41013Sevilla,Spain
bDepartamentodiGene´tica,FacultaddeVeterinaria,UniversidaddeCo´rdoba,Edif.GregorMendel,
Pl.baja,CampusUniversitariodeRabanales,Ctra.Madrid-Ca´diz(N-IV)km396a,
14071Co´rdoba,Spain
cEscolaSuperiorAgra´riadeElvas,RuadeAlcamim,19Apartado254,
7350-903Elvas,Portugal
Received4August2004;receivedinrevisedform4April2005;accepted15September2005
Availableonline2November2005
Abstract
ThelengthofgestationinAndalusian,orSpanishPurebred(SPB)andArabian(AB)maresrearedinSpain
wasanalysed,basedon766spontaneousfull-termdeliveriesappertainingto141maresofSPBbreedand72
maresofABbreedin31breedingseasons.ThedatawereobtainedfromtheYeguadaMilitardeJerezdela
FronterastudfarminCa´diz,Spain.Themeanlengthofgestationwasof336.8±0.48daysintheSPBmares
and340.3±0.63daysinABmares.Toassesstheaccuratepredictionoftimeofbirththepotentialeffectof
anumberoffactorswasinvestigated.Theinfluencesofthebreed,mare,monthandyearofmating,ageof
themother,numberofbirthsandsexofthefoalwerestatisticallysignificant.Thefactorhavethegreatest
influenceoverthegestationlengthwasthemareitself,withacorrelationamongconsecutivebirthsofaround
0.4.Theeffectofinbreeding,bothofthemareandfoal,wasnegligible.Gestationlengthshortenedasthe
breedingseasonprogressed:inbothbreeds,adelayof1monthinmatingcorrespondedtoadecreaseof3
daysinthegestationlength.Accordingtoourresults,gestationlengthdecreaseasthemaregetsolder,with
theshortestgestationperiodswhenthemareis10–12yearsold,andfromthispointon,itslowlyincreases.
Thegestationperiodshortensasthe4thor5thbirthapproaches,andthengetsprogressivelylonger.The
rangeofvariationingestationlengthduetothenumberofbirthstothemareisof2.9daysfortheABmares,
and2.2daysforSPBmares.TheheritabilityforthegestationlengthforABandtheSPBbreedswas0.2,with
arepeatabilityof0.36and0.37,forSPBandABbreeds,respectively.Withthedatafrombothbreeds,and
usingaclassicalapproach,theresponsetoselectionwasestimatedifmareswithextremegestationlengths
wereculled,i.e.lengthswhichareunder310days,orover360days.Accordingtoourresults,inthecaseof
∗ Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddress:[email protected](R.D.Santos).
0378-4320/$–seefrontmatter©2005ElsevierB.V.Allrightsreserved.
doi:10.1016/j.anireprosci.2005.09.008
76 M.Valeraetal./AnimalReproductionScience95(2006)75–96
SPB,adecreaseof14–45%wouldoccurinthenumberofextremegestationlengths,whileintheABbreed,
thisvaluewoulddecreasefrom2to39%.
©2005ElsevierB.V.Allrightsreserved.
Keywords: Equine;Heritability;Foaling;Repeatability
1. Introduction
Thelengthofgestationisaphysiologicalvariableofeconomicalimportanceinmostdomestic
species. The length of full-term pregnancies varies within a short range of days, even though
a certain variability can be appreciated and justified genetically as much as by environmental
factors.Inhorses,therangeofvariabilityofthegestationlengthislargerthaninotherdomestic
species(BosandVanDerMey,1980),probablyduetothefactthattheirlonggestationperiodis
influencedbyphysiologicalfactorsthatdonotinfluencethegestationlengthsofotherspecies(for
instance,embryonicdiapause,reportedbyLofstedt,1992).Anumberofstudieshaveconfirmed
thesewidevariationsingestationlengthforvariousbreedsofhorses(Pe´rezetal.,2003).
Additionally,thevariationinphysicalsignsofimminentparturitionmakesthepredictionof
parturitioninthemareparticularlydifficult.Themanagementofhigh-valuestockdemandsthe
precisepredictionofparturition.Eventhoughintheequinethegestationallengthisinsufficient
to determine foetal readiness for birth (Lofstedt, 1992), since, to a certain extent, the mare can
regulate the size of the foal (Allen et al., 2002), the knowledge of the gestation length and the
possibility of predicting the date of birth can be important to a successful management of the
pregnant mare. The inability to accurately predict the timing of parturition is incurred in extra
labour and veterinary costs, as well as higher risks to both mare and foal (Davies-Morel et al.,
2002). Furthermore, to coordinate deliveries in a given period of time, mares can be grouped
accordingtotheirpredictedgestationlengths.Ontheotherhand,thisenablesthecullingofmares
withverylonggestationperiodsthatresultinverylargefoals,whichcan,undersomeconditions,
causedystociclabours,aswellasofmareswithveryshortgestationperiodsthatproducevery
light-weight foals. Therefore, any factors that may be used to help determine the moment of
parturitionareofconsiderableinteresttotheindustry.
Gestation lengths from 310 to 380 days (Rossdale et al., 1984) have reportedly resulted in
viable foals, although generally gestation lengths of 320–360 days are considered acceptable
(Panchaletal.,1995).Thislargevariabilityoftimeinwhichviablefoalscanbeborn,indicates
that gestation length in the mare may be highly susceptible to both genetic and environmental
factors.
Accordingtopublishedstudies,themainenvironmentalfactorsinfluencinggestationperiod
in a certain breed are related to age of mother, number of foalings, nutrition, sex of the foal,
year and month of conception, season of conception and photoperiodic influence (Howell and
Rollins, 1951; Pozo-Lora, 1954; Flade and Frederich, 1963; Hevia et al., 1994; Panchal et al.,
1995; Davies-Morel et al., 2002; Vassilev et al., 2002). Nevertheless, other factors have been
analysed,likehaircolour(Dringetal.,1981;Blesaetal.,1999)ormooncyclephase(Blesaet
al.,1999),eventhoughtheresultsobtainedwerenotsignificant.
From a genetic point of view, the studies that have been conducted up till now show values
of heritability high enough to expect a significant response to selection (Vassilev et al., 2002).
In this sense, to determine the best selection method and to estimate the genetic progress in a
M.Valeraetal./AnimalReproductionScience95(2006)75–96 77
specificbreed,itisessentialtoestimategeneticparametersanddefinethegeneticstructureand
environmentalbreedingconditionsinthisbreed.
Theaimofthisstudywastoanalysethemainenvironmentalandgeneticfactorsinfluencing
gestationlengthandtoestimatetheheritabilityandrepeatabilityofthegestationlengthsinthe
SpanishpopulationofSpanishPurebred(SPB)andArabian(AB)breeds.Thefinalobjectivewas
totestwhetherselectionwouldresultinacorrespondingdecreaseinthepercentageofanimals
withshortorprolongedgestationlengthsinthenextgeneration.
2. Materialandmethods
ReproductiverecordsweretakenfromthepopulationsofSpanishPurebredandArabhorses,
belonging to the Yeguada Militar de Jerez de la Frontera, Cadiz, stud farm, situated at latitude
◦ (cid:3) ◦ (cid:3)
36 41Nandlongitude06 09W.Throughoutitshistory,thisstudfarmhasstronglyinfluenced
the SPB breed. In it were born several stallions that were among the most influential on the
breed (Valera et al., 2005). It is a breeding farm and, simultaneously, a stallion deposit, with
bothSPBandABstallionsavailabletobreedinstudfarmsthatrequestedthem.Assuch,ithas
maintainedindividualsthatrepresentmostgeneticlinesfrombothbreeds(Valeraetal.,2003).
YeguadaMilitardeJerezdelaFronteracan,therefore,beconsideredasagoodrepresentativeof
theexistingvariability,forbothSPBandAB,inSpain.Furthermore,itsbreedingmanagement
controlissuperiortomostSpanishstudfarms.
Thereportedanalyseddatarepresents31breedingyearsandweregroupedin20classes(<1980,
1980–1998).Matingwasbynaturalmount.Until1980,gestationlengthswerecalculatedasthe
time from mating to parturition. This was assumed to be equivalent to ovulation to parturition,
even though sperm may survive for up to several days within the mare’s tract and still achieve
fertilization(Newcombe,1994).Fromthisyearon,inallgestations,themomentofovulationwas
determinedusingultrasoundscanning.Themareswerecoveredwithintheimminentovulation
period, considering the time of ovulation coincident with that of fertilization (Davies-Morel et
al.,2002).TheadventofultrasonicscanninginSpainbeganinthisstudfarmandso,from1980
onwards,theaccuratetimingofovulationand,hence,theprecisecalculationofgestationlength,
couldbemade.
Inthiswork,weonlyconsideredgestationswithnormalparturitionandviablenewborn.The
766 analysed births represent a population of 213 mares (141 SPB and 72 AB). These births
resultedin49.1%malesand50.9%females.Theageofthemaresatdeliveryrangedfrom2to
23years(mean7.6,95%boundariesbetween3and16).Themeannumberofbirthswasof3.8
perSPBmareand3.3perABmare.Asforstallions,themeanwasof5.6foalsperSPBstallion
and5.4foalsperABstallion.
2.1. Analysisoffactorsinfluencinggestationlengthandstatisticalproceduresused
Withthisinformation,wehavestudiedthedescriptivestatisticsofthegestationlengthvariable
fortheglobalpopulation,aswellasforeachbreed.
Toanalysethepossibleinfluenceofthedifferentenvironmentalfactorsongestationlengthof
themares,wecarriedoutanANOVAfactorialmodel,whichincludedasfactorstheageofthe
mother (13 classes: under 4 years old, 4, 5 and up to 14 years old and over 14 years old), the
month (7 classes: December–June) and year of conception (20 classes: <1980 and yearly from
1980to1998),andthesexofthefoal,aswellasdifferentinteractionsbetweenthesefactors.A
previousmodel(datanotshown)includedthenumberofbirthsasafactor,insteadoftheageof
78 M.Valeraetal./AnimalReproductionScience95(2006)75–96
themare.Becauseofbothfactorsaresohighlycorrelated(r=0.85,0.90toABand0.82toSPB),
andalsobecauseofthehighercoefficientofdeterminationofthemodelthatincludestheageof
themare,wechosetoincludethelatterinthedefinitivemodel.
Thosefactorsthatwerefoundtobesignificantweresubmittedtoavariancecomponentestimate
forthequantificationofthevarianceofgestationlengthexplainedbyeachfactor.
Tounderstandtowhatextentthefoalinglengthofamarecouldbeusedtoestimatethelength
of her next foaling, a regression analysis was carried out on the length of a gestation over the
lengthofthepreviousone.Thiskindofanalysisisaclassicalmethodtoestimatetherepeatability
ofacertaincharacter.Weightedcorrelationsbetweenadjacentrecordsofthesamemarewerealso
computed,anotherclassicmethodofestimatingrepeatability(Bliss,1967).Thiswasaccomplished
byestimatingthecorrelationcoefficientbetweenthelengthofthefirstandthelengthofthesecond
foaling, and also the correlation coefficient between the lengths of two consecutive foalings of
thesamemare.
Wealsostudiedthepossibleinfluenceoftheinbreedingcoefficientofthemareandthefoal
over the gestation length, through the analysis of the correlation between gestation length and
inbreeding coefficients of both mare and foal. An ANOVA test was used to compare the mean
gestationlengthsofdifferentinbreedinglevelsforbothmareandfoal:lowFbeingunder6.25%,
moderate F between 6.25 and 12.5% and high F over 12.5%. For the individual inbreeding
coefficient estimation (Wright, 1931), genealogical trees were reconstructed to the last known
generationusingthestudbooksofbothbreeds.Theywerethenenteredintoadatabasewith8439
records(5823forSPB,and2616forAB).
Finally,weanalysedthepredispositionofthemaresthatpresentedatleastonegestationlength
overorunderthe95%limitstohavelongerorshortergestationperiods.Forthis,weestimated
themeangestationlengthforthosemares,removingallrecordsaboveandunderthoselimits.
The statistical analyses were achieved using the Corr, Glm, Means, Tabulate and Varcomp
proceduresoftheStatisticalAnalysisSystem(SAS)package,Version6.0.
2.2. Geneticparametersestimation
To estimate of heritability and repeatability parameters, we designed a best linear unbiased
predictor(BLUP)univariateanimalmodel,inwhichweconsideredthelengthofgestationasa
character of the mare, and, as fixed environmental factors, the year of conception, the delivery
season and the sex of the foal. In this case, the age of the mare was considered a co-variable
(linearandquadratic).Therandomfactorincludedinthemodelwastheadditivegeneticeffect
oftheanimalitself(themare),andthepermanentenvironmenteffectofthemareasuncorrelated
randomeffect.
Toobtaintherelationshipmatrix,weexpandedthegenealogicaltreesuptothethirdknown
generation (great grandparents), through the data gathered from the SPB and AB studbooks,
creating a database with a total of 1253 genealogic entries (841 belonging to SPB and 412
belongingtoAB).
To estimate of heritability and repeatability parameters, we used Groeneveld (1998) VCE
program,Version4.0,whichfollowsaREMLiterativeprocedure.
Finally,andusingtheseparametersandthestatisticalvaluesthatcharacterizethefrequencydis-
tribution(meanandstandarddeviation)ofbothbreeds,withaclassicalapproach(seeFalconerand
Mackay,1996;Roff,1997;LynchandWalsh,1998),weestimatedthegeneticprogress(meanand
variance)thatwouldbeobtainedifweeliminatedmareswithgestationlengthsundertheinferior
limitofthefrequencydistribution(premisea),orwithgestationlengthsoutsidethe310–360days
M.Valeraetal./AnimalReproductionScience95(2006)75–96 79
Table1
DescriptivestatisticsofthegestationlengthinSpanishPurebred(SPB)andArabian(AB)mares
Breed N Mean±S.E.M. Median 95%Populations Coefficientof
boundaries variation(%)
SPB 532 336.75±0.480 337 311–358 3.29
AB 234 340.27±0.632 341 313–357 2.84
boundaries(premiseb).Havingestimatedtheprogressinthefollowinggeneration,wecalculated
thepercentageofanimalswithgestationlengthsstandingoutsideoftheseboundaries.
3. Results
Wehaveobtainedameangestationlengthof336.8daysforSPBmaresand340.3daysforAB
mares(Table1).AwiderrangewasfoundamongtheSPBmares,withboundariesat95%of47
days,andadifferenceof79daysbetweenextremevalues,whencomparedtothe44daysfound
intheABbreedwith54daysbetweenextremevalues.
Inbothbreeds,thegreatestfrequencywasfoundinthe335–340modalclass,with23%ofthe
gestations (Fig. 1). In both breeds, the coefficients of variation were very small, indicating the
lowvariabilityofthisreproductiveparameter,muchlowerthanforotherreproductiveparameters
inthehorse(Langlois,1973).
3.1. Analysisofthemainfactorsinfluencinggestationlength
Table2givesthesignificancelevelsofthemultifactorANOVAmodelsofthosefactorsthat
significantly affected the length of the gestation period. According to our results, the pregnant
mareitselfisthemostinfluentialfactoringestationlength.Thus,globally,themare’sinfluence
represents25.8%(Table2)ofthisvariable’svariability,whichmeansover50%ofthevariability
takenupbytheanalysedmodel,47.8%ofthegestationlengthvariability.Ifthissameanalysisis
doneseparatelytoeachofthebreeds,wewillfindthatthemareexplains25.7%ofthevariability,
i.e. 56% of the variability taken up by the model, in the SPB, while in the AB, it accounts for
26.4%ofthevariability,i.e.59%ofthevariabilitytakenupbythemodel.
Fig.1. HistogramofgestationlengthforSpanishPurebred(SPB)andArabbreed(AB).
80 M.Valeraetal./AnimalReproductionScience95(2006)75–96
Table2
SignificancelevelofANOVAfactorialsignificantfactorsandexplainedproportionofvarianceofgestationlengthin
SpanishPurebredandArabianmares
Factors Significancelevel(P) Variancecomponents %Ofvariancecomponents
Breed <0.001 6.789 5.40
Mare <0.001 32.429 25.81
Ageofmare <0.001 2.402 1.41
Sexoffoal <0.05 0.546 0.43
Yearofconception <0.001 5.533 4.40
Monthofconception <0.001 6.888 5.48
Year×monthofconception <0.001 5.434 4.32
Fromtheremaininganalysedeffects,thosewithgreatersignificancelevelsarethebreed,which
explains5.4%ofthevariability,theyearofconception,themonthofconceptionandtheinteraction
betweenboththesefactors.Globally,thefactormonth–yearofconceptionaccountsfor14.2%of
thevariability.Thefactorswithlesserlevelsofsignificancearetheageofmare(1.41%)andthe
sexofthefoal(0.43%).
3.1.1. Mare
AsshowninTable2,ifweconsiderthefactorsinherenttothemareitself(maturity,physio-
logicalstatusandsoon)togetherwithitsage,wecanaccountfor27.2%ofthevariabilityofthe
gestationlength.
Theregressionofthelengthofasecondgestationoverapreviousgestationhadamoderate
magnitude,butthiswas,statistically,highlysignificant(b=0.359intheglobalsimple,0.338in
SPBand0.365inAB).Thecorrelationbetweenthelengthoftwoconsecutivegestationperiods
wasof0.383(0.394inSPBand0.380inAB).
Finally,westudiedmaresthatpresentedatleastonegestationperiodoutofthe95%boundaries
(Table1)toanalysetheirpredispositiontopresentlongerorshortergestationperiods.Wefound
areducednumberofmares(26),withatotalof96births,soresultsshouldbeconsideredwith
extremecaution(8ABmaresand18SPBmares).Ofthese26mares,14presentedanextremely
short gestation period (4 AB mares and 10 SPB mares), while 12 presented an extremely long
gestationperiod(4ABand8SPB).
We estimated the mean gestation lengths for these mares, excluding the extreme gestation
periods.Themeangestationlengthformarespresentinganextremelyshortgestationperiodwas
of336.9fortheSPBand337.2fortheABbreed.Inthecaseofmarespresentinganextremelylong
gestationperiod,themeanwasof342.4inSPBmaresand354.2inABmares.Thedifferences
betweenthemeansofthisgroupandthegeneralgroupwerenon-significantintheSPB,whereas
in the AB the differences were significant to a 99% level. When both breeds are included, the
meanswerealsostatisticallydifferent.
From our results, seven of the eight AB mares presented mean gestation periods above or
belowthebreedmeanvalues,asexpected.InthecaseofSPB,however,only10ofthe18mares
had means that fitted the expected values, although in the other cases, they were very near the
breedmeanvalues.
Asregardstheageofthemare,thegreatestnumberoffoalingsoccurredwhenthemarewas
between4and9yearsoldinthecaseofSPBmares(66.9%),andbetween4and7yearsoldfor
Arabianmares(55.3%).Thelowestnumberoffoalingshappenedinmaresolderthan14inthe
SPBbreed(6.6%)andolderthan11intheABbreed(15.2%).
M.Valeraetal./AnimalReproductionScience95(2006)75–96 81
Fig.2. EvolutionofmeangestationlengthaccordingtoageofthemareinSpanishPurebred(SPB)andArabbreed(AB).
Accordingtoourresults,thegestationlengthdecreaseasthemaregetsolder,withtheshortest
gestationperiodswhenthemareis10–12yearsold;fromthispointonitslowlyincreases(Fig.2).
Differencesof10daysfortheSPBand9daysfortheABbreedwerefound.Uppervalueswere
foundwhenmaresare4yearsold,presentingmeangestationlengthsof341.6daysforSPBand
343.1daysforAB.Ontheotherhand,valuesfortheshortestgestationperiodswerewhenmares
are10and12yearsoldforSNBandAB,respectively,withmeanvaluesof330.9and334days,
respectively.Thesevaluesadjustthemselvestoasecond-degreepolynomialequation.
The number of foalings is highly correlated with the age of the mare (r=0.85 in the global
simple, 0.90 in AB and 0.82 in SPB). It was noticed that the gestation period shortens with
the approach of the 4th or 5th birth, and then gets progressively longer. The range of variation
undergonebythegestationlengthduetothenumberofbirthsis2.88daysforABmaresand2.22
daysforSPBmares.
Asconcernstheinfluenceofinbreedingoverthegestationlength,negligibleresultswerefound
inthecorrelationsbetweenboththemares’andthefoals’inbreedingcoefficientsandthegestation
length(−0.0002and0.0222,respectively).
Themeaninbreedingcoefficientforthe376SPBmaresthatwereinbred(71%oftheanimals)
was0.070±0.0035(withamaximumof0.5).IntheABmare,themeancoefficientforthe211
inbredanimals(90%ofthewholesample)wasof0.083±0.0032(withamaximumof0.5).
ThemeaninbreedingcoefficientfoundtheinbredABfoals(120animals,representing51%
oftheABfoalssample)wasof0.086±0.0035,withamaximumvalueof0.252,whereasinthe
SPBbreed,themeanvaluewasof0.073±0.0036,withamaximumof0.316,forthe234inbred
foals,representing44%oftheSPBfoals.
To determine whether the low correlation values found were due to a lack of associa-
tion of the variables, or if the range of inbreeding coefficients was too low to show signs of
82 M.Valeraetal./AnimalReproductionScience95(2006)75–96
Fig.3. EvolutionofmeangestationlengthaccordingtoyearofconceptioninSpanishPurebred(SPB)andArabianmares
(AB).
the inbreeding effect, a simple ANOVA test was performed to compare the mean gestation
lengths of mares grouped under three levels of inbreeding coefficient: low (F<6.25%), aver-
age (6.25%≤F≤12.5%) and high (F≥12.5%). Our results showed no significant differences
(F(2, 272)=2.0860, p=0.06729). The same results were obtained when using the same test to
analysetheinfluenceofthefoal’sinbreedingcoefficient(F(2,272)=1.8680,p=0.15640).
3.1.2. Monthandyearofconception
Fig.3showsthegestationlengthevolutionaccordingtotheyearofconceptionintheanalysed
period.Itcanbeseenthatthereisagreatvariabilitywithmaximumdifferencesbetween2-year
periods, which range between 11 days in AB (between 1981 and 1982) and 12 days for SPB
(between1983and1984).Ingeneral,therangeinvariationofthegestationlengthsamongyears
was5daysforSPBmaresand3.7daysforArabianmares.
Fig. 4 shows the gestation length evolution according to the breeding season and month of
conception. The greatest number of conceptions occurs between the months of February and
May (84% in SPB and 88% in AB), the natural breeding season in Spain (Pe´rez et al., 2003),
correspondingtobirthsfromJanuarytoApril.Transitionalperiodconceptions(fromSeptember
to January) were only 13 and 9% for SPB and AB mares, respectively. Finally, the end of the
breedingseason(JuneandJuly)representedtheremaining3%ofconceptionsinbothbreeds.
Itwasfoundthatthegestationlengthsdecreaseasthemonthsofconceptionprogress,reaching
theiruppervaluewhenconceptionhappensinthemonthsofDecemberandJanuary,andthelower
value when it happens in the months of May and June (Fig. 4). The comparison of means test
forthethreematingseasonsresultedinhighlysignificantstatisticaldifferences(F(2,760)=9.08,
p=0.00013;verysignificantinSPB,F(2,529)=5.88,p=0.002107andhighlysignificantinAB
(F(2,231)=7.64,p=0.00061orp<0.001),witha3-daydifferencebetweentransitionalperiod
M.Valeraetal./AnimalReproductionScience95(2006)75–96 83
Fig.4. EvolutionofmeangestationlengthaccordingtothebreedingseasonandmonthofconceptioninSpanishPurebred
(SPB)andArabianmares(AB).
and natural breeding season in SPB mares and an equal difference between natural breeding
seasonandtheendofbreedingseason.Thesedifferencesreach5and10days,respectively,inthe
ABbreed.
Therangeofvariationofgestationlength,accordingtotheconceptionmonth,is8daysinSPB
and17daysinAB.So,whentheSPBmaresconceiveinJanuary,thegestationperiodincreasesup
to340days,butwhenthemaresconceiveinMay,thisvariabledecreasesto332days.WhenAB
maresconceiveinDecember,thegestationperiodincreasesto346days,andwhentheyconceive
inJune,thisvariabledecreasesto329days.Finally,theaveragedifferenceamongthegestation
periods,correspondingtotwoconsecutiveconceptionmonths,fluctuatesaround3daysinboth
breeds.
3.1.3. Sexofthefoal
In the populations of SPB and AB, 50.9 and 44.9% of gestations, respectively, resulted in a
colt,while49.1and55.1%resultedinafilly.Ourresultsshow(Table3)largergestationlengths
forthecolts(337.9daysand341daysforSPBandAB,respectively)thanforthefillies(335.5
and339.6days,respectively),resultinginadifferenceof2.37daysbetweenthegestationperiods
ofbothsexesinSPB,and1.41daysinAB.
3.2. GeneticparametersofthegestationlengthinSPBandABmares
TheheritabilityobtainedusingaBLUPanimalmodelforthegestationlengthparameterwas
0.21inbothbreeds(Table4).Repeatabilitywasof0.36forSPBand0.37forAB.
Table3
DescriptivestatisticsofgestationlengthaccordingtothesexoffoalonSpanishPurebred(SPB)andArabbreed(AB)
Breed Sexoffoal N Mean±S.E.M. 95%Sample Coefficientof
boundaries variation(%)
SPB Male 271 337.91±0.674 310–359 3.28
Female 261 335.54±0.676 313–356 3.25
AB Male 105 341.05±0.956 313–359 2.87
Female 129 339.64±0.842 313–357 2.82
84 M.Valeraetal./AnimalReproductionScience95(2006)75–96
Table4
GeneticparametersforgestationlengthinSpanishPurebred(SPB)andArabbreed(AB)
SPB AB
Geneticadditivevariance(days2) 22.12 19.15
Permanentenvironmentalvariance(days2) 15.36 13.74
Environmentalvariance(days2) 68.32 56.66
Phenotypicvariance(days2) 105.80 89.55
Heritability 0.21±0.013 0.21±0.027
Repeatability 0.36±0.048 0.37±0.062
Table5showstheexpectedresultsofgestationlengthafteracycleofselectionunderthetwo
premisesthatwereanalysedinbothbreeds.
It can be seen that the percentage of mares to cull would be under 1% for both breeds in
premise A (culling of mares with gestation lengths under 310 days) and over 2% in premise B
(cullingofmareswithgestationlengthsunder310andover360days).Theresponsetoselection
hasdeterminedaveryslightincreaseinthegestationperiodmeaninbothbreedsunderpremise
A,andaveryslightdecreaseinthisvalueforbothbreedsunderpremiseB.Asfortheexpected
reductioninthephenotypicvarianceofthisvariableinthenextgeneration,itvariedfrom0.9%(AB
breed,premiseA)to17.8%(SPBbreed,premiseB).Withbothparameters(meanandphenotypic
variance),inthenextgenerationonecanexpectfrom1.3%(ABbreed,premiseB)to2.2%(SPB
breed,premiseB)ofanimalswithgestationlengthsoutsidetheseboundaries.Whencomparedto
animalswithextremegestationperiodsintheparentalpopulation(2.58%inSPBand2.14%in
AB),theexpectedresponsewillbeadecreaseof2–45%inextremeanimals,dependingonthe
consideredbreedandpremise.
4. Discussion
A number of studies have confirmed wide variations in gestation length for different breeds
of horses (Salerno and Montemurro, 1965b; Pe´rez et al., 2003). Hence, even though the mean
gestation found in literature for different breeds is generally between 335 and 345 days (see
Pe´rez et al., 2003, for a review), an equine gestation length between 320 and 360 days can be
considered normal (Laing and Leech, 1975; Rossdale et al., 1984; Rossdale, 1993; Immegart,
1997). Gestations of less than 320 days are generally considered short and resulting foals may
present,amongothercharacteristicscoveredbytheterm‘prematurity’,predispositiontoillness,
lowbirthweightandinabilitytostand.Gestationsshorterthan300daysarenotviablebecause
vital foetal organs are not fully developed (Rossdale, 1976), even though ‘full-term’ gestations
havebeendescribedlastingfrom294to386days.ItshouldbepointedoutthatintheSPBbreed,
thereweretwofoalscomingfromgestationsunder300daysthatsurvivednormally,whichseems
to disagree with previously reports on foal viability and the classification of dysmature foals
(Rossdale,1976).
Likewise,agestationperiodover360daysisconsideredlongor‘prolonged’.Insuchcases,
the newborn foal is ‘post mature’ and might display weakness, large size with poor muscular
development,amongotherabnormalities(Rossdale,1993).Nevertheless,therearereferencesof
gestationperiodsover400daysresultinginalivefoal:403days(Braunton,1990)or419days
(West,1994).Inoursample,nogestationlastedover360daysintheABbreedandonly5didin
theSPBbreed(<1%).ThisvaluewasslightlylowerthanthatfoundbyPe´rezetal.(2003)inthis
samebreed,butintheCarthusianlineage,thatpointedto1.10%ofprolongedgestations.