Proceedings of the 5th International Symposium on EQUINE ...

Equine Embryo TransferThe embryo recovery rate was similar in premares inseminated one or 2 times, both for singleovulating mares (52/62, 84% and 34/41, 83%respectively) and for double ovulating mares(15/11, 136% and 50/37, 135%, respectively).There was no difference in embryo recovery rateamong single ovulating mares last bred 0.5, 1, 1.5,2, or 2.5 days before ovulation (29/35, 83%; 4/6,67%; 31/37, 84%; 6/8, 75% and 2/2, 100%,respectively).DISCUSSIONThese findings indicate that post ovulationinsemination results in reduced embryo recovery,but no increase in embryo mortality after transfer.Although reduced, <strong>the</strong> recovery rate <strong>of</strong> 63%justifies using this procedure ra<strong>the</strong>r thanabandoning <strong>the</strong> cycle. For <strong>the</strong> 8 stallions used inthis programme, <strong>the</strong>re was no advantage ininseminating 2 times ra<strong>the</strong>r than once, or fortiming insemination closer than 2.5 days beforeovulation.There have been conflicting reports in <strong>the</strong>literature regarding <strong>the</strong> fertility <strong>of</strong> post ovulationbreeding in <strong>the</strong> mare. Huhtinen et al. (1996) foundthat embryo recovery rate decreased as time fromovulation to insemination increased, but maresinseminated within 16 h <strong>of</strong> ovulation had similarrecovery rates to mares inseminated beforeovulation. Woods et al. (1990) reported thatinsemination within 12 h <strong>of</strong> ovulation did notsignificantly decrease pregnancy rates. However,<strong>the</strong>se studies involved few mares (14–20 pergroup) which may have limited <strong>the</strong> power to detectdifferences between treatments. In <strong>the</strong> study <strong>of</strong>Woods et al. (1990), pregnancy rates achieved inmares inseminated 0–6 h post ovulation and 6–12h post ovulation (79% and 65% respectively) weresimilar to our embryo recovery rates for pre andpost breedings (83% and 63%, respectively),suggesting that most fertilisation failure/embryoloss occurs in mares inseminated 6–12 h afterovulation.The delay from ovulation to fertilisation inpost breedings incorporates both <strong>the</strong> time neededfor <strong>the</strong> sperm to arrive at <strong>the</strong> site <strong>of</strong> fertilisationand time needed for capacitation. Oocyte agingmay result in lack <strong>of</strong> fertilisation or in reducedviability <strong>of</strong> <strong>the</strong> fertilised eggs. It is also possiblethat some <strong>of</strong> <strong>the</strong> embryos from post ovulationbreeding lack <strong>the</strong> ability to signal <strong>the</strong> oviduct fordescent into <strong>the</strong> uterus, and thus perish in <strong>the</strong>oviduct. The embryos recovered from postbreedings, even though <strong>the</strong>y were recovered oneday later, were at an earlier stage <strong>of</strong> developmentthan were embryos from pre breedings. However,<strong>the</strong>re was no significant difference in quality.These findings agree with those <strong>of</strong> Huhtinen et al.(1996), and also with <strong>the</strong> ultrasonographicfindings <strong>of</strong> Woods et al. (1990), who reported thatpost embryos were more than one day smaller indiameter than pre embryos at Days 11–15 <strong>of</strong>pregnancy. Since insemination was performedwithin 12 h <strong>of</strong> ovulation, <strong>the</strong> >1 day developmentaldelay in post embryos may reflect <strong>the</strong> time neededfor <strong>the</strong> sperm to capacitate before fertilisation, orpossibly retardation <strong>of</strong> embryo growth because <strong>of</strong>compromised oocyte viability. The time requiredfor <strong>the</strong> sperm to capacitate is not known in <strong>the</strong>stallion. When mares were bred before ovulation,anaphase was recognised by 6 h after ovulation(Bezard et al. 1989). However in mares bred postovulation, telophase was not seen until 10 h postcoitum (Enders et al. 1987). Although <strong>the</strong> data arescant, this suggests about a 4 h delay relating totransport and capacitation <strong>of</strong> sperm afterinsemination. The contrast between our results and<strong>the</strong>se previous data suggest that more work isneeded on <strong>the</strong> physiology <strong>of</strong> in vivo spermtransport and capacitation in <strong>the</strong> horse, and on <strong>the</strong>effects <strong>of</strong> oocyte aging on embryo development.REFERENCESBezard, J., Magistrini, M., Duchamp, G. and Palmer, E.(1989) Chronology <strong>of</strong> Equine fertilisation andembryonic development in vivo and in vitro. Equinevet. J. Suppl. 8, 105-110.Enders, A.C., Liu, I.K.M., Bowers, J., Lantz, K.C.,Schlafke S. and Suarez, S. (1987) The ovulatedovum <strong>of</strong> <strong>the</strong> horse: cytology <strong>of</strong> non-fertilized ova topronuclear stage ova. Biol. Reprod. 37, 453-466.Huhtinen, M., Koskinen, E., Skidmore, J.A. and Allen,W.R. (1996) Recovery rate and quality <strong>of</strong> embryosfrom mares inseminated after ovulation.Theriogenol. 45, 719-726.Riera, F.L and MacDonough, J. (1993) Commercialembryo transfer in polo ponies in Argentina. Equinevet. J. Suppl. 15, 116-118.Woods, J, Bergfelt, D.R. and Gin<strong>the</strong>r, O.J. (1990) Effects<strong>of</strong> time <strong>of</strong> insemination relative to ovulation onpregnancy rate and embryonic loss rate in mares.Equine vet. J. 22, 410-415.90