Reproduction in Domestic Animals

262 MG Diskin and DG MorrisTable 1. Probability of embryo survival as a function of ovulationrate a in naturally mated ewes and does (Source: Hanrahan 1994)No. ofcorpora luteaEwesProbability of embryo survival bDoes2 0.82 (5069) c 0.82 (306)3 0.74 (884) 0.60 d (58)4 0.65 (270) –5 0.55 (91) –6 0.45 (38) –a Based on summary of published and unpublished information on ovulation rateand the number of foetuses ⁄ off-springs.b About ‡250 and 0.06 to 0.08 otherwise.c No. of females involved (n).d Includes observations on some does with >3 corpora lutea.rate (Hanrahan 1994) and data are summarized inTable 1. While the information on goats is limitedrelative to sheep, both sets of data show that theprobability of embryo survival declines as ovulationrate increases. From a comparison of the publishedliterature on embryo survival in mated ewes or in ewesfollowing embryo transfer, Hanrahan (1994) concludedthat the change in pattern of embryo survival withincreasing number of ova shed or embryos transferredwas similar, and the clear negative relationshipbetween the number of ovulations and subsequentembryo survival is a direct consequence of the numberof embryos entering the uterus rather than anyproblems with embryo quality, the time of ovulationor uterine environment. He also concluded that therewas no evidence of an inherent physiological or geneticrelationship between ovulation rate and embryo survival.A direct consequence of these conclusions is thatinformation on embryo survival in sheep and goatsshould be examined in relation to ovulation rate andthe interpretation of differences due to management orenvironmental factors should allow for this directeffect of ovulation rate. Extrapolation of the datacontained in Table 1 provides estimates ofembryo ⁄ foetal loss of 12%, 18% and 26% for eweswith one, two or three ovulation, respectively. Incomparison with cattle, which are predominatelymono-ovular, the embryo loss rate of 12% formono-ovular sheep is much lower than comparablefigures of about 40% and 60% for heifers and highproducingdairy cows, respectively.In camelids the proportion pregnant at about60 days post-mating is about 65% of those thatovulate (Adams et al. 1991), although much lowerfigures have been recorded. Given that fertilizationrates are similar in all species, it seems that embryo lossrates are much higher in camelids than in sheep orgoats. While twin ovulations occur quite frequently incamelids, twin births are rare indicating that themechanisms that control embryo survival are probablydifferent in camelids to those in cattle, sheep and goats.One of the factors which may contribute to the higherembryo loss rate is the fact that in all camelids, over98% of the foetuses implant in the left uterine horndespite the fact both ovaries contribute almost equallyto ovulation. It appears that the right uterine horn isunable to sustain a pregnancy after about 50 days ofgestation (Arthur et al. 1982).Causes of Embryo or Foetal LossSeveral factors have been implicated in embryo andfoetal loss, usually with a lack of supporting experimentaldata. The main factors implicated are normallycategorized as those of genetic, physiological, endocrineand environmental origin (see reviews by Ashworth1994; Bazer 1994; Nancarrow 1994; Zavy 1994). Onlysome possible genetic and environmental factors areconsidered here.Genetic causesGenetic causes of embryo death include chromosomaldefects, individual genes and genetic interactions (Van-Raden and Miller 2006). The 1 ⁄ 29 Robertsonian chromosomaltranslocation (Gustavsson 1979) present inseveral beef breeds and in the Scandinavian Red breeds,but not in the Holstein breed and has been suggested asa cause of reduced fertility of males and femalesheterozygous for 1 ⁄ 29 translocation. Sweden has successfullyimplemented a testing programme to eliminatefrom AI bulls that are carriers for this conditionresulting in a significant improvement in fertility (Gustavsson1979).In the Holstein breed, two major recessive defectsaffecting embryo ⁄ foetal survival have been detected.Deficiency of uridine monophosphate synthase(DUMPS) (Robinson et al. 1984), a homozygous recessivecondition, causes foetal death a 40–50 days ofgestation (Shanks and Robinson 1989). Testing of AIsires for DUMPS has significantly reduced the frequencyof heterozygous sires and of homozygousrecessive embryos and has now almost eliminated thisas a cause of fertility losses (VanRaden and Miller2006). Complex vertebral malformation is a lethalrecessive condition that causes late foetal death in cattle.This defect was disseminated through the widespreaduse of the Holstein Carlin-M Ivanhoe Bell as a maternalsire of sires. In a Swedish study, Persson (2003) recordedthat carrier bulls had significantly lower breeding valuesfor 168-day non-return rate than non-carriers at168 days following insemination but were not differentat 28 and 56 days indicating that it primarily affects laterather than early foetal mortality. Again it is possible togreatly reduce the incidence if carriers for this conditionby testing sires.Several reproductive traits are adversely affected byinbreeding. Maternal inbreeding has been reported todecrease the 56–70-day non-return rates by between 1%(Wall et al. 2003) and 2% (Cassell et al. 2003) per 10%inbreeding of the dam. On the other hand, inbreeding ofthe embryo has bee reported to reduce the 70-day nonreturnrate by 1% for each 10% increase in the level ofinbreeding of the embryo (Cassell et al. 2003; VanRadenand Miller 2006). Genetic variation in embryo survivalmay be attributable to genetic constitution of theembryo itself and or the genetic differences among damswith respect to their ability to provide an appropriateintraovarian and uterine environment. In sheep, BodinÓ 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag